Tabell – Oversikt

Adventdalen Group

Name

The group is named after a major valley in central Spitsbergen.

Type area

The type area is Central Spitsbergen.

Thickness

Thickness on Svalbard: ca. 750-1600 m, known thickness on the Barents Sea Shelf: ca. 1000-1750 m.

Lithology

The Adventdalen Group comprises shales, siltstones and sandstones of Late Jurassic to Early Cretaceous age in Svalbard and throughout the Barents Sea Shelf.

Distribution

The group is widely exposed along the margins of the Central Tertiary Basin on Spitsbergen, as well as in eastern Spitsbergen (Sabine Land) and on Kong Karls Land. It continues across the Barents Sea Shelf to the Bjarmeland Platform, around the Loppa High and into the Hammerfest and Nordkapp basins. The Adventdalen Group was eroded down to varying levels during the late Cretaceous uplift. On the southem Barents Sea Shelf, this hiatus comprises only the Cenomanian and part of the Turonian, while the entire Upper Cretaceous is lacking on Svalbard.

Age

Late Jurassic - Early Cretaceous.

Depositional environment

The group is dominated by dark marine mudstones, but includes also deltaic and shelf sandstones as well as thin, condensed carbonate beds. Important hydrocarbon source rocks occur in the Upper Jurassic succession, both in Svalbard and in the Barents Sea (Agardhfjellet, Fuglen and Hekkingen formations). A Barremian sandstone unit, (Helvetiafjellet Formation) in Svalbard is the result of local uplift and deltaic progradation, while a coeval, condensed limestone interval in the Barents Sea ( Klippfisk Formation ) grades into marls and calcareous mudstones in the basins. A hiatus occurs around the Jurassic - Cretaceous boundary.

Subdivision

Six formations are defined within the group on the Barents Sea Shelf: the Fuglen , Hekkingen , Klippfisk , Knurr , Kolje and Kolmule formations.

Compiled from

Agat Formation

Name

Named after the gas-condensate Agat Discovery in Norwegian block 35/3.

Well type section

Norwegian well 35/3-4 from 3589 m to 3345 m, coordinates N 61°51'54.54", E 03°52'26.99" (Fig 5.19) , 95 m of cores, mainly from the lower half of the formation.

Well reference section

Norwegian well 35/3-5 from 3620 m to 3219 m, coordinates N 61°47'46.71",E 03°54'44.01" (Fig 5.20) . 65 m of cores from the upper part of the formation.

Thickness

In the type well the gross thickness of the formation is 244 m, and in the reference well 401 m. The gross thickness varies in that range in the wells in block 35/3.

Lithology

In the type well the formation consists of white to light grey, fine- to medium-grained, moderately to well-sorted sandstones alternating with grey claystones. The sandstones are usually micaceous and glauconitic and sometimes contain small amounts of pyrite. The sandstones in the type well are carbonate- and silica-cemented in zones. In the reference well, the upper part of the formation consists of medium- and coarse-grained to pebbly sandstones and conglomerates alternating with dark grey claystones. The conglomerates are both matrix- and grain-supported. The claystones are often found as 0.5-5 m thick layers between the sandstones. They are dark grey, usually calcareous and contain varying amounts of siltstone. They may occasionally pass into light grey, micaceous, calcareous and glauconitic siltstones.

Basal stratotype

The lower boundary is defined where sandstones become the dominant lithology and is placed at the base of the first marked coarsening-upwards sandstone unit or distinct sand body. On logs it shows as an upward reduction in gamma-ray response (Fig 5.19) and (Fig 5.20) and most often an increase in velocity (Fig 5.20) .

Characteristics of the upper boundary

The upper boundary is placed at the top of the upper sandstone layer. This boundary is especially distinct on the gamma-ray log since the overlying sediments are dominated by calcareous shales with a low sandstone content. The overlying sediments are represented either by the Rødby Formation (well 35/3-1 and 35/3-2 ), or by the Svarte Formation (well 35/3-4 and 35/3-5 ), (Fig 5.19) . (Fig 5.20) .

Distribution

The formation is encountered in the area around the Måløy Fault Blocks in Norwegian blocks 35/3-36/1 (Fig 5.21) and is expected to be present along the western boundary of the Fennoscandian Shield. It is assumed to pass into shales towards the west (Fig 5.21) .

Age

Aptian-Albian (possibly Early Cenomanian).

Depositional environment

Marine environment influenced by gravity flows of sediment.

Source

Akkar Member

Origin of name

Norwegian for “squid”.

Well type section

Norwegian well 7121/5-1 coordinates 71° 35'54.88"N, 21° 24'21.78"E (Fig 4.46) .

Well reference section

Norwegian well 7120/12-1 coordinates 71°6'48.71"N, 20° 45'20.13"E (Fig 4.47) .

Thickness

The gross thickness of the member is 55 m in the type well, and 38 m in the reference well.

Lithology

Grey to dark grey shales, interbedded sandstones, coal.

Lower boundary definition

The base of the Akkar Member (and the Fruholmen Formation ) is defined by a marked increase in gamma ray and neutron porosity logs, but often more moderate increases in interval transit time and bulk density readings.

Age

Norian (based on palynology).

Depositional environment

Open marine.

Compiled from

Alge Member

Name

Norwegian for “alga”.

Well type section

Norwegian well 7119/12-1 coordinates N 71°6'08.00'' E 19°47'40.29''.

Reference well section

Norwegian well 7120/12-1 coordinates N 71°6'48.71'' E 20°45'20.13''.

Thickness

35 m in the type well and 50 m in the reference well.

Lithology

The Alge Member forms the lower high gamma intensity part of the Hekkingen Formation and consists of black paper shales rich in organic material.

Lower boundary definition

The base is defined by the transition from carbonate cemented and pyritic mudstones to poorly consolidated shales, producing a sudden increase in interval transit time and an abrupt decrease in bulk density values.

Age

Late Oxfordian – Kimmeridgian, based on ammonites and palynology.

Depositional environment

The member was deposited in restricted shelf environments.

Compiled from

Alke Formation

Amundsen Formation

Name

Named by Deegan & Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2993m to 3051m, coord N 61°08'06", E 01°43'36.5" (Fig 3.10) .

Well reference sections

Norwegian wells 33/9-1 (Mobil) from 2838 m to 2923m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) , 31/2-1 (Shell) from 2272.5 m to 2292.5 m and from 2134 m to 2176 m, coord N 60°46'19,16", E 03°33'15.87" (Fig 3.13) . and 30/6-7 (Norsk Hydro) from 3023 m to 3151.5 m, coord N 60°38'39.49", E 02°45'21.74" (Fig 3.12) .

Thickness

58 m in the type well, 85 m in well 33/9-1 , 128.5 m in well 30/6-7 and 62 m in well 31/2-1 , where the formation is split by the Johansen Formation , forming an upper (42 m) and a lower (20 m) unit.

Lithology

In the well type section the formation consists of light to dark grey, firm, non-calcareous siltstones and shales, in part carbonaceous and pyritic. Thin, fine to coarse grained, grey calcareous and glauconitic sandstone beds are present in the marginal areas of the basin. In the Norwegian sector the formation is more calcareous, especially in the lower part.

Boundaries

The base of the formation is the base of the Dunlin Group (defined above). The top is marked by the change to the more regular gamma ray and sonic log response of the overlying Burton Formation . Where the Burton Formation is missing (on the Horda Platform), the upper boundary is the base of the Cook Formation .

Distribution

The formation is widely distributed in the East Shetland Basin and Viking Graben north of 59°N. Towards the northwest the formation appears to overstep the Statfjord Formation and rests on the Hegre Group .

Age

Probably Hettangian to Sinemurian or Early Pliensbachian.

Depositional environment

The formation contains exclusively marine sediments, representing deposition on a shallow marine shelf.

Source

Vollset, J. and Doré, A. G. (eds.) 1984: A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No. 3, 53 pp.

Andrew Formation

Name

Named by Deegan & Scull (1977) after the Andrew Field in UK blocks 18/27 and 16/28.

Well type section

UK well 14/25-1 from 2199 m to 1897 m, coordinates N58°01'11.40", E 00°00'56.40". No cores.

Well reference section

UK well 21/10-1 from 2464 to 2370 m, coordinates N 57°43'50.37", E 00°58'29.19" (Fig 5.44) . No cores.

Thickness

The Andrew Formation is 302 m thick in the type well and 94 m in the reference well, which represents the general thinning southwards from the East Shetland Platform. Close to the Fladen Ground Spur, the formation reaches thicknesses of approximately 1200 m.

Lithology

The Andrew Formation consists of sandstones with claystone interbeds. The sandstones are generally very fine to medium grained and are composed of subangular to subrounded, clear to orange-stained quartz and feldspar grains. The sandstones are poorly sorted and often have a calcareous cement. Thin stringers of limestone occur.

Basal stratotype

The Andrew Formation overlies the shales, marls and limestone interbeds of the Våle Formation , reflected by a higher gamma-ray level in the Andrew Formation. It may also rest on the Maureen Formation , and in that case the boundary is still seen as an increase in gamma-ray readings upwards into the less calcareous Andrew Formation (Fig 5.44) .

Characteristics of the upper boundary

The Lista Formation usually overlies the Andrew Formation, and the boundary is characterized by higher gamma-ray readings and lower velocity upwards into the Lista Formation . Where the Forties Formation directly overlies the Andrew Formation, the boundary may be difficult to define, but the Forties Formation generally has a lower velocity than the Andrew Formation (Fig 5.44) . This boundary is very difficult to determine in the Norwegian sector.

Distribution

The Andrew Formation was deposited as an elongated lobe trending southeastwards from the western part of the Fladen Ground Spur into the Central Trough. The formation is not well developed in the Norwegian sector, even though the distal parts of the lobe may be expected to occur. Its approximate distribution is shown in (Fig 5.47) .

Age

Paleocene.

Depositional environment

The Andrew Formation was deposited as submarine fans.

Source

Balder Formation

Name

Named by Deegan & Scull (1977) from the Balder Field in Norwegian blocks 25/10 and 25/11. Balder was a son of Odin, and one of the most famous gods in Norse mythology.

Well type section

Norwegian well 25/11-1 from 1780 m to 1705 m, coordinates N 59°10'57.39", E 02°24'28.18" (Fig 5.57) . Cores.

Well reference sections

Norwegian well 30/2-1 from 1993 m to 1917 m, coordinates N 60°52'05.42", E 02°38'49.16". Cores. Norwegian well 15/9-17 from 2253 to 2204 m, coordinates N 58°26'44.19", E 01°56'53.58" (Fig 5.58) . No cores.

Thickness

The Balder Formation is 75 m thick in the type well. Generally its thickness varies from less than 20 m to more than 100 m. Normally it is between 40 and 60 m.

Lithology

The Balder Formation is composed of laminated varicoloured, fissile shales with interbedded grey, green and buff, often pyritic, sandy tuffs and occasional stringers of limestone, dolomite and siderite. Sandstones are locally present, as shown in the type well (Fig 5.57) .

Basal stratotype

The lower boundary to the Sele or Lista formations is generally identified on logs as an upward decrease in gamma-ray response and an increase in velocity (Fig 5.56) (Fig 5.58) . This probably reflects the increase in the tuffaceous component of the Balder Formation.

Characteristics of the upper boundary

The upper boundary is defined at the transition from the laminated shales of the Balder Formation to the non-laminated, often glauconitic, occasionally reddish, overlying sediments. On logs this can normally be seen as an upward reduction in gamma-ray response and a decrease in velocity (Fig 5.56) . When the Balder Formation is overlain by the Frigg Formation the boundary is seen as a decrease in gamma-ray response and an increase in velocity into the Frigg Formation (Fig 5.62) . The log response of the Balder Formation is often described as bell-shaped (Fig 5.58) .

Distribution

The Balder Formation is distributed over most of the North Sea, and may correspond in part to the Mo Clay Formation in Denmark.

Age

Paleocene - Early Eocene.

Depositional environment

The Balder Formation was deposited in a deep marine setting, mainly as hemipelagic sediments. Some turbiditic sands occur locally. There was probably more than one volcanic source.

Source

Basement

On the Norwegian Continental Shelf bedrock of pre-Devonian age are regarded as basement in a petroleumgeological context. More than 50 wells in the North Sea, Norwegian Sea and Barents Sea have penetrated pre-Devonian rocks. Most of these wells were drilled on structural highs with depth to top basement quite evenly distributed between 1200 and 4000m.

The thicknesses drilled have generally been in the order of a few meters. 5 wells have drilled more than 100m of basement rock:
2/6-3 (142m),
16/1-4 (146m),
16/1-5 (195m),
35/3-2 (233m),
6306/10-1 (207m).

Some of the pre-Devonian rocks have been dated by the K/Ar-, Ar/Ar- or Rb/Sr-methods, indicating pre-Middle Devonian metamorphic ages. The age of formation of the rocks are ranging from the Precambrian to the Early Paleozoic.

The pre-Devonian rocks normally encountered include diverse rocks of low to high metamorphic grade, including metamorphosed igneous rocks such as granitic gneiss, and metasedimentary rocks such as mica schist, greenschist, quartzite and marble. The existing data indicate that these rocks are highly fractured and record a history of polyphase deformation from at least late Proterozoic times onward. In some wells weathered rock material forms an intermediate zone between basement and overlying sediments.

In more than 30 of the wells basement rocks were cored. The cores were taken to confirm that the wells had reached basement, and coring was therefore usually carried out below any weathering. All wells have information from cuttings and logs.

Gas condensate was tested from fractured crystalline basement in well 16/1-4 . Minor shows in basement fractures are recorded in various wells. Highly fractured and/or weathered basement rocks can have some potential as conduits for hydrocarbon migration or as reservoirs.

Compiled from

Billefjorden Group

Name

Cutbill & Challinor (1965) introduced the term “Billefjorden Group” for a suite of nonmarine sediments now recognised to be of late Devonian to early Carboniferous age. The type area is in the environs of Billefjorden in central Spitsbergen. The Billefjorden Group is a well-established lithostratigraphic unit and its overall facies development and depositional evolution is well known (e.g. Gjelberg 1981; Steel & Worsley 1984). Lower Carboniferous units in the southern Norwegian Barents Sea are herein also assigned to the group. The offshore development of the group resembles that described from onshore with one major exception: the upper part of the group includes some shallow marine deposits in the southeastern Finnmark Platform. As yet enigmatic red-bed sequences on the Loppa High are also tentatively assigned to the group, but need further investigation (see below).

Offshore reference areas

The Barents Sea subsurface reference area is located on the eastern Finnmark Platform (Fig 9.1 , 9.2 , 9.7) , where the Billefjorden Group has been penetrated in its entirety in wells 7128/4-1 from 2503 to 2058 m (Fig 9.8) . and 7128/6-1 from 2533.5 m to 2150 m (Fig 9.9) . Southwards, towards the Norwegian mainland, the Upper Palaeozoic succession subcrops against the Pliocene/Pleistocene unconformity and the IKU shallow cores 7127/10-U-02, 7127/10-U-03 and 7029/03-U-01 penetrated different intervals of the group (Bugge et al.1995). Present seismic coverage and quality provides reasonable control on the seismic-scale lateral development of the group on the Finnmark Platform.

Thickness

Thicknesses of 445 m and 384 m in wells 7128/4-1 and 7128/6-1 respectively should be compared to cumulative thicknesses of up to 2500 m in the type area of Billefjorden and of 590 m on Bjørnøya (Dallmann et al. 1999; Worsley et al. 2001). As elsewhere, the group’s sediments were deposited in the early phase of a period with active rifting, and both wells are located over the crests of structural highs in the southern part of the Finnmark Platform; seismic data indicate that the group is more than 600 m thick in the adjacent halfgrabens. Well 7120/2-1 on the Loppa High penetrated an 847 m thick succession of sediments tentatively assigned herein to the Billefjorden Group; the succession was encountered from 2624 m to 3471 m, before the well was terminated in a possible dolerite (Fig 9.10) , see also discussion in next section).

Lithology

On the Finnmark Platform, medium- to coarse-grained, occasionally conglomeratic, sandstones and minor siltstones and coals dominate the basal part of the succession represented by the Soldogg Formation . These are overlain by a succession of stacked metre-scale fining-upward cycles of sandstone, siltstone, claystone and coal assigned to the Tettegras Formation . The upper part of the group, represented by the Blærerot Formation , is characterised by a basal unit of fossiliferous limestones, overlain by marine shales and fine- to medium-grained, fluvial and shallow marine sandstones. Sediments tentatively assigned to the group are very differently developed on the Loppa High where well 7120/2-1 penetrated 847 m of varicoloured arkosic breccias, conglomerates, ignimbrites and other types of volcanoclastic deposits (Fig 9.10) . The basal 115 m are dominated by brownish siltstone and mudstone. The red nature of these sediments, together with their high content of volcanoclastics, makes them anomalous representatives of the Billefjorden Group in this area, although palynomorphs indicate an early Carboniferous age (Viséan PU to VF Miospore zones from 3467 to 2682 m and early Serpukhovian TK Zone from 2645 to 2630 m) (Lindström, in press). The only other arctic areas where lower Carboniferous red beds have been found are in the northern part of eastern Greenland (Stemmerik et al. 1993), but even there the change to red beds appears to have occurred in the late Tournaisian – i.e. older than the earliest datings on the Loppa High. The Loppa sequence’s overall lithology and tectonic setting seems rather to suggest closer affinities to the lowermost clastics of the overlying mid-Carboniferous Gipsdalen Group . More work is clearly needed on this problem, especially in view of this representing the only occurrence of volcanoclastics in the Upper Palaeozoic of the Barents Shelf; we note that dolerite dykes of probable mid- to late Carboniferous age have been reported by Lippard & Prestvik (1997) on Magerøy in Finnmark and mid-Carboniferous volcanics have also been identified on the adjacent Kola peninsula in northern Russia (Ulmishek 1982). In view of these uncertainties, this sequence has not yet been defined as a formational unit, and its assignment to the Billefjorden Group is still tentative.

Lateral extent and variation

The group is generally difficult to map outside the Finnmark Platform and little is known about its regional distribution and variation, although the overall impression is that the Billefjorden Group represents thick siliciclastic-dominated wedges that fill developing Carboniferous half-grabens in the southwestern Barents Sea. On the Finnmark Platform itself seismic mapping of the group suggests pronounced lateral variations in thickness due to infill of local half grabens resulting from Viséan–Serpukhovian rifting. Most of the thickening and probably most of the lateral facies changes are in the uppermost part of the group. The rift event appears to be less pronounced east of approximately 29º 30' on the Finnmark Platform where the base of the group is poorly defined seismically. A possibly pre-Viséan sedimentary succession is present locally in this eastern part of the platform. The group’s representatives also seem to infill local half-grabens on the Loppa High and the Norsel High (central Bjarmeland Platform), and deeply buried half-graben systems appear to be also present further to the east on the Bjarmeland Platform. Thick wedge-shaped units corresponding to the Billefjorden Group are observed on seismic lines along the margins of the Nordkapp Basin, suggesting that the basin already formed a major depocentre at that time.
Sediments assigned to the Billefjorden Group are generally separated from the underlying strata by an angular unconformity, as seen in wells 7128/6-1 , 7128/4-1 and core 7029/03-U-01. In 7128/6-1 , the group rests on Precambrian metasandstones (Røe & Roberts, 1992) at 2533.5 m RKB, with a 45 m thick transitional zone of conglomeratic sandstones interpreted as weathered and reworked basement rocks. These sediments are included in the Billefjorden Group (see definition of Soldogg Formation ). Upwards, they pass into more mature sandstones with rare siltstone and coal beds defined by a sharp upward decrease in bulk density and sonic velocity at 2488.5 m.

Age

The Billefjorden Group has been assigned to the Famennian to Viséan in the onshore areas of Bjørnøya and Spitsbergen (Dallmann et al. 1999; Worsley et al. 2001). The offshore development has been dated to the Viséan to early Serpukhovian. On the Finnmark Platform, palynomorphs suggest that the basal part of the group is of middle to late Viséan age (Bugge et al. 1995), i.e. significantly younger than the Famennian to Tournaisian age recorded for the basal sediments onshore. The upper part of the group is apparently of late Viséan to early Serpukhovian age (Bugge et al. 1995; Simon-Robertson 1992 and Geochem Group 1994). As noted above - the succession on the Loppa High has been dated to the Viséan to early Serpukhovian (Lindström, in press).

Depositional environments

The Billefjorden Group is characterised by an overall transition from continental fluvially dominated deposits of the Soldogg and Tettegras formations into transitional continental to marginal marine deposits of the Blærerot Formation on the eastern Finnmark Platform. The presence of coal indicates deposition in overall humid climatic conditions in contrast to the overlying Gipsdalen Group that is characterised by sediments deposited in more arid climates.
Shallow core data from 7029/03-U-01 suggest that the lower parts of the Soldogg Formation represent basement wash and braided river deposits. These pass upwards into delta/coastal plain sandstones, siltstones, claystones and coals of the Tettegras Formation , and are overlain by marine and transitional continental to marginal marine deposits of the Blærerot Formation in well 7128/6-1 . The transitional nature of this upper part is demonstrated by rare coal beds in the lowermost part of the formation at well 7128/6-1 . The Blærerot Formation appears to be missing in 7128/4-1 either as the result of local uplift and erosion or because the marine transgression never reached the high on which the well was drilled. This depositional area was separated by a major fault southwest of well 7128/4-1 from the provenance areas of basement rocks and metasediments towards the Finnmark coast (Gabrielsen et al. 1990). Shallow cores 7127/10-U-02 and 7127/10-U-03 were taken in a proximal position, 2-3 km away from this main fault and record a thick development of Viséan syn-rift fluvial deposits (Bugge et al. 1995).
The succession on the Loppa High apparently represents deposition in alluvial fans and proximal braided river systems in a rapidly subsiding sub-basin. Local volcanic activity is suggested on the basis of the large amount of volcanoclastic material in well 7120/2-1 . The depositional environments recorded from the Finnmark Platform generally resemble those recognised in the onshore areas of Spitsbergen and Bjørnøya. The most important difference is the evidence of marine flooding of the eastern Finnmark Platform, perhaps suggesting more prevalent marine conditions in the contemporaneous Nordkapp Basin, with transgression from the east. The lithofacies and depositional environments of the Billefjorden Group on Spitsbergen and Bjørnøya are summarised by Gjelberg (1981), Steel & Worsley (1984) and Harland (1997), all emphasising the considerable facies variations related to local variations in tectonic regime. The reservoir potential of the group’s sandstones has been noted by several authors (Grønlie et al. 1980, Steel & Worsley 1984, Worsley et al. 2001).

Formations assigned to the group

The Billefjorden Group is represented by three formations on the Finnmark Platform and these are formally defined and described herein. Formational names are selected from land plants found in northern Norway that utilise nourishment from insects that stick to their leaves. The succession in well 7120/2-1 on the Loppa High is not yet given any formal formational status.

Source

Bjarmeland Group

Name

Bjarmeland was used by the Vikings to describe the area immediately south of the Barents Sea. The area was visited and described by the Norwegian Viking, Ottar, in the 9th century. The name was more recently used to name a structural element on the Barents Shelf: the Bjarmeland Platform (Gabrielsen et al. 1990). The group was introduced and briefly reviewed by Dallmann et al. (1999) in the knowledge of the ongoing more detailed work presented herein.

Type area

The Bjarmeland Platform in the southern Norwegian Barents Sea is here defined as type area for the group since the offshore successions are best displayed in wells from this area, including the eastern flanks of the Loppa High (Fig 9.38) . Three wells show typical developments of the group: 7124/3-1 (4271 m to 3900 m), 7226/11-1 (4334 m to 4103 m) and 7121/1-1 R (3990 m to 3502 m): the base of the group is defined by the basal stratotype of the biohermal Polarrev Formation in well 7229/11-1 on the northern Finnmark Platform.

Reference areas

Well 7228/9-1 S (4361 m to 4065 m) (Fig 9.38) . located on the Finnmark Platform’s northern margins towards the Nordkapp Basin illustrates the group’s development in a deeper water basinal setting. The Finnmark Platform itself is an important reference area since the group has been drilled in a variety of settings from the outer platform areas to the north ( 7229/11-1 , 4282 m to 3970 m, (Fig 9.38) . across the central platform ( 7128/4-1 , 1820 m to 1704 m) and 7128/6-1 , 1835 m to 1745 m) to the southern updip areas represented by core 7128/12-U-01, 569.2 m to 557.5 m). The succession assigned to the group in 7128/4-1 and 7128/6-1 corresponds to lithological unit L-8 of Ehrenberg et al. (1998a). Hambergfjellet on the southern mountain massif of Bjørnøya is designated as an onshore reference area. Hambergfjellet is the type area of the Hambergfjellet Formation, the only onshore unit to be assigned to the Bjarmeland Group at the present time.

Thickness

The group attains a maximum thickness of 488 m in well 7121/1-1 R at the eastern flank of the Loppa High. It is thinner, 233-371 m, in wells 7226/11-1 and 7124/3-1 on the Bjarmeland Platform (Fig 9.38) . On the Finnmark Platform the group thins from 312 m in well 7229/11-1 to 116-89 m in wells 7128/4-1 and 7128/6-1 central on the platform and less than 50 m in the IKU cores further updip. The Hambergfjellet Formation on the southern cliffs of Bjørnøya shows a similar thickness of up to 60 m, but this unit wedges out and disappears northwards on the island.

Lithology

The group is dominated by white to light grey bioclastic limestones containing a typical cool-water fauna of crinoids, bryozoans, brachiopods and siliceous sponges. Silty, dark grey to black, locally bituminous limestones characterise the deeper-water succession. Minor cherts occur, especially in the uppermost part. Siliciclastics are rare, except on the Polheim Subplatform where the group is unusually developed and dominated by fine-grained siliciclastics and marls (well 7120/1-1 R2 ). The Hambergfjellet Formation on Bjørnøya consists of basal sandstones which onlap all older units from basement to Gipsdalen Group , passing up into sandy bioclastic limestones with a fauna dominated by crinoids, bryozoans and brachiopods (Worsley et al. 2001).

Lateral extent and variation

The group is most thickly developed at the eastern flanks of the Loppa High and eastward across the Bjarmeland Platform. The thickest development is in outer platform settings north and south of the Nordkapp Basin where thick bryozoan-dominated buildups occur as isolated mounds or merge to form elongated complexes (Gerard & Buhrig 1990; Nilsen et al. 1993). Intermound and basinal areas are dominated by more fine-grained and thinly bedded limestones, which in well 7120/1-1 R2 are interbedded with siliciclastic shales. The platform areas of the eastern Finnmark Platform are characterised by relatively uniform successions of bedded crinoid- and bryozoan-dominated packstones and grainstones.
The group is seen to onlap palaeohighs and the margins of the depositional basin such as the eastern flank and crestal areas of the Loppa High and the southern parts of the Finnmark Platform. It is missing in wells 7120/12-2 and 7120/12-4 from the southern Hammerfest Basin – western Finnmark Platform and onshore it is only known from the Hambergfjellet Formation on Bjørnøya, - although future work may well demonstrate that the uppermost Gipsfjorden Formation and the Vøringen Member (Kapp Starostin Formation) of the Tempelfjorden Group both age- and facies-wise represent lateral equivalents of parts of the group on Spitsbergen.

Age

Fusulinids suggest a mid-Sakmarian to late Artinskian age in 7128/6-1 (Ehrenberg et al. 2000). The base of the group is thought to be highly diachronous, oldest in the more distal settings and youngest on the platforms (Fig 9.4) . The Hambergfjellet Formation on Bjørnøya is dated as late Artinskian based on fusulinids and conodonts (Nakrem 1991; Nakrem et al. 1992). Fusulinids indicate a similar age in cores 7128/12-U-01 and 7129/10-U-01 (Bugge et al. 1995; Ehrenberg et al. 2000).

Correlation

The lower, Sakmarian to early Artinskian, part of the group may correlate to the uppermost Gipsfjorden Formation of Spitsbergen. The upper, late Artinskian, part - including the Hambergfjellet Formation of Bjørnøya - perhaps should be correlated to the transgressive Vøringen Member of the Kapp Starostin Formation on Spitsbergen (Dallmann et al. 1999; Worsley et al. 2001).

Depositional environments

The group is characterised by deposition of carbonates dominated by crinoids, bryozoans, brachiopods and siliceous sponges. The fauna is markedly different from the foraminifer-dominated warm-water fauna of the underlying Gipsdalen Group and is believed to reflect deposition in more temperate cool-water environments (Stemmerik 1997). Deposition took place in a variety of cool-water carbonate environments and deposits range from shallow inner shelf bioclastic grainstones to outer shelf bryozoan-dominated buildups and thinly bedded bioclastic wackestones and packstones. Siliciclastic input to the basin was limited - except locally in the west where deeper water shales are interbedded with resedimented carbonates in 7120/1-1 R2 ; sand input was also significant on Bjørnøya, immediately adjacent to the subaerially exposed parts of the Stappen High. The bryozoan-dominated carbonate buildups formed along the margins of the Nordkapp Basin on the outer part of the platforms. They are often located above older buildups. Distally to the trend of build-ups more marly sediments have been recorded in well 7228/9-1 S .

Formations assigned to the group

The Bjarmeland Group is represented by three formations in the offshore areas of the southern Norwegian Barents Sea. The formations are formally described herein and named after predators common to Arctic Norway. The Polarrev and Ulv formations show an interfingering of the carbonate buildups of the former and the inter-buildup lithofacies of the latter formation. The Ulv Formation was also developed in the outer platform and basinal areas throughout deposition of the group, while the uppermost Isbjørn Formation in inner shelf areas overlies earlier buildups but does not extend into deeper waters characteristic of the Ulv Formation . The Hambergfjellet Formation of Bjørnøya, defined by Worsley & Edwards (1976) is included in the group, as it appears to represent a lithologically similar but highly condensed (<60 m thick) development of the Isbjørn Formation .

Source

Blodøks Formation

Name

Named after Eirik Haraldson Blodøks, a Norwegian king who reigned in Norway (A.D. 930-934) and in Northumberland (A.D. -954).

Well type section

Norwegian well 25/1-1 from 3807 m to 3790 m, coordinates N 59°53'17.40", E 02°04'42.70" (Fig 5.33) . No cores.

Well reference sections

Norwegian well 35/3-2 from 3207 m to 3190 m, coordinates N 61°51'05.98", E 03°46'28.22" (Fig 5.34) . No cores. Norwegian well 1/3-1 from 4371 m to 4343 m, coordinates N 56°51'21.00", E 02°51'05.00" (Fig 5.24) . No cores. Danish well BO-1 from 2220 m to 2213 m, coordinates N 55°48'02.22", E 04°34'18.66" (Fig 5.27) . Cored throughout.

Thickness

The formation is 17 m thick in the type well ( 25/1-1 ), 17 m in well 35/3-2 , 28 m in well 1/3-1 and 7 m in well BO-1. It rarely exceeds 20 m in thickness.

Lithology

The formation consists of red, green, grey and black shales and mudstones which are non-calcareous to moderately calcareous. In the central North Sea the formation may show a varied influx of marls, limestones and chalky limestones.

Basal stratotype

The lower boundary is generally characterized by a distinct log break with an upward increase in gamma-ray intensity and a distinct decrease in velocity from the Svarte Formation (Fig 5.33) . or Hidra Formation (Fig 5.24) . into the Blodøks Formation. This is due to the lower content of carbonate in the Blodøks Formation.

Characteristics of the upper boundary

The upper boundary shows a decrease in gamma-ray intensity and an increase in velocity from the Blodøks Formation upwards into the more calcareous Tryggvason Formation (Fig 5.34) . or the chalky Hod Formation (Fig 5.24) .

Distribution

The formation is present throughout the North Sea, lacking only on local highs such as the Sørvestlandet High, the Utsira, Mandal, Jæren and Sele highs and the Grensen Ridge as well as above many salt diapirs.

Age

Latest Cenomanian to early Turonian.

Depositional environment

The formation was deposited during a period characterised by anoxic bottom conditions (e.g. Hart & Leary 1989). Presence of carbonates may indicate periods of more oxic conditions or supply of allochthonous limestones and chalks (e.g. Norwegian wells 1/3-1 and 2/5-1 ).

Remarks

The Blodøks Formation is equivalent to the former Plenus Marl Formation and the informal “formation B” of Deegan and Scull (1977) (Fig 5.6) . A black shale of early Turonian age is also widespread outside the North Sea, e.g. the Yorkshire Black Band in England (Jeffries 1963) and similar facies on Helgoland and in northwestern Germany (Schmidt and Späth 1980).

Source

Blærerot Formation

Name

From the Norwegian name for Bladderwort (Utriculária vulgáris).

Definition

The basal stratotype is defined at 501.8 m in IKU core 7029/03-U-01 on the Finnmark Platform (Fig 9.12) , (Fig 9.17) ; Table 9.1). Increased sonic velocities, imaging the contrast between the underlying porous sandstones and overlying tight carbonates, define the transition from the Tettegras Formation into the basal beds of the Blærerot Formation. Bedrock is overlain by glacial drift at 436 m in the core so that the formation’s upper part and total thickness is unknown in the type section (Fig 9.12) . Bugge et al. (1995) noted that the upper 15 m of the core, dated by them to the Serpukhovian, appear to show facies characteristic for both the Billefjorden and Gipsdalen groups, “reflecting a probable gradual transition between the two groups” in this area.

Reference section

A reference section is defined in the interval from 2202 m to 2150 m in well 7128/6-1 (Fig 9.9) ; Table 9.1). No cores were cut in this well, but logs show the same pattern as in the type section.

Thickness

The preserved thickness is 65.8 m in the type well and the total thickness 52 m in the reference well.

Lithology

The lowermost five metres of the Blærerot Formation consist of intensely bioturbated grey to yellowish brown limestone and sandy dolomites in core 7029/03-U-01. Bugge et al. (1995) described these as partially dolomitised mudstones and wackestones containing gastropods, brachiopods, bivalves, trilobites, foraminifers and crinoids. Large, laminated irregular nodules, interpreted as oncoids, are present in the lower part. The carbonates are overlain by a 23 m thick coarsening-upward succession of dark grey silty shale with a total organic carbon (TOC) content of 3-4% in the lower part. The shale becomes more bioturbated and less organic-rich upwards. It contains much of the same marine fossils as in the underlying carbonate unit, but in addition abundant terrestrial plant remains are present. There is sharp transition towards the overlying 22 m thick sandstone dominated unit, which consists of two coarsening-upwards cycles with basal dark grey siltstones. The sandstones are fine- to medium-grained and contain low-angle trough cross lamination and wave ripples (Fig 9.17) . Yellowish-brown silty shales with some coal abruptly overlie the apparently shallow marine succession of the lower Blærerot Formation and these are interpreted as coastal plain deposits (Fig 9.18) . Log correlation suggests that the same overall lithologies are present in the reference well 7128/6-1 . Cuttings from the basal carbonate bed in this well include a fauna very similar to that described from the type section.

Lateral extent and variation

The formation is only known from 7029/03-U-01 and 7128/6-1 . It is thinnest in 7128/6-1 , which is located over the crest of a rotated fault block. The formation is missing from 7128/4-1 on the crest of an adjacent uplifted block. Available biostratigraphic data suggest that the Blærerot Formation interfingers laterally with the Tettegras Formation towards the south (see Bugge et al. 1995).

Age

Palynomorphs in the type section indicate a late Viséan – early Serpukhovian age in the type section (Bugge et al. 1995). A similar age range is indicated for the formation in well 7128/6-1 (Simon-Robertson 1992).

Depositional environments

The carbonate beds at the base of the formation record the first marine flooding of the Finnmark Platform. Initial deposition of shallow marine platform carbonates was followed by deposition of shale in lower shoreface environments. The sandstones in the top of the type section are of upper shoreface to possibly fluvial origin (Bugge et al. 1995). The entire formation represents deltaic or shoreface progradation as the depositional response to a rapid marine transgression and its development resembles that of the classical Yoredale cycles of the UK (see e.g. Elliott 1975).

Correlation

Marine sediments are not known from the Billefjorden Group onshore Spitsbergen and Bjørnøya. Age-equivalent, non-marine sediments are widespread in the region and lacustrine organic-rich shales have been reported from the Sverdrup Basin (Goodarzi et al.1987; Davies & Nassichuk 1988). However, this unit probably represents the farthest extent of a marine transgression, presumably from the Timan-Pechora Basin to the east (c.f. Alsgaard 1993; Johansen et al. 1993) and correlative marine sequences should be expected to be present at depth in the Nordkapp Basin.

Source

Boknfjord Group

Name

From the main fjord in Rogaland, Norway.

Type area

The Fiskebank and Egersund Sub-Basins comprise the type area. The group is illustrated in the Norwegian well 9/4-3 (Conoco), (Fig 3.36) .

Thickness

In the type area wells show thicknesses of up to 500 m. In well 9/4-3 the thickness of the group is 290 m. Towards the basin margins the section thins considerably.

Lithology

The sediments of the group are dominated by shales. Varying amounts of siltstone, sandstone, limestone stringers and differences in organic content make it possible, however, to subdivide the group into formations (Olsen and Strass, 1982).

Boundaries

The lower boundary is characterized by a distinct log break with the underlying sandstones of the Vestland Group . The upper boundary is usually characterized by abrupt changes in log response to lower gamma ray and interval transit times in the overlaying Valhall Formation1). In the Egersund Sub-Basin this boundary may be difficult to identify due to a large supply of clastic material.

Distribution

The group is confined to the Fiskebank and Egersund Sub-Basins although the upper two formations extend further westwards than those lying below.

Age

The group ranges in age from Callovian to Ryazanian.

Subdivisions

The group can be subdivided into four formations, the Egersund (base), Tau , Sauda and Flekkefjord formations (top).

Remarks

The term "Bream Formation" was first used by Deegan and Scull (1977) to describe a Callovian-Portlandian (Volgian) sequence, mainly clay-stones and siltstones, distributed throughout the Norwegian-Danish Basin. The formation comprised the Egersund Member, Børglum Member and Fredrikshavn Member. The Bream Formation was adopted with some modification by Michelsen (1978) for the Danish Sub-Basin where it comprises the Børglum and Fredrikshavn Members. Recent correlation work in the Egersund and Fiskebank Sub-Basin (e.g. Olsen and Strass, 1982) shows the existence of four argillaceous units, ranging in age from Callovian-Ryazanian, which are considered stratigraphically useful. They are widespread enough to deserve formation status and different enough from the Danish deposits to merit separate nomenclature. These units are the Egersund , Tau , Sauda and Flekkefjord formations. The formal definitions of the unit in this volume outline their relationship to the Egersund/Børglum/Fredrikshavn Members of Deegan and Scull (1977). Note that the Flekkefjord Formation was formerly part of the early Cretaceous Vallhall Formation, also defined by Deegan and Scull.
The four formations fall naturally into a single "claystone" group. It is not however considered proper to elevate the former Bream Formation to a "Bream Group" which would encompass these units, since it may still desirable to retain the formation status of the Bream Formation and the member status of the Børglum/Fredrikshavn Members in the Danish sector.
It is therefore proposed that the term "Bream Formation" should be abandoned for the Fiske-bank and Egersund Sub-Basins, and replaced by the Boknfjord Group, which is defined above.

Source

Footnotes

Brent Group

Name

Named by Deegan and Scull (1977). According to Norwegian usage, the unit earlier had formation status and the type well was UK well 211/29-3 (Shell). In this report the unit is given group status, also in the Norwegian sector.

Type area

The type area is the East Shetland Basin. The following wells are used to illustrate the Brent Group: UK well 211/29-3 (Shell), Norwegian wells 33/9-1 (Mobil), 30/6-7 (Norsk Hydro) and 31/4-4 (Norsk Hydro).

Thickness

The thickness of the group varies considerably. In UK well 211/29-3 (Brent Field) it is 226.5 m, while the Norwegian well 33/9-1 in the Statfjord Field has 204 m of Brent Group sediments. Wells used to illustrate the group on and around the Horda Platform have thicknesses between 159 m ( 30/6-7 ) and 78 m ( 31/4-4 ). Thicknesses of 200 m or more are present to the north in quadrant 35.

Lithology

The group consists of grey to brown sandstones, siltstones and shales with subordinate coal beds and conglomerates.

Boundaries

In the Brent-Statfjord area the group normally rests with a minor disconformity on the predominantly argillaceous Dunlin Group. To the west and in the southern Viking Graben it cuts down onto lower levels within the Dunlin Group . On the Horda Platform the lower boundary is either picked at the base of a "coarsening upward" log motif, with underlying marine shales, or at the base of a homogenous sandstone with a "blocky" log appearance. The upper boundary of the Brent Group may vary in nature due to post-middle Jurassic tectonism and erosion. Variable amounts of the group may be missing, particulary, towards the crests of tilted fault blocks, but the contact is nevertheless easy to pick where the sandy Brent Group is overlain by Heather Formation shales.

Distribution

The Brent Group is recognizable over most of the East Shetland Basin and the northern part of the Horda Platform. It passes southwards into the Vestland Group south of the Frigg Field area. The absence of the basal marine sandstones is considered to be the distinguishing feature of the Vestland Group . Northwards, the Brent Group shales out within the East Shetland Basin between 61°30' N and 62°N.

Age

Mainly Bajocian to Early Bathonian but including Late Toarcian to the east.

Subdivision

The group is divided into five formations. These are: the Broom (base), Rannoch , Etive , Ness and Tarbert (top) formations.1) According to Norwegian usage, the units earlier had a member status (Deegan and Scull, 1977). In this report it is proposed to give the units formation status also in the Norwegian sector. All formations are recognizable in the Brent-Statfjord area. However, difficulties are met when moving away from the type area. On the Horda Platform the Broom Formation appears to be absent, and the presence of the Rannoch Formation is under debate.

Source

Footnotes

Reference

Broom Formation

Name

Named by Deegan and Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2818 m to 2829 m, coord N 61°08'06", E 01'43'36.5" (Fig 3.10) .

Well reference section

Norwegian well 33/9-1 (Mobil) from 2664 m to 2668 m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) .

Thickness

11 m in the type well and 4 m in 33/9-1 . In the Brent-Statfjord area it varies from a few meters to about fifteen meters in thickness.

Lithology

In the type well it is a pale grey to brown, coarse-grained poorly sorted conglomeratic sandstone containing shale clasts.

Boundaries

The Broom Formation is distinguished from the underlying Dunlin Group and the overlying Rannoch Formation by its irregular, but generally lower, gamma ray readings.

Distribution

The Broom Formation is easily identified in the Brent - Statfjord area. In parts of the East Shetland Basin a thin distal equivalent of the Broom Formation is present within the shales of the Drake Formation .

Age

Late Toarcian to Bajocian.

Depositional environment

The Broom Formation is a shallow marine deposit, and is a precursor of the regression which characterizes the overlying Rannoch Formation .

Source

Brygge Formation

Name

From the Norwegian name for wharf or quay. The formation corresponds to the informally used Narvik Formation (H6-1).

Well type section

Well 6407/1-3 (Statoil), coordinates 64°52'25.48"N, 07°02'53.47"E, from 2212.5 m to 1762.5 m (Fig 4.37) . No cores.

Thickness

450 m in the type well.

Lithology

Mainly claystone with stringers of sandstone, siltstone, limestone and marl. Pyrite, glauconite and shell fragments are seen in the sandstones.

Basal Stratotype

The base is defined by a decrease in interval transit time shown on the sonic log and by an increase in the density log readings.

Lateral extent and variation

The Brygge Formation is ubiquitous across Haltenbanken, but it is absent on the crest of the Nordland Ridge. A sand-dominated succession of Oligocene age, termed the Røyrvik formation, has been mapped (but not formally defined) off the Møre-Trøndelag coast (Askvik and Rokoengen, 1985). The Rørvik formation probably interfingers with the fine grained Brygge Formation to the west.

Age

Early Eocene to Early Miocene.

Depositional environment

The sediments were deposited in marine environments.

Source

Bryne Formation

Name

From a town in the south-western part of Norway. This new formation represents the lower part of the Haldager Formation as described by Deegan and Scull (1977).

Well type section

Norwegian well 9/4-3 (Conoco) from 2507.5 m to 2613 m, coord N 57°36'54.5", E 04°18'57.7" (Fig 3.29) .

Well reference section

Norwegian well 8/12-1 (Conoco) from 2710.5 m to 2813 m, coord N 57°13'18.6", E 03°46'45.13" (Fig 3.30) .

Thickness

The formation is 105.5 m thick in the type well and 102.5 m in the reference well. It shows local variations in thickness which probably reflect both Middle Jurassic syndepositional structuring and later erosion.

Lithology

The Bryne Formation comprises interbedded sandstones, siltstones, shales and coals. The sandstones are white to grey, very fine to coarse grained, poorly sorted, friable to hard and occasionally kaolinitic. The shales are generally grey to brown, micaceous, occasionally silty, non-calcareous and often carbonaceous.

Boundaries

The base of the Bryne Formation is unconformable and represents the contact with the partly eroded shales of the Fjerritslev Formation or with arenaceous Triassic rocks. The boundary with the Fjerritslev Formation is usually clearly defined on both gamma ray and sonic logs, whereas the boundary with the Lower Jurassic/ Triassic sandy sequences ( Gassum and Skagerrak formations) often gives no appreciable log breaks. However, on most logs the appearance of these sediments is marked by an overall sonic log shift to higher interval velocities. Where the formation is overlain by the shales of the Boknfjord or Tyne groups, clear breaks can be observed both on gamma ray and sonic logs. However, where the formation is overlain by the Sandnes or Ula formations the boundary is not so easily defined due to internal facies changes within these two formations (see description of the Sandnes and Ula formations).

Distribution

The Bryne Formation is present in the Norwegian-Danish Basin and in the Central Graben. Two main Middle Jurassic depocentres are recognized; one in the Danish Sub-Basin and another in the Fiskebank Sub-Basin (Hamar et al., 1982). The Bryne Formation is equivalent to the Haldager Sand Member of the Haldager Formation as described in Denmark (Larsen, 1966; Michelsen, 1978).

Age

Mainly Bajocian to Bathonian, but may locally be older in the Norwegian-Danish Basin.

Depositional environment

The Bryne Formation represents deposition in a fluvial/deltaic environment.

Remarks

The Bryne Formation as described above is ap proximately equivalent in age and lithofacies to the Sleipner Formation of the Southern Viking Graben. So far it is not possible to demonstrate a connection between the two deposits, and this constitutes the reason for use of separate nomenclature. The Bryne Formation represents the lower part of the Haldager Formation, extended into the Norwegian sector from the Danish sector by Deegan and Scull (1977). Having defined exten-sive marine sands worthy of formation status ( Ula , Sandnes formations) comprising the upper part of Deegan and Scull's Haldager Forma tion, workers on this project saw the need for a separate formation, defining the lower non-marine sands. The term "Haldager Formation" could not be used since it essentially equates to the Haldager Sand Member, the lower part of the Haldager Formation in the Danish sector (Michelsen, 1978). (The upper part of the Danish Haldager Formation is the Flyvberg Member, a marine sandstone/siltstone unit approximately time-equivalent to the Sandnes Formation of the Norwegian-Danish Basin). When a previously established formation is sub divided into new units which are formally given formation status, the original formation with its original name should be either raised to group rank or abandoned; the old name should not be retained for any of the divisions of the original unit (Hedberg, 1976). It was therefore thought expedient to abandon the name "Haldager Formation" in the Norwegian sector. It is recognised, however, that there is probably complete lithological continuity between the new Bryne Formation of the Norwegian sector and the Haldager Sand Member of the Danish Haldager Formation.

Source

Burton Formation

Name

Named by Deegan and Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2950.5 m to 2993 m, coord N 61°08'06", E 01°43'36.5", (Fig 3.10) .

Well reference section

Norwegian well 33/9-1 (Mobil) from 2801 m to 2838 m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) .

Thickness

42.5 m in the type well and 37 m in the reference well.

Lithology

In the type well it consists of a uniform development of dark grey to reddish-grey, soft non-calcareous claystone and shale, which may be in part slightly carbonaceous.

Boundaries

The formation is normally represented on both gamma ray and sonic logs by smooth linear almost constant readings, reflecting the lithological uniformity. The upper and lower contacts are identified where this log character changes.

Distribution

Although the log character changes slightly away from the type section, the formation can be recognised over a wide area of the northern North Sea. The formation has not been identified on the Horda Platform. It represents essentially a basinal facies and passes laterally into the Amundsen Formation in marginal areas.

Age

Sinemurian to Pliensbachian.

Depositional environment

The formation is believed to represent open marine basinal deposits.

Source

Børglum unit

This unit was first described by Larsen (1966) as the Børglum Formation. Later it was amended several times, the latest being by Michelsen (1978) who redefined it and reduced it to member status. It has been informally redesignated a formation by Hamar et al. (1982).
In the type area in Jutland, Denmark, the formation is of Kimmeridgian-Volgian age and consists of homogeneous shaly claystones, olive grey to blackish grey, slightly calcareous to non-calcareous, with mica, pyrite, shell fragments and lignite. This description is generally valid also in the eastern and southern parts of the Fiskebank Sub-Basin, as illustrated by the Norwegian well 10/5-1 (Conoco) from 1396 m to 1472.5 m, (Fig 3.39) . Further to the west a gradual change in lithology can be observed. The amount of carbonaceous material in the upper claystones increases, Norwegian well 9/12-1 (Shell), from 2011 m to 2038 m (Olsen, 1980) resulting in the transition to the Tau Formation in the Fiskebank Sub-Basin and in the Egersund Sub-Basin. The lower part of the Børglum unit as observed in the eastern areas is probably equivalent to the silty Egersund Formation in the north-west.
It is therefore recommended that the Danish nomenclature (the Børglum Member) should be applied in the south-eastern Norwegian Danish Basin and that the Norwegian nomenclature should be applied in the north-western area.

Source

Båt Group

Name

Norwegian name for boat. The Båt Group comprises the lower and middle parts of the earlier used informal Halten Group (H1).

Type area

The transition from the Halten Terrace to the Trøndelag Platform in Blocks 6507/11 and 6507/12.
The group's development is illustrated by the type section of its basal unit in well 6507/12-1 (Saga Petroleum), coordinates 65°07'01.62"N, 07°42'42.61"E from 2920 m to 2213 m (Fig 4.7) . and in the reference well 6407/1-2 (Statoil), coordinates 64°47'50.61"N, 07°02'23.76"E from 4548 m to 3907 m (Fig 4.8) .

Thickness

707 m in well 6507/12-1 and 641 m in well 6407/1-2 .

Lithology

The Båt Group consists of alternating sandstone and shale/siltstone units, with sandstone as the dominant lithology.

Basal Stratotype

The base of the Båt Group is defined by the base of the Åre Formation as described herein.

Lateral extent and variation

The Båt Group is present in most of the wells drilled on Haltenbanken and Trænabanken, with maximum thicknesses in excess of 700 m. The upper part of the succession is progressively truncated towards the crestal part of the Nordland Ridge and the whole group is absent on the highest parts of the ridge. Shallow boreholes off the Trøndelag coast (Bugge et al. 1984) indicate that the Åre Formation may be absent and younger strata within the Båt Group onlap directly on Precambrian basement.

Age

Rhaetian to Toarcian.

Depositional environment

Shallow marine to deltaic environments dominated during deposition of the group.

Correlation

The group corresponds to the mid- Fruholmen to mid- Stø Formation sequence in the Realgrunnen Group1) of the Hammerfest Basin. The group is also partially equivalent to the Statfjord Formation and the Dunlin Group of the North Sea.

Subdivision

The group comprises four formations (Fig 4.7 , 4.8) 4.8) all of which are defined herein.

Source

Footnotes

Footnotes

Cook Formation

Name

Named by Deegan and Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2887 m to 2950.5 m, coord N 61°08'06", E 01°43'36.5" (Fig 3.10) .

Well reference sections

Norwegian wells 33/9-1 (Mobil) from 2715 m to 2801 m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) . 30/6-7 (Norsk Hydro) from 2975 m to 3023 m, coord N 60°38'39.49'' E 02°45'21.74" (Fig 3.12) . 31/2-1 (Shell) from 2093 m to 2134 m, coord N 60°46'19.16'' E 03°33'15.87" (Fig 3.13) . and 30/7-7 (Norsk Hydro) from 4735 m to 4801 m, coord N 60°16'19.30" E 02°16'07.30" (Fig 3.14) .

Thickness

63.5 m in the type well, 86 m in 33/9-1 , 66 m in 30/7-7 , 48 m in 30/6-7 and 41 m in 31/2-1 .

Lithology

In the type well section the formation is dominantly a marine siltstone with minor grey, silty claystone intercalations. The siltstones and claystones may contain sandy streaks, becoming more prominent away from the type well, especially in Norwegian waters. On the Horda Platform and along its western margin, sandstones are the dominant lithology in the formation. The sands are white to greyish brown, very fine to fine grained, subangular to subrounded and well sorted. Occasionally thin layers of medium to coarse grained sandstones are found. The sandstones are hard to friable. Silica is the most common cement. Mica, glauconite, carbonaceous material and calcareous cement may be present.

Boundaries

The formation can be distinguished from the more uniform and more argillaceous sediments above and below by a decrease in gamma ray response and an increase in velocity.

Distribution

The formation appears to be present throughout the East Shetland Basin and on the northern part of the Horda Platform.

Age

Pliensbachian to Toarcian.

Depositional environment

The sandstones can be divided into three types, related to depositional environment and basin geometry. In the Statfjord Field area the sandstones are believed to represent marine shoal sands (e.g. ref. well 33/9-1 ). On the Horda Platform and along its western margin the sandstones represent prograding shelf sands and several cycles can be identified within the formation (ref. 31/2-1 and 30/6-1 ). In the graben area the sands are thinner bedded, and the shale intercalations show no gradations into the sands (ref. 30/7-7 ). These sandstones are believed to represent redeposited sands from the edge of the shelf (the Horda Platform and East Shetland Basin west of the graben area).

Source

Cromer Knoll Group

Name

From the Cromer Knoll buoy in the southern North Sea. Named by Rhys (1974) in the southern North Sea and extended in use to the northern North Sea by Deegan and Scull (1977). The group as now used off mid-Norway corresponds to the informal Finnvær Group (H3).

Type area

The type area is in the southern North Sea. Rhys (1974) used UK well 48/22-2 to illustrate a typical section of the group, and Deegan & Scull (1977) used UK wells 29/25-1, 22/1-2A and 3/29-1, and Norwegian well 2/11-1 . In the North Sea the following Norwegian wells have been used to illustrate local developments of the group: 2/6-2 , 2/7-15 , 2/11-1 , 7/3-1 , 17/11-2 , 34/10-18 , 35/3-4 , 35/3-5 , 31/6-3 , 24/12-2 and 17/4-1 ; the Danish well DK1-1 has also been used.

In the Norwegian Sea the following wells are used to illustrate the local development of the group: 6506/12-4 (Statoil), coordinates 65°12'46.97"N, 06°43'30.37"E, from 3855 m to 3132.5 m and 6506/12-1 (Statoil) coordinates 65°10'07.58"N, 06°43'44.07"E, from 3835 m to 3175 m (Fig 4.25 , 4.26) The base of the Lyr Formation marks the lower boundary of the Cromer Knoll Group on Halten Terrace. In basinal areas to the west the Lange Formation is expected to rest directly on shales of the Viking Group (Fig 4.5) (Fig 4.6) .

Thickness

The thickness of the group varies considerably since the sediments were deposited in response to an active Late Jurassic tectonic phase. In the Viking Graben, the Åsta Graben and locally in the Central Trough the thickness is often more than 600 m, gradually thinning towards the basin margins. The group is 667 m thick in Norwegian well 2/11-1 and 643 m thick in Norwegian well 17/11-1 . Seismic data indicate that the group is thickest in the Sogn Graben, where it probably reaches up to 1400 m. In the Norwegian Sea 722.5 m are recorded in well 6506/12-4 and 660 m in well 6506/12-1 .

Lithology

The Cromer Knoll Group consists mainly of fine-grained, argillaceous, marine sediments with a varying content of calcareous material. Calcareous claystones, siltstones and marlstones dominate, but subordinate layers of limestone and sandstone occur. The claystones are generally light to dark grey, olive-grey, greenish and brownish, often becoming light grey, light greenish-grey and light olive-grey marlstones. Mica, pyrite and glauconite are common. Generally, marlstones become the more dominant lithology in both the upper and lower parts of the group. On Haltenbanken, the group consists of calcareous and non-calcareous claystones inter-bedded with marls and stringers of carbonates and sandstones. Sandstones are more common in the upper part of the group.

Basal stratotype

The lower boundary is usually well defined and is recognised by a distinct decrease in gamma-ray response and an increase in velocity when passing upward from the generally more organic-rich shales of the underlying Upper Jurassic formations (Fig 5.12, 5.13 , 5.14 and 5.22)

Characteristics of the upper boundary

South of approximately 59° N (Fig 5.32ab) , the upper boundary is the base of the chalk facies of the Shetland Group , defined by the onset of a decrease in gamma-ray response and an increase in velocity into the overlying carbonates (Fig 5.12 , 5.13 ,5.14 ,5.15 and 5.22 , 5.23 , 5.24 , 5.25 , 5.26 , 5.27 , 5.28) . The uppermost Rødby Formation of the Cromer Knoll Group often appears on logs as a transition between the overlying carbonates of the Shetland Group and the more argillaceous parts of the Cromer Knoll Group (Fig 5.12 , 5.22). Further north, the upper boundary is the base of the siliclastic facies of the Shetland Group (Fig 5.32ab) . This boundary is normally also shown by a decrease in gamma-ray response and an increase in velocity when passing into the overlying, generally more calcareous, Svarte Formation of the Shetland Group (Fig 5.33 , 5.36) However, the opposite log response at the boundary is also observed (Fig 5.37) .

Distribution

The group is widely distributed in the Norwegian sector of the North Sea. It is absent from the highest parts of the Mandal High, Jæren High, Utsira High and Lomre Terrace in the Troll area and locally from the Tampen Spur (Fig 5.7 , 5.8 , 5.9 , 5.10 , 5.11) . The group's representatives are found throughout the Halten Terrace and in basinal areas to the west. There is little lateral variation apart from a gradual increase in sandstone content to the north. The sequence is generally several hundred metres thick on the Halten Terrace and within the Helgoland Basin, but it is much thinner and locally absent on the Trøndelag Platform. The group is not preserved on parts of the Nordland Ridge nor on local highs along the western flank of the Trøndelag Platform.

Age

Ryazanian (usually late) to Albian/Early Cenomanian (Noth Sea). Ryazanian to Turonian (Norwegian Sea).

Depositional environment

Open marine, with generally low energy (North Sea). The sediments were deposited in shallow to deep marine environments (Norwegian Sea).

Subdivision

Six formations are defined within the group in the Norwegian sector of the North Sea (Fig 5.7 , 5.8 , 5.9 , 5.10 , 5.11) : the Åsgard , Tuxen (Jensen et al. 1986), Mime , Sola (Hamar & Hesjedal 1983; Jensen et al. 1986), Rødby and Agat formations. In addition we recognise a need to introduce the Ran sandstone units (Fig 5.7 , 5.8 , 5.9 , 5.10 , 5.11). Fig 5.4 shows an idealised development of the Cromer Knoll Group in the northern and central North Sea. In the Norwegian Sea the Cromer Knoll Group comprises three formations in the area (Fig 4.25 , 4.26) : Lyr , Lange and Lysing formations.

Remarks

The group was erected by Rhys (1974) to embrace three marine, arenaceous, argillaceous to marly formations of mainly Early Cretaceous age recognisable onshore and offshore. Deegan & Scull (1977) formally defined the group to include the sediments between the underlying Humber Group and Bream Formation and the overlying Shetland and Chalk groups. Vollset & Doré (1984) replaced the Humber Group of the northern North Sea by the Viking Group , and the Bream Formation in the Central Trough and the Norwegian-Danish Basin by the Tyne and Boknfjorden groups, respectively. The tops of the Draupne Formation of the Viking Group , the Mandal Formation of the Tyne Group and the Flekkefjord Formation of the Boknfjorden Group define the base of the Cromer Knoll Group. The Cromer Knoll Group is partly equivalent to the Rijnland Group of the Dutch sector (NAM & RGD 1980, Crittenden 1982) and the Speeton Clay Formation together with the Red Chalk Formation of the UK sector (Rhys 1974). The subdivision in this paper can be used for the Danish sector (see also Jensen et al. 1986).

Compiled from

Delfin Formation (informal)

The Delfin Formation has been encountered in several wells in the northern Vøring Basin. It comprises sandstones of Early Campanian and Late Santonian age and is interpreted as a basin floor fan intercalated into shales of the Nise Formation . Wells show thicknesses of up to more than 1000 metres of good sandstone. The development of the Delfin Formation is illustrated by well 6707/10-1 . Well 6704/12-1 is situated in a partly different sub-basin. The sandstones encountered in this well are derived from a slightly different source area.
The Delfin Formation is named informally, no well type nor well reference sections have been established.

Drake Formation

Name

Named by Deegan and Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2829 m to 2887 m, coord N 61°08'06", E 01°43'36.5" (Fig 3.10) .

Well reference sections

Norwegian wells 33/9-1 (Mobil) from 2668 m to 2715 m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) . 30/6-7 (Norsk Hydro) from 2786 m to 2975 m, coord N 60°38' 39.49'', E 02°45'21.74" (Fig 3.12) . 31/2-1 (Shell) from 1985 m to 2093 m, coord N 60°46'19.16", E 03°33'15.87" (Fig 3.13) . 25/2-4 (Elf) from 3876 m to 3895 m, coord N 59°58'44.09", E 02°22'58.88" (Fig 3.15) .

Thickness

58 m in the type well, 47 m in 33/9-1 , 189 m in 30/6-7 , 108 m in 31/2-1 and 19 m in 25/2-4 .

Lithology

In the type well the lower part of the formation consists of medium grey, slightly sandy, calcareous claystone. The upper part is dark grey to black, fissile, micaceous shale containing calcareous nodules. On the Horda Platform and along its western margin white to grey, fine to coarse sandstones are found within the formation. The sandstones are often hard and calcite cemented. They also show and "upward coarsening" on the gamma ray log. The claystone is often silty.

Boundaries

In the basinal areas, where the sandstones are not present, the formation has more regular gamma ray and sonic log responses than that of the underlying formation, the gamma ray response being uniformly higher and the velocity lower. The lower velocity is also apparent where sandstone beds are present. The upper boundary is marked by the presence of arenaceous sediments of the overlying Brent Group , and the upper boundary is placed at the base of the upward coarsening sequence of the Rannoch Formation of the Brent Group . The Brent Group shows a more erratic sonic log pattern than the Drake Formation. In the south the top of the formation is marked by an unconformity (e.g. well 25/2-4 , (Fig 3.15) . In the northernmost area where the Brent Group is not recognised, the Dunlin Group is often unconformably overlain by the Viking Group .

Distribution

The formation is widely distributed throughout the East Shetland Basin and northern Horda Platform. It thins towards the west and south, where it may be absent due to erosion. The sand developments within the formation seem to be a function of marginal position within of the basin.

Age

Toarcian to Bajocian.

Depositional environment

The Drake Formation is generally considered to have been deposited in prodelta and delta front environments.

Source

Draupne Formation

Name

From a gold ring owned by Odin (the mythologic Viking God) which "dripped" 7 new rings, of the same weight as the original, every 9th day. The name is considered particularly appropriate in view of the Draupne Formation's role as a prolific hydrocarbon source in the Northern North Sea. It replaces the name "Kimmeridge Clay Formation" as used by Deegan and Scull (1977) in the Northern North Sea (see remarks).

Well type section

Norwegian well 30/6-5 (Norsk Hydro) from 2452 m to 2615 m, coord N 60°41 '20.6", E 02°57'11.09", (Fig 3.22) .

Well reference sections

UK wells 211/21-1A (Shell) from 2729.5 m to 2810 m, coord N 61°11'09.6", E 01°06'45", (Fig 3.18) . and 210/30-1 (Arpet), from 3330 m to 3717.5 m, coord N 61°04'05.4", E 00°54'14.4'', (Fig 3.20) .
Norwegian wells 33/9-1 (Mobil) from 2443 m to 2450 m, coord N 61°15'07.5", E 01°50'25.8", (Fig 3.11) . 15/9-2 (Statoil) from 3397 m to 3478 m, coord. N 58°25'34.06", E 01°42'28.2", (Fig 3.26) . and 15/3-1 S (Elf) from 3947 m to 4754 m, coord N 58°50'57.00", E01°43'13.25", (Fig 3.19) .

Thickness

163 m in the type well, 80,5 m in 211/21-1A, 387,5 m in 210/30-1, 7 m in 33/9-1, 81 m in 15/9-2 and 807 m in 15/3-1 S .

Lithology

The formation consists of dark grey-brown to black, usually non-calcareous, carbonaceous, occasionally fissile claystone. It is characterized by very high radioactivity (often above 100 API units) which is a function of organic carbon content. It has anomalously low velocity, density and high resistivity. Minor limestone streaks and concretions occur throughout the formation. In addition interbedded sandstones and siltstones can cause a reduction in gamma ray response. The reference well 15/3-1 S is an atypical example of the Draupne Formation which has been chosen to illustrate these arenaceous intercalations.
An informal three-fold subdivision has been identified, and is best recognizable along the basin rim areas where a middle high gamma ray zone separates two zones of lower gamma ray response (e.g. 210/30-1).

Boundaries

These are marked by distinct log breaks due to the very high gamma ray response and low velocity shown by the Draupne Formation.
In basinal areas the formation generally has a diachronous contact with the Heather Formation . On marginal highs the formation onlaps pre-Upper Jurassic rocks. On the northern Horda Platform, Upper Jurassic sandstones of the Sognefjord Formation mark the base of the Draupne Formation.
The upper boundary is often an unconformity or discontinuity, usually overlain by Cretaceous sediments which have a higher velocity and lower gamma ray response than the Draupne Formation. The importance and regional validity of this unconformity is under debate; see for example Rawson and Riley (1982).

Distribution

The formation is found in the East Shetland Basin, the Viking Graben and over the Horda Platform.

Age

The formation ranges from Oxfordian to Ryazanian in age.

Depositional environment

The Draupne Formation was deposited in a marine environment with restricted bottom circulation and often with anaerobic conditions. In places the formation may contain sandstones which are generally considered to be of turbiditic origin (De`Ath and Schuyleman, 1981; Harms et al, 1981).

Remarks

The Norwegian Lithostratigraphic Nomenclature Committee formally proposes abandonment of the name "Kimmeridge Clay Formation" for Upper Jurassic claystones encountered north of the Mid-North Sea High. The committee further recommends substitution of the names "Mandal Formation" (in the Central Graben area) and "Draupne Formation" (in the Northern North Sea) for intervals formerly termed Kimmeridge Clay Formation in those areas. The principal reasons for this revision are:

The type area for the Kimmeridge Clay Formation is Kimmeridge Bay in Dorset, southern England (e.g. Arkell 1947). There the unit consists of dark, organic-rich claystones of Kimme-ridgian to Volgian age (using Boreal stage terminology). The term was extended by Rhys (1974 and 1975) into the Southern North Sea to describe a unit of comparable age overlying Oxfordian limestones (Corallian Formation) in type well 47/15-1. Deegan and Scull (1977) further extended the Kimmeridge Clay into the Northern North Sea to denote a generally highly radioactive, generally low velocity claystone sequence. The age range of the formation was broadened to include part of the Oxfordian and Ryazanian. It was remarked that the sediments in the Southern North Sea were less radioactive (less than 60 A.P.I, units compared with 100-200 A.P.I, units in the north). However, the low seismic velocities shown by both units were considered sufficient grounds to justify a correlation.
Deegan and Scull did not specifically discuss the application of the term "Kimmeridge Clay" to the Central Graben region of the Central North Sea. Nevertheless, the name is in common usage there (e.g. Ofstad, 1983). It has, among other meanings, been taken to denote:

To the northwest, in the Norwegian-Danish Basin, a similar deposit is present (the Tau Formation , described in this publication). This unit has never been referred to the Kimmeridge Clay Formation but is nevertheless a typical Upper Jurassic organic shale. It is of Kimmeridgian — Volgian age and occupies a lower relative position in the overall Upper Jurassic shale sequence than the Central Graben "hot shales". (Table 3.4). A connection between the two deposits across the Southern Vestland Arch is most unlikely.
In extending the Kimmeridge Clay Formation from the type area to the Northern North Sea (a distance of approximately 1300 km) the stratigraphic meaning of the name has varied considerably (Table 3.5). It is recognised that deposits of this type were laid down over a very wide area in intervals of the late Jurassic, the widespread deoxygenated bottom conditions reflecting the combined effect of paleo-geography with general sea level rise (e.g. Tyson et al., 1979; Ziegler 1982). However, acceptance of this overall regime should not alone be taken as a basis for lithostratigraphic classification. The nature and timing of the organic shales appears to have varied according to the evolution of individual basins. Use of the generic term Kimmeridge Clay Formation is a potential cause of mistaken correlation and does not further understanding of inter-basin relationships. Replacement of the name is a more palatable alternative to its continued extension northwards to the Arctic and Russia, where alternative nomenclature already exists.
It therefore seems appropriate to limit the "Kimmeridge Clay Formation" of the North Sea to to the area south of the Mid-North Sea High, as described by Rhys (1974). The Norwegian Lithostratigraphic Nomenclature Committee proposes substitution of the name "Draupne Formation" for the "Kimmeridge Clay Formation" of the Northern North Sea as defined by De-egan and Schull (1977). It is suggested that the name "Mandal Formation" should be applied specifically to the "hot" shales of Volgian — Ryazanian age in the Central Graben region (Table 4), following the precedent set by Hamar et al., (1982).

Source

Dunlin Group

Name

Named by Deegan and Scull (1977). According to earlier Norwegian usage the unit had formation status, and the type well was UK well 211/29-3. In this report the unit is given group status in Norwegian as well as UK usage.

Type area

The type area is the East Shetland Basin, in particular the region of the Brent Field. The group is illustrated in the following wells: UK well 211/29-3 (Shell), Norwegian wells 33/9-1 (Mobil), 30/6-7 (Norsk Hydro), 31/2-1 (Shell) and 25/2-4 (Elf).

Thickness

222 m in the UK well 211/29-3, 255 m, 365.5 m, 308 m and 204 m in Norwegian wells 33/9-1 (Mobil), 30/6-7 (Norsk Hydro), 31/2-1 (Shell) and 25/2-4 (Elf) respectively. The group is thickest in the northern Viking Graben area.

Lithology

The group consists mainly of dark to black argillaceous marine sediments, but in the marginal areas of the basin marine sandstones are well developed at several stratigraphic levels and can extend a considerable distance into the basin. The sandstones are white to light grey, very fine to medium grained and generally well sorted. The group tends to be more calcareous in the Norwegian sector, and in places limestone beds, some of which contain chamosite and siderite ooliths, are found.

Boundaries

The lower boundary with the Statfjord Group and the upper boundary with the Brent Group are clearly marked by gamma ray log breaks. The Dunlin Group generally has a more regular log character than the underlying and overlying sediments. In the northernmost area where the Brent Group is not recognised, the Dunlin Group is often unconformably overlain by the Viking Group .

Distribution

The group is more widespread than the underlying Statfjord Group and is thickest in the Viking Graben area, east of the Statfjord and Brent fields. It is recognizable over most of the East Shetland Basin and northern part of the Horda Platform. In places the Dunlin Group rests with an apparent unconformity on the Statfjord Group . In the western part of the basin higher formations within the group are thought to be transgressive onto pre- Statfjord Group sediments. Variation in thickness on tilted fault blocks probably reflects syndepositional movement.

Age

The group ranges from Hettangian to Bajocian in age.

Subdivisions

The Dunlin Group is divided into five formations. These are named the Amundsen (base), Johansen , Burton , Cook and Drake (top) formations and can be clearly differentiated on sonic and gamma ray logs. The Amundsen , Cook and Drake formations are found throughout the East Shetland Basin. The Burton Formation is found over most of the area but is not present on the Horda Platform. The Johansen Formation , on the other hand, has so far only been found on the Horda Platform. It should be stressed that the upper part of the calcareous sands in the Statfjord Group passes laterally into the lower part of the calcareous silts and shales of the Amundsen Formation in the central parts of the basin.

Source

Egersund Formation

Name

Named by Deegan and Scull (1977) who gave the unit member status.

Well type section

Norwegian well 9/4-1 (Amoseas) from 2251 m to 2288 m, coord N 57°35'02", E 04°01'13" (Fig 3.34) .

Well reference section

17/12-1 R (Phillips) from 2215 m to 2290 m, coord N 58° 11' 15.4'', E 03°56'22.2'' (Fig 3.35) .

Thickness

37 m in the type well and 75 m in the reference well.

Lithology

In the type well the formation consists of dark grey micromicaceous shales and siltstones with brownish, locally oolitic, microcrystalline carbonate beds and occasionally sandstone streaks. The latter become more important east and north-east of the type well.

Boundaries

The lower boundary is also the junction of the Boknfjord Group with the Vestland Group and is described above. At the base of the formation a more radioactive part is recognizable in several wells. The upper boundary is marked by the appearance of the dark grey to black organic-rich shales of the overlying Tau Formation . The shales of the Tau Formation have a high radioactivity and a low velocity, and therefore the upper boundary is marked by strong log breaks.

Distribution

The formation is distributed throughout Fiskebank and Egersund Sub-Basins.

Age

Callovian to Kimmeridgian.

Depositional environment

The Egersund Formation was deposited in an open marine, generally low energy basinal environment.

Source

Egga Formation (informal)

The Egga Formation (also referred to as Egga Member or Egga sandstone unit) has been encountered in a number of wells in the Møre Basin and Slørebotn Sub-basin. It comprises sandstones of Danian age and is interpreted as deepmarine massflow deposits. The development of the Egga Formation is illustrated by well 6305/7-1 . Wells on the Ormen Lange Field where it is the main reservoir unit have shown thicknesses of 40 to 80 m, while wells further east and south have encountered thicker sand-rich intervals. The Egga Formation is named informally, no well type nor well reference sections have been established.

Eiriksson Formation

Name

Named after Leiv Eiriksson, the discoverer of North America in the year 1000, according to the Norse Sagas. He was the son of Eirik Raude.

Well type section

Norwegian well 33/12-2 (Mobil) (Fig 1.17-18) . from 2719 m to 2790 m below KB.

Well reference section

UK well 211/24-1 (Conoco/Gulf/NCB) (Fig .17-18) .

Thickness

71 m in the type well. In the reference well the formation is 157 m thick. The thickness is relatively constant over the area of the Statfjord Field but the formation thickens towards the south and west (Fig .17-18) . where sandstone characteristic of this formation occur at lower levels in the section.

Lithology

In the type well the formation is characterised by massive sandstone beds, generally correlatable between the wells, interbedded with hard grey shales. The sandstones are white to light grey, medium to very coarse grained with thin horizons of granules, pebbles, and lignite fragments, often concentrated in channels and along cross-bedding foresets. They contain slightly less kaolinite matrix, mica and rock fragments than the sandstones of the Raude Formation . The shales are slightly and commonly micaceous and carbonaceous. In the area of the Statfjord Field the sandstone beds average about 5 m in thickness and the shale beds average about 2.5 m. The sediments of this formation appear more mature than those of the Raude Formation , and marine fossils and glauconite are present near the top of the formation in the type well.

Boundaries

The originally Eiriksson Member of the Statfjord Formation was elevated to formation level by Lervik, 2006. There is usually a sharp upward transition from the Raude Formation reflecting a change to a more humid climate as well as a tectonic uplift of the hinterland. Sandstones are more abundant and thicker. The modified climate resulted in a change in the colour of the siltstones and mudstones from red to green and grey, and carbonate nodules and soils that formed in the previous evaporitic environment were replaced by coal- and carbonaceous-rich beds. The lower boundary is formed by the base of the lowest massive sandstone which can be well correlated. The upper boundary is marked by the base of the distinctive sandstones of the Nansen Formation which are frequently calcareous. The Eiriksson Formation has a characteristic blocky gamma ray and sonic log response but the boundaries may not always be marked by prominent log breaks.

Distribution

The formation is present over much of the northern North Sea, but not in the southern part of the East Shetland Basin.

Age

In the type well it is Hettangian, possibly extending into the early Sinemurian. However, to the west and south the base of the formation appears to be progressively older.

Depositional environment

Nystuen and Fält (1995) reported vertically stacked channel-sandstones forming multi-storey sandstone units up to 30-40 m thick. Cross-bedded sandstones with basal channel-lag conglomerates of coarse- to very coarse-grained sandstones with pebbles of quartz and gneiss (Nystuen and Fält, 1995) associated with coal facies are interpreted as having a fluvial origin (Røe and Steel, 1985). Røe and Steel (1985) discussed the sheet-like extent of the sand bodies, interpreting them as having formed in a relatively high-gradient, braided-alluvial setting. The Eiriksson Formation is interpreted as the more proximal braided-stream deposit of a fan-delta system associated with coastal to shallow marine elements. Deegan and Scull (1977) reported marine fossils and glauconite near the top of the fromation in well 33/12-2 , suggesting a marginal marine environment varying from coastal backswamp and river mouth situations to coastal barriers.

Compiled from

Ekofisk Formation

Name

Named after the Ekofisk Field in Norwegian block 2/4 (Deegan & Scull 1977).

Well type section

Norwegian well 2/4-5 from 3164 m to 3037 m, coordinates N 56°34'29.77", E 03°12'13.03" (Fig 5.30) . No cores.

Well reference sections

Norwegian well 1/3-1 from 3354 m to 3258 m, coordinates N 56°51'21.00", E 02°51'05.00" (Fig 5.24) . No cores. UK well 22/1-2A from 2982.5 m to 2935 m, coordinates N 57°56'12.20", E 01°02'55.80" (Fig 5.25) . No cores. Norwegian well 2/5-1 from 3132 m to 3041 m, coordinates N 56°38'19.95", E 03°21'07.94" (Fig 5.31) . Cored through the upper 78 m.

Thickness

The formation is 127 m thick in the type-well, 96 m in 1/3-1 , 47.5 m in 22/1-2 A and 91 m in 2/5-1 . In the Norwegian sector, seismic interpretation indicates that a thickness of more than 150 m is found in the northwestern part of the Central Trough.

Lithology

In the type well, the formation comprises white, tan or beige, hard, dense, sometimes finely crystalline limestones, although softer chalky textures are also present. The formation usually consists of white to light grey, beige to brownish, mudstones or wackestones with occasional packstones/grainstones and pisolitic horizons, often alternating with argillaceous chalks, chalky limestones or limestones. Thin beds of grey, calcareous, often pyritic shales or clays are most common in the lower part while brownish-grey cherts occur rarely to abundantly throughout the formation.

Basal stratotype

The lower boundary is marked by a change in gamma-ray readings from a constant low level in the Tor Formation to a slightly lower level. The velocity may or may not show a corresponding increase. The lower boundary separates the Cretaceous and Tertiary chalks and may represent an unconformity (e.g. Norwegian well 1/9-1 , (Fig 5.29) .

Characteristics of the upper boundary

The upper boundary is defined where the gamma-ray response increases and the velocity decreases towards the marly beds of the Våle Formation . Where the marl is not present the change is more abrupt (e.g. Norwegian well 2/8-8 , (Fig 5.28) .

Distribution

The formation is widespread in the southern and central North Sea. In the Norwegian sector, it is missing from parts of the Sørvestlandet High and the Lindesnes Ridge.

Age

Danian.

Depositional environment

Open marine with deposition of calcareous debris flows, turbidites and autochthonous periodites (Skovbro 1983, d'Heur 1986, Hatton 1986).

Remarks

Two zones of the formation are readily correlatable within the Central Trough area (Hatton 1986, (Fig 5.24 , 5.29 , 5.30 , 5.31)

LOWER MEMBER OF THE EKOFISK FORMATION

The lowermost part consists of a low porosity to tight zone with a higher terrigenous clay content, and is informally termed the Ekofisk tight zone. The larger part consists of the informal Ekofisk reworked zone with mainly reworked Maastrichtian chalks ( Tor Formation ) deposited as various mass flows and periodite-facies chalks. This lower member is present in Norwegian wells 1/3-1 from 3354 m to 3307 m, 1/9-1 from 3104 m to 3072 m, 2/4-5 from 3164 m to 3106 m and 2/5-1 from 3132 m to 3099 m.

UPPER MEMBER OF THE EKOFISK FORMATION

This zone is composed of mainly homogenous chalks with a low clay content, debris flows of reworked Danian chalks and minor turbiditesc A lower tight to low porosity zone, informally termed the Tommeliten tight zone, is present in parts of the Central Trough. The zone is found in Norwegian wells 1/3-1 from 3311 m to 3258 m, 1/9-1 from 3072 m to 3036 m, 2/4-5 from 3102 m to 3037 m and 2/5-1 from 3094 m to 3041 m.

Source

Eldfisk Formation

Name

From the Eldfisk Field in Norwegian block 2/7.

Well type section

Norwegian well 2/7-3 (Phillips) from 3626 m to 3695m, coord N 56°23'02.9", E 03°14'45.9" (Fig 3.40) .

Well reference section

Norwegian well 1/9-3 R (Statoil) from 4359.5 m to 4386.5 m, coord N 56°24'56.2", E 02°54'15.15" (Fig 3.43) .

Thickness

69 m in the type well 2/7-3 and 27 m in the reference well.

Lithology

The Eldfisk Formation consists predominantly of sandstone but contains substantial interbeds of shale. In the type well the sandstone is dark yellowish brown, fine to coarse grained, poorly sorted and generally angular, while the shale is medium light grey to dark grey. Both the sandstone and the shale contain calcareous streaks which produce high amplitude peaks on the sonic log.

Boundaries

The sands of the Eldfisk Formation are entirely enclosed within the thick upper Jurassic shale sequence of the Central Graben. The Eldfisk Formation is therefore easily distinguished from the underlying Haugesund Formation and the overlying Farsund Formation by its lower gamma ray readings.

Distribution

As defined at present, the main development of the Eldfisk Formation is confined to the region of the Eldfisk Field , although thin time equivalent sands are present in other parts of the Central Graben.

Age

Kimmeridgian.

Depositional environment

The Eldfisk Formation represents an influx of sand into the axial portions of the Central Graben at a time of regression, and for this reason it is postulated that the formation is turbiditic in origin. However, no conventional cores have been taken in the sands and there is no definitive sedimentological evidence.

Source

Etive Formation

Name

Named by Deegan and Scull (1977) who gave it "sub-unit" status.

Well type section

UK well 211/29-3 (Shell) from 2772 m to 2783 m, coord N 61°08'06", E 01°43'36.5" (Fig 3.10) .

Well reference sections

Norwegian wells 33/9-1 (Mobil) from 2575 m to 2602 m, coord N 61°15'07.5", E 01°50'25.8" (Fig 3.11) . 30/6-7 (Norsk Hydro) from 2727 m to 2786 m 1), coord N 60°38'39.49", E 02°45'21.74" (Fig 3.16) . and 31/4-4 (Norsk Hydro) from 2721 m to 2758 m, coord N 60°40'01.12", E 03°06'54.12" (Fig 3.17) .

Thickness

11 m in the type well, 27 m in 33/9-1 , 59 m in 30/6-7 and 37 m in 31/4-4 .

Lithology

The formation consists of massive grey-brown to clear, fine to coarse, occasionally pebbly and cross-bedded sandstones. The mica-content is generally low. Calcite cemented stringers are also present, especially on the Horda Platform.

Boundaries

The formation is characterised by low gamma ray readings. This characteristic and the low mica content distinguish it from the underlying Rannoch Formation. The lower boundary may, however, be transitional in places. The formation is often found to cut into and occasionally through the underlying formations, thus giving a “blocky” gamma ray log character either above a truncated Rannoch Formation , or directly above marine shales of the Dunlin Group (ref. well 31/4-4 ). The upper boundary is taken at the first significant shale or coal in the overlying Ness Formation .

Distribution

The distribution of the Etive Formation is essentially the same as that of the Brent Group .

Age

Bajocian.

Depositional environment

The formation has been interpretated as upper shoreface, barrier bar, mouth bar and distributary channel deposits.

Source

Footnotes

References

Falk Formation

Name

From the Norwegian name for a falcon (4 species, all belonging to the genus Falco, are found in northern Norway).

Definition

The type section is defined in the interval from 2221 m to 2024 m (log depth) in well 7120/2-1 , on the Loppa High (Fig 9.20) ; Table 9.1). The base corresponds to 2225, 7 m in core depth (Fig 9.24) . The formational base reflects an abrupt change from non-marine, reddish brown conglomerates of the Ugle Formation to dark grey, marine shales and marks the first marine transgression in the area; this gives a log response characterised by a change toward overall lower readings on the gamma ray log as a result of the transition into carbonates (Fig 9.20) . The formation otherwise shows a noisy log pattern, reflecting the intercalation of siliciclastics and carbonates.

Reference sections

Reference sections are found from 2150 m to 2050 m in well 7128/6-1 (Fig 9.25) . 2058 m to 1952 m in 7128/4-1 (Fig 9.26) . 498.6 m to 363.5 m in core 7029/03-U-02 (Fig 9.27) . and from TD at 481.7 m to 458 m in 7030/03-U-01 (Fig 9.28) . all located on the Finnmark Platform. In 7029/03-U-02 the base also represents an abrupt change in both colour and overall lithofacies from non-marine, red and green mottled silty shales to marine, greenish-grey, bioturbated silty shales. In wells 7128/4-1 and 7128/6-1 the formation corresponds to units L1 and L2 of Ehrenberg et al. (1998a).

Thickness

The formation is thickest in the type well (201.7 m); it is 135 m thick in 7029/03-U-02 on the southern Finnmark Platform and thins northwards to 100 m in well 7128/6-1 and 58 m in 7128/4-1 (Fig 9.19) . The formation is missing in 7226/11-1 where carbonates of the overlying Ørn Formation rest directly on basement, while wells 7124/3-1 , 7121/1-1 R , 7228/9-1 S and 7229/11-1 reached TD higher in the Gipsdalen Group .

Lithology

The formation consists of a mixture of shallow marine sandstones, marine siltstones and shallow marine carbonates. In 7120/2-1 , the lower 51 m consists of stacked, less than 5 m thick rhythms of coarse-grained pebbly sandstone with minor shale and dolomite (Fig 9.24) . Trough cross-bedding and horizontal lamination is common. This lower development is overlain by a 125 m thick unit of rhythmically interbedded shales, fossiliferous dolomitic mudstones to packstones (locally with anhydrite or chert nodules), and fine- to medium-grained sandstones with a few pebbly sandstone beds. Crinoids, brachiopods, fusulinids, small foraminifers and corals are the most abundant fossils, together with occasional phylloid algae and palaeoaplysinid plates. In the lower part of core 7029/03-U-02 and in 7030/03-U-01, the Falk Formation consists of 1 to 5 m thick fining upward units of light grey, medium- to coarse-grained, pebbly, trough cross-bedded to planar-laminated sandstones grading upwards into laminated greenish silty shales. Each unit has a sharp and erosive lower boundary. Marine fossils are limited to very rare brachiopods. The upper part of the formation in 7128/6-1 and 7029/03-U-02 consists of cycles of fine- to very fine-grained sandstone, green silty shale and carbonate wackestones to boundstones (Fig 9.25), (Fig 9.27), (Fig 9.29).

Lateral extent and variation

The base of the formation represents a major transgression of the platform areas as seen in 7120/2-1 from the Loppa High and in the Finnmark Platform wells 7128/4-1 7128/6-1 , and 7029/03-U-02, where marine siliciclastics overlie continental deposits or basement. The top of the Falk Formation is likely to be highly diachronous as it reflects the differing times when local siliciclastic source areas were drowned and the mixed siliciclastic-carbonate depositional system was replaced by carbonates. The formation is accordingly expected to be thickest in proximal platform areas and around tectonically active highs; thinnest developments are expected distally on the platforms, and either highly condensed or missing in the basins; these prognoses are supported by the gross wedge-shaped geometry of the formation seen on the Finnmark Platform. The formation is missing in areas that have been sheltered from siliciclastic supply, like the local high on the southern Bjarmeland Platform where well 7226/11-1 was drilled: in this location carbonates of the overlying Ørn Formation rest directly on basement .

Age

Stemmerik et al. (1995, 1998) suggested a late Bashkirian to early-middle Gzelian age based on fusulinid data. The formation is of late Bashkirian to Moscovian age in 7120/2-1 , where the lower part of the overlying Ørn Formation is dated as being of late Moscovian age (Stemmerik et al. 1998). The top of the formation in 7030/03-U-01 apparently coincides with the Kasimovian-Gzelian boundary whereas an even younger age is indicated in 7029/03-U-02 where the uppermost part of the formation extends into the early to middle Gzelian (Stemmerik et al. 1995; Bugge et al. 1995). In 7128/6-1 , the formation is of late Moscovian to early Gzelian age (Ehrenberg et al. 1998a).

Depositional environments

The Falk Formation is characterised by sediments deposited as a response to high frequency and high amplitude fluctuations in sea level (see e.g. Stemmerik et al. 1998; Stemmerik & Worsley 2000). Deposition also took place during an overall rise in relative sea level in shallow shelf environments ranging from offshore silt-dominated to shoreface sand-dominated lithofacies during deposition of the lower part of the formation. Sediments in the upper part of the formation suggest that the relative sea level rise had by then flooded most platform areas so that lithofacies there are characterised by more fine-grained siliciclastic input, deposited in offshore to lower shoreface environments, and by subtidal carbonates. The presence of caliche indicates periods of subaerial exposure of the carbonates, and during sea level lowstands the platform areas apparently formed vast lowlands.

Correlation

Mixed siliciclastics and shallow marine carbonates are common in the lower part of the Gipsdalen Group in the onshore areas of Svalbard. The formation correlates to the Kapp Kåre and Kapp Hanna formations on Bjørnøya (Worsley et al. 2001; Stemmerik & Worsley 2000), perhaps to the uppermost red-bed Hyrnefjellet Formation and lower Treskelodden Formation in Hornsund, the Tårnkanten/Schleteligfjellet and lower Wordiekamen formations of western Spitsbergen, and the Minkinfjellet, Malte Brunfjellet, Hårbardbreen and lowermost Wordiekammen formations of central to eastern Spitsbergen and Nordaustlandet (Dallmann et al. 1999: (Fig 9.6) . In contrast to the offshore and most onshore areas, both Bjørnøya and Hornsund were characterised by significant syndepositional tectonism at the time.

Possible members

No formal members are proposed herein, but the work of Ehrenberg et al. (1998a) and seismic mapping on the Finnmark Platform indicates that the formation in this area may comprise two distinctive units (“L-1” and “L-2”) separated by a hiatus spanning the lower Kasimovian in well 7128/6-1 . This corresponds to a period of tectonic activity on Bjørnøya (Worsley et al. 2001). On a seismic scale, however, these two informal units appear to be conformable on the Finnmark Platform.

Source

Fangst Group

Name

Norwegian name for catch. Previous informal name was the Tomma Formation (H1-4).

Type area

The Halten Terrace. The group's development is illustrated by the type section of its basal unit in well 6507/11-3 (Saga Petroleum), coordinates 65°01'59.8"N, 07°30'42.34"E, from 2536m to 2412 m (Fig 4.16) . Reference wells for the group are 6407/1-2 (Statoil), coordinates 64°47'50.61"N, 07°02'23.76"E, from 3907 m to 3658.5 m (Fig 4.17) and 6507/12-1 (Saga Petroleum), coordinates 65°07'01.62"N, 07°42'42.61"E, from 2213 m to 2094 m (Fig 4.18) .

Lithology

The Fangst Group typically comprises three lithological units: a lower fine to medium-grained sandstone with numerous shaly interbeds, a middle mudstone, and an upper relatively massive fine to coarse-grained sandstone. Each of these units are defined as formations herein.

Basal Stratotype

The base of the group is defined by the base of the Ile Formation as described below.

Lateral extent and variation

The Fangst Group is represented in most of the Trænabanken-Haltenbanken area except on the highest parts of the Nordland Ridge where its constituent units have been eroded. Time-equivalent sandstone-dominated sequences subcrop on the sea-floor along the inner part of the Trøndelag Platform (Bugge et al. 1984) and outliers of Middle Jurassic sediments are present east of the Froan islands and beneath Beitstadfjorden in Trøndelag. The latter probably represent a continental facies equivalent to the dominantly marine Fangst Group.
Along the southern margin of the Nordland Ridge (e.g. the Heidrun Field ) the succession is much thinner than on Halten Terrace and the threefold lithologic division is not so obvious. Further north the Trænabanken wells show a lateral facies change to marine mudstone of the Viking Group and only the lower unit of the Fangst Group (the Ile Formation ) is recognized.

Age

Late Toarcian to Bathonian.

Depositional environment

Shallow marine to coastal/deltaic facies dominate sequences on the Halten Terrace. Increasing continental influence is inferred towards the Trøndelag Platform to the east, especially in the lower part of the group. Upper parts interfinger with marine shales to the northeast in the Trænabanken area.

Correlation

The Fangst Group corresponds generally to the Brent Group in the North Sea and to the Stø Formation on Tromsøflaket. However, the basal part of Stø Formation is older and the base of the Brent Group is slightly younger than the Fangst Group.

Subdivision

Although three formations are described herein it is clear that increasing knowledge will lead to the establishment of a more varied framework to reflect the lateral facies changes seen in the area.

Source

Farsund Formation

Name

After the town of Farsund on the south-west coast of Norway.

Well type section

Norwegian well 2/7-3 (Phillips) from 3414 m to 3626 m, coord N 56°23'02.9", E 03°14'45.9" (Fig 3.40) .

Well reference sections

Norwegian well 7/12-2 (BP) from 3306 m to 3378.5 m, coord N 57°06'41.34", E 02°50'50.73" (Fig 3.32) and 2/8-3 (Amoco) from 3594m to 3761 m, coord N 56°18'31", E 03°26'54.1" (Fig 3.41) .

Thickness

200 m in the type well and 72.5 m ( 7/12-2 ) and 167 m ( 2/8-3 ) in the reference wells. The formation attains its maximum thickness in the axial part of the Central Graben and thins towards the flanking highs.

Lithology

The Farsund Formation consists predominantly of medium to dark grey shale. The shale is often well laminated and contains frequent calcareous streaks. Sandstone stringers are common in the type well 2/7-3 , particularly in the lowermost part of the sequence. In the reference well 7/12-2 , closer to the flank of the Southern Vestland Arch, a thinner Farsund Formation is present as a clear "coarsening upward cycle", becoming consistently less radioactive towards the top of the unit.

Boundaries

In the type well the base of the Farsund Formation occurs at the top of the sandy Eldfisk Formation and as a consequence is a pronounced gamma ray marker. Similarly, in the reference well the Farsund Formation overlies the sandy Ula Formation and is easily distinguished on logs. In several wells within the Central Graben, the Farsund Formation overlies the shaly Haugesund Formation with no intervening sand (e.g. reference well 2/8-3 . Here the base of the Farsund Formation is picked directly above the gamma ray minimum which forms the top of the Haugesund Formation .
The top of the Farsund Formation is marked by a further gamma ray minimum. Above this occurs the distinct log motif of the Mandal Formation , with its high gamma ray and inter val transit time readings.

Distribution

The formation is present throughout the Central Graben but thin or absent over the Southern Vestland Arch and intra-basinal highs.

Age

Kimmeridgian to Volgian.

Depositional environment

The Farsund Formation shales were mainly deposited in a low-energy marine environment. The gamma ray log profile suggests that the formation represents an initial period of deepening followed by gradual shallowing. In parts of the Central Graben the occurence of thin sand stringers in the lower part of the formation probably represent minor turbidite influxes from the adjacent shelf, where time equivalent sands of the Ula Formation were being deposited.

Source

Fensfjord Formation

Name

After a fjord on the west coast of Norway, adjacent to the type area in Quadrant 31.

Well type section

Norwegian well 31/2-1 (Shell) from 1594.5 m to 1741.5 m, coord N 60°46 '19.16", E 03°33' 15.87", (Fig 3.21) .

Well reference section

None at present.

Lithology

The formation consists of sandstones, grey-brown in colour, fine to medium grained, well sorted and moderately friable to consolidated. Calcite cemented sandstones occur in bands containing common bioclastic material. In the type well it is often carbonaceous and occasionally micaceous. Minor shale intercalations occur throughout. The formation has a "serrate" log character, composed of 3-5 m thick units arranged in several cycles.

Boundaries

The formation has an overall higher gamma ray intensity and larger FDC-CNL separation than the underlying Krossfjord Formation . The top of the Fensfjord Formation is characterized by a transition in the gamma ray log from a high intensity, serrate log shape to a high intensity but smooth outline in the overlying Heather Formation .

Distribution

The formation has only been clearly recognized in the Troll Field area.

Age

Callovian.

Depositional environment

The formation was deposited in a coastal shallow marine environment.

Source

Fiskebank Formation

Name

From the Fiskebank (Fisher Bank), off the shore of southern Norway. Named by Deegan & Scull (1977).

Well type section

Norwegian well 9/11-1 from 1483 m to 1335 m, coordinates N 57°00'41.40", E 04°00'33.52" (Fig 5.54) . No cores.

Well reference section

Norwegian well 8/9-1 from 1399 m to 1307 m, coordinates N 57°26'27.28", E 03°51'03.48" (Fig 5.55) . No cores.

Thickness

The formation is 148 m thick in the type well and 92 m thick in the reference well.

Lithology

In the type section the major lithology is very fine grained, well sorted, slightly silty sandstone, which occasionally has calcareous cement.

Basal stratotype

The basal contact of the Fiskebank Formation is defined by the boundary between the shales of the Lista Formation and the coarser sediments of the Fiskebank Formation. The difference between the two formations is not well defined on the logs. The boundary is placed where the gamma-ray readings decrease and the velocity increases somewhat upwards into the Fiskebank Formation (Fig 5.54, 5.55)

Characteristics of the upper boundary

The Fiskebank Formation is overlain by the shales of the Balder Formation . The boundary is generally seen as an upward decrease in gamma-ray response and an increase in velocity (Fig 5.55) .

Distribution

The formation is encountered in the Norwegian-Danish Basin.

Age

Late Paleocene.

Depositional environment

The formation is probably a basin-margin deposit, and appears to be mostly time-equivalent with the Sele Formation .

Source

Fjerritslev Formation

Name

After the village of Fjerritslev, Jutland, Denmark, (Larsen, 1966)

Well type section

Fjerritslev No 2 well, Jutland, (Larsen 1966).

Well reference sections

In Norwegian waters wells 17/9-1 R (Esso) from 2835 m to 2992 m, coord N 58°28'27.26", E 03°50'16.18" (Fig 3.25) . and 7/9-1 (Conoco) from 2524 m to 2601 m, coord N 57°20'37.10", E02°51'21.4" (Fig 3.24) .

Thickness

In the Norwegian reference wells the thickness of the Fjerritslev Formation is 157 m ( 17/9-1 R ) and 77 m ( 7/9-1 ).

Lithology

The formation consists predominantly of grey to dark grey or greyish brown marine claystone. It is variably calcareous and pyritic. Silty intervals occur frequently, grading into grey or buff micaceous siltstone. In the Danish area the Fjerritslev Formation is divided into four members according to degree of siltiness (Michelsen, 1978). However, such a subdivision is not merited in the Norwegian sector.

Boundaries

The formation is distinguished from the underlying sandy deposits of the Gassum and Skagerrak formations, and from the overlying sands of the Vestland Group , by its higher gamma ray and lower sonic log velocity readings.
In reference well 17/9-1 R the Fjerritslev Formation is overlain by a sequence of interbedded lavas and sediments over 500 m thick which are probably of Lower-Middle Jurassic age. This sequence has only been identified in one well. The boundary between the Fjerritslev Formation and the volcanics is again made by an upward change to lower gamma ray readings and higher sonic log velocities, (Fig 3.25) .

Distribution

The Fjerritslev Formation is only patchily developed in the Norwegian sector. The developments which are present probably represent those remnants of a once more widely distributed deposit which survived the mid-Jurassic erosional episode. The formation has been penetrated in two distinct and separate areas; around the Southern Vestland Arch (e.g. blocks 7/9 and 7/12) and in the Egersund Sub-Basin (e.g. block 17/9).

Age

The formation ranges in age from Hettangian to Pliensbachian. It is approximately equivalent to the Lower Jurassic Dunlin Group of the Northern North Sea, although no direct connection between the two sequences is thought likely.
In reference well 17/9-1 R a dyke immediately below the Fjerritslev Formation has been dated as Pliensbachian (Fumes et al., 1982).

Depositional environment

The claystones of the Fjerritslev Formation are shallow marine sediments deposited during a widespread marine transgression.

Remarks

In block 17/12 (Bream area) a sequence of continental clastics has been dated as Pliensbachian to Toarcian (Olsen and Strass, 1982). They are therefore partially age-equivalent to the Fjerritslev Formation, but cannot on lithological grounds be referred to the latter, (Table 3.4). These deposits have not been named by the present nomenclature group.

Source

Fladen Group

Name

From the Fladen Ground Spur, a structural feature.

Type area

The group is typically developed in the UK sector of the Central North Sea particularly in the area between the Piper and Forties oilfields.

Thickness

Within the volcanic province the thickest section penetrated so far is 1100 m and in this well the base of the volcanic was not reached. A considerable amount of thickness information was presented by Howitt and others (1975).

Lithology

The group embraces both volcanic and non-volcanic formations. Therefore the group contains a wide range of lithologies including basalts, tuff agglomerates, and normal continental to shallow water sediments.

Boundaries

The group normally rests on pre-Jurassic rocks, frequently Triassic sediments, and the lower boundary is marked either by the incoming of igneous rocks or the change from Triassic continental sediments to paralic sediments containing tuff horizons. The upper boundary is the contact with the marine sediments of the Humber Group. The Piper or Kimmeridge Clay formations may overlie the Fladen Group and in some sections the Heather Formation may be present but as noted earlier the complex interdigitations of the component formations of the Humber Group are not precisely known for this part of the Central North Sea.

Distribution

The group is largely restricted to the area of UK quadrants 14, 15, 16, and the more northerly parts of 21 and 22.

Age

Middle Jurassic.

Subdivision

As noted above the rocks of the Fladen Group are divided into two formations which are essentially end members of an interdigitating sequence. These are named the Rattray and Pentland formations. For present purpose if the encounters section contains more than half volcanic it is assigned to the Rattray Formation while if it contains more than half sedimentary rocks then it is placed in the Pentland Formation. As more data become available it will almost certainly be possible to subdivide the two formations further, and a proper understanding of the relationship between these end members may necessitate revision of the nomenclature in the future.

Source

Flekkefjord Formation

Name

From a town on the south-west coast of Norway. This formation was formerly included in the Valhall Formation of the Cromer Knoll Group (Deegan and Scull, 1977). The same unit was defined as the Flekkefjord Member by Rawson and Riley (1982), and is here elevated to formation status.

Well type section

Norwegian well 9/4-2 (Texaco) from 2155 m to 2208m, coord N 57°41'11.05", E 04°02'34.85" (Fig 3.38) .

Well reference section

Norwegian well 8/1-1 (Phillips) from 2379 m to 2425m, coord N 57°51'43.53", E 03° 12'27.64" (Fig 3.37) .

Thickness

In the type well it is 53 m, and in the reference well it is 46 m. This is also the approximate thickness in most wells within the Egersund Sub-Basin.

Lithology

The formation consists of dark grey shales which are variably carbonaceous, pyritic and may contain thin limestone stringers.

Boundaries

The lower boundary of the formation is clearly defined both on gamma and sonic logs by the contact with the more silty Sauda Formation . The upper boundary usually appears as a distinct log break, with higher radioactivity and higher interval transit times in the Flekkefjord Formation. In the Egersund Sub-Basin, which is situated closer to the source area, the boundary may be difficult to identify on logs.

Distribution

The formation is present in the Norwegian-Danish Basin. It is time-equivalent to the upper part of the Mandal Formation in the graben areas to the west and to the Fredrikshavn Unit C in the Danish sector to the east (Michelsen, 1978).

Age

Ryazanian.

Depositional envitronment

The Flekkefjord Formation was deposited in a marine, low-energy, basinal environment.

Source

Forties Formation

Name

Named by Deegan & Scull (1977) from the Forties Field in UK block 21/10.

Well type section

UK well 21/10-1 from 2370 m to 2131 m, coordinates N 57°43'50.37", E 00°58'29.19" (Fig 5.44) . Cores.

Well reference section

Norwegian well 7/11-1 from 3069 m to 2904 m, coordinates N 57°04'15.60", E 02°26'24.40" (Fig 5.46) . No cores.

Thickness

The Forties Formation is 239 m thick in the type well and 165 m thick in the reference well. The thickness decreases eastwards and southwards into the Norwegian sector.

Lithology

The formation typically consists of interbedded sandstones, siltstones and claystones, becoming predominantly sandy higher in the section. The sand is fine to coarse grained, poorly to moderately sorted and contains minor amounts of lignite, pyrite, glauconite and mica. The sands encountered in the Norwegian sector were deposited distally in a lobe, and consist of very fine to fine, angular to subangular grains often with mica and a calcareous cement.

Basal stratotype

Where the Forties Formation rests on the Andrew Formation (Deegan & Scull 1977) its lower boundary is defined by a decrease in velocity into the sandstones of the Forties Formation (Fig 5.44) . This boundary may be difficult to define on logs. Eastwards the Forties Formation overlies the argillaceous Lista Formation , and the boundary is characterised by decreasing gamma-ray and increasing velocity readings into the Forties Formation.

Characteristics of the upper boundary

The upper boundary is defined as the break between the Forties sandstones and the shales of the more or less time-equivalent Sele Formation . The log response changes from low gamma-ray readings and high velocity to higher gammaray readings and lower velocity in the Sele Formation (Fig 5.44) . As the Forties Formation passes into shales eastwards it may be enveloped by the Sele Formation .

Distribution

The Forties Formation extends as a large lobe from the area south of the Halibut Horst to the northwestern part of the Central Trough. Its approximate distribution on the Norwegian continental shelf is shown in (Fig 5.47) .

Age

Late Paleocene.

Depositional environment

The Forties Formation was deposited as submarine fans.

Source

Frigg Formation

Name

Named by Deegan & Scull (1977) after a Norse goddess, the wife of Odin.

Well type section

Norwegian well 25/1-1 from 2115 m to 1836 m, coordinates N 59°53'17.40", E 02°04'42.70" (Fig 5.62) . 42 m of cores (1868-1910 m).

Well reference section

Norwegian well 30/7-6 from 1923 m to 1783 m, coordinates N 60°29'29.82", E 02°03'26.14" (Fig 5.63) . No cores.

Thickness

The formation has a thickness of 279 m in the type well and 140 m in the reference well. A depocentre with a maximum thickness of approximately 300 m lies in Norwegian block 25/1.

Lithology

The formation consists of sandstones with some lenses and streaks of silty claystone. The sandstones are poorly consolidated, light brown to buff, micaceous and carbonaceous, and very fine to medium, occasionally coarse grained. Some layers have a calcareous cement. Traces of glauconite are present. The silty claystones are green to grey and carbonaceous.

Basal stratotype

The lower boundary normally shows a decrease in gamma-ray intensity and an increase in velocity from the Balder Formation into the Frigg Formation (Fig 5.62) .

Characteristics of the upper boundary

The top of the formation is placed where the sandstones give way to light grey to brown, occasionally green claystone of the Hordaland Group . The boundary is seen on logs as an increase in gamma-ray response and a decrease in velocity (Fig 5.62) .

Distribution

The Frigg Formation is found in the southwestern part of quadrant 30, the northwestern part of quadrant 25, and in adjacent areas in the UK sector. The Frigg sands of the Beryl and Brace Fields just extend into the Norwegian sector at about 59°30'N.

Age

Early Eocene.

Depositional environment

The Frigg Formation was deposited as submarine fans, by gravity flows. The mode of deposition led to the formation varying in thickness over short distances. The source was the East Shetland Platform to the west.

Source

Fruholmen Formation

Name

From a lighthouse on a skerry north of Ingøy in Finnmark. The present formational concept embraces the two units T2-1 and T2-2 (Helgøy and Ytterøy formations) of earlier informal usage.

Well type section

Well 7121/5-1 (Statoil), coordinates 71°35'54.88"N, 21°24'21.78"E, from 2793 m to 2572m (Fig 4.46) .

Well reference section

Well 7120/12-1 (Norsk Hydro), coordinates 71°06'48.7"N, 20°45'20.1"E, from 2535 m to 2337 m (Fig 4.47) .

Thickness

From 221 m in the type well to 198 m in 7120/12-1 .

Lithology

Basal grey to dark grey shales pass gradually upwards into interbedded sandstones, shales and coals. Sand dominates in the middle of the formation in several wells, while the upper part is more shaly, prompting a tripartite subdivision into (ascending order) the Akkar (Squid) Reke (Prawn) and Krabbe (Crab) members. The extent of these members in the type and reference wells are shown in (Fig 4.46 , 4.47).

Basal Stratotype

The base of the formation (and of the Akkar Member) is defined by a marked increase in gamma ray and neutron porosity logs, but often more moderate increases in interval transit time and bulk density readings.
The middle Reke Member is characterized by a lower gamma ray response, but its base can be best defined by a carbonate bench above which the separation between density and porosity logs decreases markedly. This separation increases again at the base of the uppermost Krabbe Member . A characteristic feature of the Reke and Krabbe members are funnel-shaped gamma ray responses.

Lateral extent and variation

No marked trends in lateral variation are suggested by present data, although few wells have yet penetrated the entire formation; the thickest sequence (262 m) is seen in well 7120/9-2 . The unit may be represented in its entirety further to the north in the Hammerfest Basin by the more shaly marine lithofacties of the Akkar Member passing up into sands of the overlying Tubåen Formation .

Age

The base of the formation is early Norian. The top corresponds in general to the Triassic/ Jurassic transition, but available data indicate that it is somewhat diachronous.

Depositional environment

Open marine shales of the Akkar Member pass up into coastal and fluvial sandstone dominated sequences of the Reke Member . These represent northward fluviodeltaic pro-gradation with a depocentre to the south. As the main deltaic input shifted laterally, most of the central and southern parts of the basin became the site of flood-plain deposition, with more marine environments to the north.

Subdivision

Present information suggests subdivision into three members as described above.

Source

Fuglen Formation

Name

Fuglen lighthouse is situated on the western tip of Sørøy at 70°40'N and 21°55'E. The unit corresponds to T3-1 or Risfjord Formation of earlier informal schemes.

Well type section

Well 7120/12-1 (Norsk Hydro), coordinates 71°06'48.7"N, 20°45'20.1"E, from 2047 m to 2019 m (Fig 4.50) . The lowermost 5 m and the contact with the underlying Stø Formation are cored in this well.

Well reference section

Well 7119/12-1 (Statoil), coordinates 71°06'08.00"N, 19°47'40.29"E, from 2658 to 2610 m (Fig 4.51) .

Thickness

28 m in the type well and 48 m in the reference well.

Lithology

Pyritic mudstones with interbedded thin limestones give characteristic gamma, sonic and density log responses. The shales are dark brown and the limestones white to brownish grey.

Basal Stratotype

The lower boundary is marked by sharp increases in gamma ray and density responses and by an accompanying decrease in interval transit time.

Lateral extent and variation

The formation is thickest in southwestern parts of the Hammerfest Basin, thinning to less than 10 m on the central highs in the basin; these areas are characterized by rare, thin limestones and by pyritic shales.

Age

Late Callovian to Oxfordian.

Depositional environment

The formation was deposited in marine environments during a highstand with ongoing tectonic movements. Coarse clastic sources were not emergent, but local block structures were the sites of low sedimentation rates.

Source

Garn Formation

Name

From the Norwegian word for a net. The unit corresponds to the upper part of the Tomma Formation (H1-4) of informal usage, or “Tomma I”.

Well type section

Well 6407/1-3 (Statoil), coordinates 64°52'25.48"N, 07°02'53.47"E, from 3704 m to 3600 m (Fig 4.20) . The lower 84 m of the unit are cored, including the base.

Well reference section

Well 6507/11-3 (Saga Petroleum), coordinates 65°01'59.8"N, 07°30'42.34"E, from 2457 m to 2412 m (Fig 4.16) . The lower 29 m of the unit are cored, including the base.

Thickness

104 m in the type well and 45 m in the reference well.

Lithology

The Garn Formation consists of medium to coarse-grained, moderately to well-sorted sandstones. Mica-rich zones are present. The sandstone is occasionally carbonate-cemented.

Basal Stratotype

The lower boundary is defined by a drop in gamma ray response. In the type well this coincides with an erosional contact at the base of a thin conglomerate.

Age

Bajocian to Bathonian.

Depositional environment

The Garn Formation may represent progradations of braided delta lobes. Delta top and delta front facies with active fluvial and wave-influenced processes are recognized.

Lateral extent and variation

The Garn Formation is encountered across most of Haltenbanken. It may be over 100 m thick on the Halten Terrace, but in structurally high positions the entire unit may be eroded. In the Trænabanken area shaly sediments are lateral equivalents of the Garn Formation sandstones.

Correlation

The Garn Formation is time equivalent to parts of the Brent Group in the North Sea, to the lower part of the Vardekløft Formation in East Greenland and to the upper part of the Stø Formation in the Hammerfest Basin.

Subdivision

The formation is not subdivided at present although two different units may be identified in the western part of Haltenbanken.

Source

Gassum Formation

Name

After the village of Gassum, Jutland, Denmark, (Larsen, 1966).

Well type section

Danish well Gassum No. 1 from 1613 m to 1643 m below ground (Ground evelation 53.3 m) (Bertelsen, 1978).

Well reference sections

In Norwegian waters wells 17/10-1 (Shell) from2682 m to 2825 m, coord N 58°01 '54", E 03°09'58" (Fig 3.23) . and 7/9-1 (Conoco) from 2601 m to 2609 m, coord N 57°20' 37.1", E 02°51'21.4" (Fig 3.24) . The section in well 17/10-1 was included by Deegan and Scull (1977) in the Skagerrak Formation .

Thickness

In the Norwegian reference wells the thickness of the Gassum Formation is 143 m ( 17/10-1 ) and 8 m ( 7/9-1 ).

Lithology

In the type well the formation consists of predominantly light grey to whitish, in places rather coarse-grained sandstones, with subordinate dark-coloured clay bands and coal lenses (Larsen, 1966). In Norwegian waters the formation is predominantly a white to light grey, mainly fine to medium grained sandstone, but frequently contains coarse sand and gravel. It is often calcite cemented and in some instances contains glauconite.

Boundaries

In the Norwegian sector the lower boundary may be characterized by a general lowering of the velocity when entering the underlying Skagerrak Formation . Often this boundary coincides with a distinct horizon with high gamma ray response (e.g. well 17/4-1 (Elf) and well 9/12-1 (Shell)).
The upper boundary of the Gassum Formation is easily picked where it is overlain by Lower Jurassic shales of the Fjerritslev Formation or Upper Jurassic shales of the Boknfjord or Tyne groups. Where the Gassum Formation is overlain by the Middle Jurassic Bryne Formation (Norwegian sector), the boundary may only show a slight decrease in the gamma ray response and an overall decrease in velocity marking the appearance of the Bryne Formation .
In the reference well 17/10-1 , where the Fjerritslev and Bryne formations are missing, the Gassum Formation is overlain by the shales of the Boknfjord Group .

Distribution

In the Norwegian sector the Gassum Formation occurs throughout the Norwegian-Danish Basin, on the Southern Vestland Arch and along the northeastern margin of the Central Graben. Its distribution towards the axial part of the Central Graben is unknown. It is often completely or partially eroded as a result of mid-Jurassic earth movements.

Age

Rhaetian in the type well, but seems to become younger northwards (Bertelsen, 1978). Sparse dating in the Norwegian sector gives Rhaetian to Sinemurian ages.

Depositional environment

The sedimentology of the Gassum Formation in the Norwegian sector has not been much studied, but is assumed to represent fluvial to marginal marine deposits laid down during a transgressive phase at the Triassic/Jurassic transition.

Source

Gipsdalen Group

Name

Cutbill and Challinor (1965) introduced the term Gipsdalen Group for a suite of rocks of mid-Carboniferous to early Permian age. The group is widely exposed on Svalbard, with its type area in central Spitsbergen. The group’s overall geological development is well known onshore, both on Spitsbergen itself (e.g. Steel & Worsley 1984; Dallmann et al. 1999) and on Bjørnøya on the Stappen High (Worsley et al. 2001). The Gipsdalen Group is here extended to cover the offshore mid-Carboniferous to early Permian succession in the southern Norwegian Barents Sea and is there dominated by red-coloured siliciclastics and warm-water, often dolomitised carbonates – also with the significant presence of evaporites and the halite diapirs in the Nordkapp Basin. Wells 7121/1-1R , 7124/3-1 and 7226/11-1 from the margins of the Loppa High and the Bjarmeland Platform record deposition in deeper marine settings than seen onshore. The formational scheme proposed herein is relatively broad and reflects three, easily recognised, highly diachronous stages of development starting with red-bed sedimentation in isolated fault-controlled basins, followed by mixed siliciclastic-carbonate deposition and terminated by carbonate-dominated sedimentation on the platforms and carbonates and evaporites in the basins.

Offshore reference areas

In the Norwegian Barents Sea, 11 wells and 4 shallow cores have penetrated strata assigned to Gipsdalen Group. The subsurface reference area is located on the eastern Finnmark Platform where this succession has been penetrated by 7229/11-1 and 7228/9-1 S on the northern margin and 7128/6-1 and 7128/4-1 in a more central position on the platform (Fig 9.19) . Further toward the south, IKU drilled three cores (7029/03-U-02, 7030/03-U-01 and 7129/10-U-02) close to the Finnmark coast where the group’s sediments subcrop against the Pliocene/Pleistocene unconformity (Bugge et al. 1995). Additional information on the group’s development comes from well 7120/12-4 on the western Finnmark Platform.
The Loppa High also forms an important reference area with good seismic coverage, including a 3D survey, and three wells, viz. 7120/1-1R2 , 7120/2-1 and 7121/1-1R , that penetrate the succession in the southern Loppa High area (Fig 9.19 , 9.20) Further to the east, the group was encountered in wells 7124/3-1 and 7226/11-1 on the southern margins of the Bjarmeland Platform. A shallow core and several minicores have also been drilled on the Svalis Dome on the Bjarmeland Platform (Nilsson et al. 1996).

Thickness

The Finnmark and Bjarmeland platforms and the Loppa High formed low-angle ramps dipping toward the Nordkapp and eastern Hammerfest basins during deposition of the Gipsdalen Group. The thickest drilled succession is from the southern flanks of the Loppa High, where the group is more than 1000 m thick in well 7121/1-1R and seismic data suggest that a further 500 m is present below TD. This is comparable to the up to 1800 m thick successions recorded locally in marked half-graben structures such as Inner Hornsund and Billefjorden on Spitsbergen. In contrast the group’s sediments are totally absent on the crest of the Loppa High – as on southern Bjørnøya on the Stappen High – while well 7120/2-1 in a near-crestal position shows a 680 m thick development, similar to the 595 m thick development on northern Bjørnøya.
The group thins from approximately 315 m in well 7128/6-1 to 250 m in 7128/4-1 on the Finnmark Platform; IKU cores suggest comparable thicknesses (Bugge et al. 1995). Further to the west, well 7120/12-4 penetrated the upper 85 m of the group. Wells 7228/9-1 S and 7229/11-1 on the northern Finnmark Platform penetrated only the upper (Moscovian-Sakmarian) part of the group: this interval is 211 m and 333 m thick respectively in these wells - significantly thicker than the corresponding interval in 7128/4-1 and 7128/6-1 further to the south. The group thickens even more towards the northwest and seismic data from the Nordkapp Basin indicate thicknesses of several hundred metres. On the Bjarmeland Platform, the group is more than 800 m thick in well 7226/11-1 , where Bashkirian carbonates rest directly on basement. A total thickness of 1000 m is suggested by the 465 m penetrated in 7124/3-1 combined with seismic data from the underlying section. The group is 670 m thick in well 7120/1-1 R2 where it rests on garnet mica schists/gneisses of Caledonian age.

Lithology

The group is composed of metre-thick to rarely tens of metre-thick rhythmic units generally showing shallowing upward trends continental red bed sandstones, siltstones and conglomerates dominate the basal part of the succession. These are overlain by mixed carbonates and siliciclastics where the siliciclastics are grey-coloured marine sandstones, conglomerates and shales and the carbonates include a variety of shallow marine facies. The upper part of the group is dominated by rhythmically bedded limestones and dolomites with occasional small phylloid algal – Palaeoaplysina buildups, and minor evaporites on the platform areas. The biota is of chlorozoan composition and dominated by algae and foraminifers (c.f. Lees & Buller 1972). Seismic data suggest that the shelf carbonates pass into several hundred metre thick successions of stacked buildups in the deeper ramp areas (Elvebakk et al. 2002). These buildups have not been drilled and their internal composition and exact stratigraphic position is therefore unknown. However, similar relationships are described from age equivalent rocks in the Sverdrup Basin where the largest build-ups occur on the basin slope (Beauchamp 1993). Evaporites dominate in the basinal areas; anhydrite occurs interbedded with carbonates near platform margins whereas halite dominates in more distal settings.

Lateral extent and variation

The group’s sediments are found throughout the Norwegian Barents Sea. Thickest developments are seen in the Nordkapp Basin and other basinal areas where the succession is dominated by evaporites. The thickest carbonate-dominated successions are found on the distal parts of the platforms, such as the eastern flanks of the Loppa High and the northern margins of the Finnmark Platform. The group thins towards structural highs and mainland Norway: it shows a clearly onlapping development, so that the lower non-marine parts were deposited in isolated half-grabens, while platforms and highs only became part of the depositional basin later, when relative second order sea-level rise led to marine flooding of the entire circum-Arctic region (c.f. onland Spitsbergen, Steel & Worsley 1984). The group’s occurrence resting directly on basement in 7226/11-1 (see above) confirms this general pattern. The considerable variations in lithology, both laterally and vertically, reflect the ongoing sea level rise and resultant varying timing of drowning of different siliciclastic provenance areas. A larger proportion of shallow marine siliciclastics are expected updip on the platforms, while carbonate buildups are best developed on basinal margins. The Loppa and Stappen highs experienced several phases of tectonism during deposition of the group, in contrast to the vast bulk of offshore platforms and basins, and onshore exposures on Bjørnøya show interesting analogues for the development expected on the Loppa High (Worsley et al. 2001).
The boundary between the Gipsdalen Group and the underlying Billefjorden Group is only known with certainty from wells 7128/4-1 and 7128/6-1 on the Finnmark Platform and from 7120/2-1 on the Loppa High. On the Finnmark Platform, the sharp contact between Lower Carboniferous grey fluvial siliciclastics with coals below and red bed facies with caliche above marks a boundary represented by a major regional unconformity in the circum-Arctic and is associated with a significant change in palaeoclimate from warm and humid to warm and arid to semi-arid (Steel & Worsley 1984; Stemmerik & Worsley 1989; Stemmerik 2000).

Age

The basal non-marine red-bed succession contains palynomorphs indicating a general Serpukhovian to Bashkirian age. Fusulinids suggest a late Bashkirian to Sakmarian age for the marine part of the group (Stemmerik et al. 1998; Ehrenberg et al. 1998a). In onshore areas of Svalbard, the group’s sediments have been dated to the late Serpukhovian to early Artinskian (Dallmann et al. 1999) (Fig 9.6)

Depositional environments

The basal non-marine red-bed succession of the Ugle Formation was deposited during active rifting in the ?late Serpukhovian to Bashkirian and cores from 7120/2-1 represent alluvial fan and braided river deposits. The overlying Falk Formation marks the transition into shallow marine deposition at a time when there still was siliciclastic supply from emergent highs. The Ørn Formation uppermost in the group was deposited in a variety of shallow to deeper marine carbonate environments during sea level highstands. The presence of extensive subaqueous anhydrite and halite deposits in the basins and sabkha evaporites on the platforms clearly suggests deposition took place in warm semi-arid to arid climates (Steel & Worsley 1984; Stemmerik 2000). The platform succession is characterised by stacked rhythmic shelf deposits often terminated by subaerial exposure surfaces, reflecting deposition during a time period characterised by high frequency and high amplitude fluctuations in sea level (e.g. Stemmerik & Worsley 1989; Pickard et al. 1996; Stemmerik et al. 1998; Ehrenberg et al. 1998a; Samuelsberg & Pickard 1999; Worsley et al. 2001). The depositional environments recorded from the platform areas generally resemble those recognised onshore Spitsbergen and Bjørnøya. The deeper marine, outer ramp and basinal deposits have no counterparts onshore.

Formations assigned to the group

Three formations are formally described below and these are named after birds of prey common to northern Norway. The still poorly known outer platform and basinal succession is provisionally included in the uppermost Ørn Formation .

Source

Grey Beds (informal)

Sediments of Triassic age beneath the Åre Formation have been encountered in a number of wells offshore mid-Norway. Based on their colour the Triassic section in the Norwegian Sea has been informally divided into “Grey Beds” and “Red Beds” . The “Grey Beds” sediments represent continental clastics of grey colour, deposited in a more humid climate compared to the “ “Red Beds” sediments. Thicknesses of more than 2500 meters have been drilled. No type well section has been established.

Grid Formation

Name

Named after a female giant in Norse mythology, who was one of the wives of Odin.

Well type section

Norwegian well 15/3-3 from 1840 m to 1470 m, coordinates N 58°52'31.25", E 01°46'46.24" (Fig 5.64) . No cores.

Well reference sections

Norwegian well 24/12-1 from 1660 m to 1502 m, coordinates N 59°02'29.80", E 01°52'57.93" (Fig 5.61) . No cores. Norwegian well 24/12-2 from 1397 m to 1282 m, coordinates N 59°12'00.75", E 01°52'53.34" (Fig 5.65) . No cores.

Thickness

In wells 24/12-1 and 24/12-2 it is 158 m and 115 m, respectively.

Lithology

The formation consists of sandstones with interbeds of claystone and siltstone. The sandstones often have a massive, "blocky", appearance as illustrated by type well 15/3-3 (Fig 5.64) . Individual sandstone beds show little, or no evidence of fining-upwards or coarsening- upwards. The sandstones are very fine to fine, sometimes medium to coarse. Sorting is generally moderate to good. Traces of mica, pyrite, glauconite and fossil fragments are common. A higher argillaceous content is found in distal areas. Well 24/12-1 illustrates the interfingering of thicker claystone units of the Hordaland Group with the Grid Formation (Fig 5.61) . Further subdivision may be possible in the future (see Remarks).

Basal stratotype

The lower boundary shows a decrease in gamma-ray response and an increase in velocity from the Hordaland Group into the sandstones of the Grid Formation (Fig 5.64) .

Characteristics of the upper boundary

The upper boundary is characterised by an increase in gamma-ray readings and a decrease in velocity from the sandstones of the Grid Formation into the clay-stones of the Hordaland Group (Fig 5.64) .

Distribution

The sandstones were probably derived from the East Shetland Platform and the formation is recognised in the Viking Graben area between 58°30'N and approximately 60°30'N (Fig 5.66) . A depocentre lies in Norwegian block 15/3 where the formation reaches a thickness of nearly 400 m. It thins eastwards and is not penetrated by wells on the Utsira High. It has been identified in some wells in the Oseberg area. In the Viking Graben north of 61° N, several sandstone bodies occur in the Hordaland Group at the same level, but it is uncertain whether they belong to the Grid Formation.

Age

Middle to Late Eocene, but wells 25/6-1 and 24/12-2 have given an Early Oligocene age.

Depositional environment

The formation is thought to have been deposited in an open marine environment during a regressive period. An eustatic fall in sea level in the Late Eocene is indicated by Haq et al. (1987).

Remarks

The formation comprises a series of sand bodies which interfinger with claystones. There is a considerable increase in thickness from less than 200 m north of 59° N (e.g. wells 24/12-1 and 24/12-2 ) to nearly 400 m south of 59° N (e.g. well 15/3-3 ). This is not due to a general increase in thickness, but rather to sand deposition having started earlier in the south. This could give grounds for erecting two formal units, a lower one confined to the area south of 59° N and probably of Middle Eocene age, and an upper one. In some areas the lower unit is separated from the upper one by a sequence of claystones which is referred to informally as the Belton member in the UK sector. However, lithological uniformity renders such subdivision impractical at present.

Source

Hardråde Formation

Name

Named after Harald "Hardråde" Sigurdsson, a Norwegian king (A.D. 1046-1066).

Well type section

Norwegian well 30/11-3 from 2892 m to 2601 m, coordinates N 60°02'38.59", E 02°32'15.47" (Fig 5.36) . No cores.

Well reference section

Norwegian well 31/6-2 from 978 m to 968 m, coordinates N 60°34'58.24", E 03°54'55.76" (Fig 5.37) . No cores.

Thickness

The formation is 291 m thick in the type well ( 30/11-3 ) and 10 m in well 31/6-2 . It is absent on tilted fault blocks in the Troll area (e.g. well 31/2-9 ).

Lithology

The formation consists generally of interbedded limestones and mudstones, except in the Troll area where it is thin and consists of a single limestone bed. The limestones are white or pale, moderately hard to very hard. The mudstones are medium to light grey, often silty and calcareous.

Basal stratotype

The lower boundary is towards the Kyrre Formation or an unconformity above older rocks. The boundary towards the Kyrre Formation is identified by the absence of relatively thick limestone beds in this formation and a lower content of calcareous material in the mudstone. This results in a decrease in gamma-ray intensity and an increase in velocity from the Kyrre Formation into the Hardråde Formation (Fig 5.36) . The formation has an unconformable lower boundary in the Troll area.

Characteristics of the upper boundary

The upper boundary is towards the Rogaland Group . When it is towards the Lista Formation it is characterized by an upward increase in gamma-ray intensity and a distinct drop in velocity due to a transition from limestones to mudstones (Fig 5.36) . An upper boundary towards the Våle Formation lacks the distinct drop in velocity. This is due to the presence of limestones and a more marly facies in the Våle Formation . An upper boundary towards the Ty Formation is shown by a change to sandstone.

Distribution

The formation is present on the Horda Platform (Fig 5.32ab) .

Age

Late Campanian to Maastrichtian.

Depositional environment

Open marine.

Remarks

The Hardråde Formation is time-equivalent with the Jorsalfare and Tor formations of the Shetland Group (Fig 5.6) .

Source

Haugesund Formation

Name

After the town of Haugesund on the west coast of Norway.

Well type section

Norwegian well 2/7-3 (Phillips) from 3695 to 4191 m, coord N 56°23'02.9", E 03°15'45.9'7, (Fig 3.40) .
See “remarks” for qualification of this well type section.

Well reference sections

Norwegian well 3/5-2 (Gulf) from 3182.5 to 3345 m, coord N 56°32', 56°32'34.46", E 04°23 '11.1", (Fig 3.42) . and 2/8-3 (Amoco) from 3761 m to 4115 m (TD.), coord N 56°18'31", E03°26'54.1", (Fig 3.41) .

Thickness

496 m in the type well, 162.5 m in 3/5-2 , and 354 m in 2/8-3 . The formation is thickest in the axis of the Central Graben and thins towards the flanking highs, where it passes partially or entirely into the sandy lithology of the Ula Formation .

Lithology

The Haugesund Formation consists predominantly of shale ranging in colour from light grey to brownish black. The shale is often carbonaceous and calcareous, and contains frequent thin sandstone interbeds. In general the upper part of the formation represents an overall "coarsening-upward cycle", becoming sandier and siltier upwards.

Boundaries

In the type well, 2/7-3 , the Haugesund Formation overlies Zechstein salt and the base of the formation is therefore obvious from both logs and cuttings. However, the Zechstein salt is almost certainly penetrative at this location and does not therefore provide a true stratigraphic base for Haugesund Formation (see “remarks”). In the reference well 2/8-3 and elsewhere in the Central Graben the base of the Haugesund Formation is often found overlying the Vestland Group . This is shown in the reference well 3/5-2 where the upward change from the sandy Bryne Formation to the shales of the Haugesund Formation produces the expected gamma ray/sonic log break (see (Fig 3.42) .
The top of the Haugesund Formation in the type well is the contact with the sandy Eldfisk Formation . In areas of the Central Graben where the Eldfisk Formation is absent, the top of the Haugesund Formation is picked at a clearly correlatable gamma ray minimum, above which the gamma ray increases to the higher values of the basal Farsund Formation (e.g. well 2/8-3 ).

Distribution

The formation is ubiquitous in the Central Graben and widely distributed around the flanks of the basin and intra-basinal highs. It is absent in the Ula Field where it is entirely replaced by time-equivalent sands of the Ula Formation , and is also absent on the crest of the Southern Vestland Arch and intra-basinal highs.

Age

Callovian to Early Kimmeridgian. In neither the type nor the reference wells have pre-late Oxfordian ages been proven but Callovian mudstones assignable to the Haugesund Formation occur in the vicinity of the reference well 3/5-2 .

Depositional environment

The bulk of the shales of the Haugesund Formation were deposited in a marine, low energy, basinal environment. The common thin sand interbeds may represent sporadic turbidite influxes emanating from the adjacent shelf where coarser elastics (i.e. the Ula Formation ) were being deposited. The "coarsening-upward" nature of the sequence represents an overall regression which was terminated by a further transgression and the deposition of the Farsund Formation shales.

Remarks

The type well 2/7-3 penetrated a thick development of the Haugesund Formation, considered to be typical of the formation as it is commonly encountered in the Central Graben. However, the inadequately defined base of the formation makes 2/7-3 ultimately unsatisfactory as a type well. None of the other Central Graben wells available to this study establish a base for this formation, and penetrations on the flanks of the basin (such as the reference well 3/5-2 are fewer, atypical and potentially controversial stratigraphically. Penetration of a well-defined base for the Haugesund Formation by a future well in the Central Graben would perhaps provide a rare instance in which replacement of a type well might be justified.

Source

Havert Formation

Name

From the seal species Halichoerus grypus: The unit corresponds to unit T1-1 or the Svolvær Formation of earlier informal usage.

Well type section

Well 7120/12-2 (Norsk Hydro), coordinates 70°7'30.03"N, 20°48'19.00"E, from 3657 m to 3552 m (Fig 4.42) .

Well reference section

Well 7120/9-2 (Norsk Hydro), coordinates 71°29'40.81"N, 20°42'05.38"E, from 4956 m to 4806 m (Fig 4.43) .

Thickness

105 m in the type well and 150 m in the reference well.

Lithology

In the type well the formation consists of medium to dark grey shales with minor inter-bedded pale grey siltstones and sandstones comprising two generally coarsening upwards sequences. The reference section further north in the Hammerfest Basin shows a more monotonous silty shale sequence with only a very weak upwards coarsening trend.

Basal Stratotype

The base is defined by increasing gamma ray and decreasing density responses above underlying mixed clastic and carbonate sequences. In the reference well the base is also easily recognised as basal silty shales directly overlie a 6 m thick limestone bed.

Lateral extent and variation

Present information suggests a fining trend from the southern margins of the Hammerfest Basin northwards. Thicknesses are moderate and show no marked trends.

Age

Palynomorphs suggest a Griesbachian to Dienerian age.

Depositional environment

The formation was deposited in marginal marine to open marine settings with coastal environments to the south and southeast.

Correlation

Palynofloras and sequence development show similarities to the Vardebukta Formation of western Spitsbergen.

Source

Heather Formation

Name

Named by Deegan and Scull (1977).

Well type section

UK well 211/21-1A (Shell) from 2810 m to 2840 m, coord N 61°11'9.6", E 01°06'5.7", (Fig 3.18) .

Well reference sections

Norwegian wells 33/9-1 (Mobil) from 2450 m to 2464m, coord N 61 °15°07.5", E 01°50'25.8", (Fig 3.11) . 31/2-1 (Shell) from 1531.5 m to 1594.5 m, coord N 60°46'19.16'°, E 03°33'15.87", (Fig 3.21) . and 15/3-1S (Elf) from 4754 m to 4986 m, coord N 58°50' 57.0'', E 01°43'13.25", (Fig 3.19) .

Thickness

30 m in the type well, but attains thicknesses in the order of a thousand metres in graben areas. In the reference wells the thicknesses are

Lithology

The formation consists of mainly of grey silty claystone with thin streaks of limestone. A further subdivision of the formation is possible (Fig 3.18) . although no formal status is proposed for this subdivision. Two divisions are commonly recognized. The lower division is light to dark grey, hard, silty claystone, often micaceous and calcareous. The upper division is separated into two further units by an unconformity detected by dipmeter data or biostratigraphical gaps, but no distinct lithological difference is noted. The lithology is dark grey silty claystone, carbonaceous in part with limestone streaks. (For further discussion see Deegan and Scull (1977) p. 18). On the Horda Platform where the Heather Formation interdigitates with sandstones of the Krossfjord , Fensfjord and Sognefjord formations, it becomes in places highly micaceous and may grade into a sandy siltstone.

Boundaries

The lower boundary is the contact with the arenaceous Brent Group . The upper boundary is the contact with the Draupne Formation , which has an anomalously high gamma ray response and low velocity. Both boundaries are therefore marked by log breaks.

Distribution

The formation can be recognized over most of the northern North Sea north of 58°N and east of the East Shetland Platform boundary faults.

Age

Bathonian to Kimmeridgian.

Depositional environment

The silty claystones of the Heather Formation were deposited in an open marine environment, brought about by the marine transgression which initially deposited the youngest formation of the Brent Group .

Remarks

Use of the Heather Formation is here restricted to the part of the North Sea north of approximately 58°N. Deegan and Scull (1977) indicated the presence of the formation in the Central Graben, but did not describe the area in any detail. Recent work (e.g. Ofstad 1983, and this report) suggests a subdivision of the Middle Jurassic-earliest Cretaceous claystones of this region into three new formations. None of these can alone be referred to the northern Heather Formation, and the name has therefore not been used in the southern area.

Source

Hegre Group

Name

From the bird (English: heron) of the same name. The pre-Rhaetian Triassic rocks of the Northern North Sea were earlier designated the Cormorant Formation by Deegan and Scull (1977), having UK well 211/21-1A (Shell) as a type well section. In this report this Triassic Unit is given group status, while the Cormorant Formation is only applied to certain areas where a subdivision of the Triassic is impossible. We suggest that the Cormorant Formation should refer to attenuated sequences confined to structural highs in the UK sector.

Type area

The type area is the East Shetland Basin, west of the Viking Graben. The group is illustrated by UK well 211/29-5 (Shell), and Norwegian wells 33/5-1 (Norsk Hydro), 33/12-2 and 33/12-5 (Mobil).

Thickness

No well has penetrated a complete Triassic succession. The maximum drilled sequence is 1839 m in well 33/12-5 .

Lithology

The Hegre Group consists of intervals of interbedded sandstones, claystones and shales associated with sequences of dominantly sand or shale/claystone. Shales and claystones usually have reddish colours whereas the sandstones show a range in colour from white, light grey, orange to brick red. The grain size varies from very fine to very coarse and the sediments are in parts of a pebbly nature. The Hegre Group also has subordinate white limestone, anhydrite and brownishred marl.

Boundaries

The Hegre Group is directly overlain by Cretaceous strata on some of the structural highs. Where Jurassic is present, the top of the Hegre Group is normally placed at the change from interbedded sandstones and shales of the Hegre Group to the relatively massive clean sandstones of the Statfjord Formation . This is normally represented by a change from an irregular sonic log response in the Hegre Group to a more regular or blocky one in the Statfjord Formation . In addition the upper boundary of the Hegre Group is often close to the top of abundant red beds in the section - see also the description of the Statfjord Formation . The base of the Triassic rocks is only penetrated on structural highs and close to the margins of the sedimentary basin. In these sections only late Triassic seems to be present, and hence the nature of the lower boundary of the Hegre Group is not yet established. It is realized that the Hegre Group cannot be given full formal definition until its base has been adequately defined, but we nevertheless offer the term for interim use.

Distribution

The Hegre Group is apparently present in the whole Northern North Sea area. Its relationship to the Triassic units defined further south by Deegan and Scull (1977) is unclear. We therefore recommend that the term Hegre Group should only be used in the area north of 60°N. It is terminated to the west by major faults along the east flank of the East Shetland Platform and to the east by the Øygarden Fault Zone (Hamar et al., 1980). In the northeastern part of the North Sea area, where Precambrian/ Caledonian basement dips gently to the west, progressively younger Triassic sediments onlap basement in an easterly direction. In the east, on the Måløy Fault Blocks, Triassic strata are probably missing, but may have been preserved from erosion in N-S elongated basins to the east of the structural highs. Alternatively Triassic sediments might not have been deposited in this area. The thickness of the Hegre Group within the East Shetland Basin shows a general increase from the western flank toward the central part of the depositional basin. On the eastern flank thick Triassic deposits are found just west of the Øygarden Fault Zone, which may indicate that the Triassic sediments were deposited in an asymmetric basin.

Age

The Triassic sections penetrated in the Northern North Sea show ages from Late Triassic (early Rhaetian) to possibly Early Triassic (?Scythian).

Subdivision

The Hegre Group is divided into three formations: the (basal) Teist Formation, the Lomvi Formation and the Lunde Formation (top). The subdivision suggested here for the Triassic of the Northern North Sea is based on information from areas to the west of the Viking Graben. However, available data from the Horda Platform indicate that a subdivision is also possible in that area.

Source

Heimdal Formation

Name

Named by Deegan & Scull (1977) after the Heimdal Field on the Norwegian continental shelf. Heimdal was a son of the Norse god Odin, and one of the principal gods in Norse mythology.

Well type section

Norwegian well 25/4-1 from 2423 m to 2067 m, coordinates N 59°34'27.30", E 02°13'22.60" (Fig 5.52) . 36 m of cores from the upper part of the formation, and 6.5 m from the lower part.

Well reference section

Norwegian well 15/9-5 from 2684 m to 2448 m, coordinates N 58°24'12.47", E 01°42'29.20" (Fig 5.42) . No cores.

Thickness

The Heimdal Formation is 356 m thick in the type well and 236 m thick in the reference well. It thins rapidly east of these wells and south of well 15/9-5 .

Lithology

The formation is dominated by thick units of poorly sorted, fine to coarse grained, poorly cemented sandstones with variable amounts of mica, glauconite and detrital lignite. The sandstone units are interbedded with grey and black shales, limestones and sandy limestones. There is a wide range in number and thickness of interbedded lithologies. In general, the amount of carbonate increases towards the base of the formation.

Basal stratotype

The lower boundary of the Heimdal Formation is usually marked by a transition from the Lista Formation into the interbedded sandstones of the Heimdal Formation. The log response is characterised by lower gamma-ray readings and higher velocities when entering the overlying Heimdal Formation (Fig 5.52) . The Heimdal Formation locally overlies the cleaner sandstones of the Ty Formation . In that case, the lower boundary is placed where clean sandstones give way to the interbedded sandstones of the Heimdal Formation. These formations are normally separated by the Lista Formation .

Characteristics of the upper boundary

The upper boundary is usually defined by a transition into the Lista Formation shales and is then characterized by higher radioactivity and lower velocity (Fig 5.52) . Locally, the Heimdal Formation is overlain by the Hermod Formation , the upper boundary therefore being defined by a change into clean, "blocky" sand.

Distribution

The sandstones of the Heimdal Formation are distributed in a broadly lobate pattern eastwards from the western margin of the Viking Graben. Their approximate distribution on the Norwegian continental shelf is shown in (Fig 5.47) .

Age

Paleocene.

Depositional environment

In the westernmost areas (East Shetland Platform/Fladen Ground Spur), the Heimdal Formation was deposited on a shallow-marine shelf under high-energy conditions. In the Viking Graben, the formation was deposited as submarine fans derived from sand accumulations on the shallow shelf to the west. The shale layers consist partly of the fine fraction of the turbidity currents and of hemipelagic mud.

Remarks

In a narrow belt extending from the eastern part of quadrant 15 (Fig 5.47) , the Heimdal Formation is developed as a clean sandstone without interbedded shales. This is described as the Meile member .

Source

Hekkingen Formation

Name

The formation name is taken from Hekkingen Lighthouse at 69°30'N and 17°40'E on the northern tip of Senja. The formation corresponds to T3-2 or Olderfjord Formation of earlier usage.

Well type section

Well 7120/12-1 (Norsk Hydro), coordinates 71°06'48.7"N, 20°45'20.1"E, from 2019 m to 1660 m (Fig 4.50) . Two short cores have been taken between 1661 and 1668 m and between 1702.6 and 1707.6 m.

Well reference section

Well 7119/12-1 (Statoil), coordinates 71°06'08.00"N, 19°47'40.29"E, from 2610 to 2497 m (Fig 4.51) .

Thickness

359 m in the type well and 113 m in the reference well.

Lithology

The formation consists of brownish-grey to very dark grey shale and claystone with occasional thin interbeds of limestone, dolomite, siltstone and sandstone. These minor clastic components are most common towards basinal margins. Lower parts of the formation show especially high gamma ray readings. This is used to differentiate the lower Alge from the upper Krill Member in the formation (Fig 4.50, 4.51)

Basal Stratotype

The base is defined by the transition from carbonate cemented and pyritic mudstones to poorly consolidated shales, producing a sudden increase in interval transit time and an abrupt decrease in bulk density values.

Lateral extent and variation

The formation is thickest in its type well. It thins northwards to less than 100 m towards the axis of the Hammerfest Basin. This pattern reflects the development of semigrabens along basin margins while doming was active along the basin axis.

Age

Palynomorphs suggest an age span of late Oxfordian/early Kimmeridgian to Ryazanian. There are local breaks in deposition at the base and top, probably most developed near the basinal axis.

Depositional environment

Marine, deep water with anoxic conditions resulted from the formation of local barriers to circulation by Kimmerian movements.

Source

Hermod Formation

Name

Hermod was a son of Odin, and was known as "the quick one".

Well type section

Norwegian well 25/2-6 from 2361 m to 2221 m, coordinates N 59°45'33.55", E 02°33'05.96" (Fig 5.56) . No cores.

Well reference section

UK well 10/1-1A from 2212 m to 2127 m, coordinates N 59°50'10.50", E 02°00'33.60" (Fig 5.48) . No cores.

Thickness

The Hermod Formation is 140 m thick in the type well and 85 m thick in the reference well. It thickens towards the centre of its distribution area (Fig 5.47) .

Lithology

The Hermod Formation consists of clean sandstones which are very fine to fine grained and clear to grey. The formation is to a limited extent interbedded with dark shales.

Basal stratotype

The lower boundary of the Hermod Formation is identified by a transition from the shales of the Lista Formation . This boundary essentially represents the boundary between the Lista and Sele formations, and the Hermod Formation may rest on shales of the Sele Formation . The log response in both cases is a sharp transition from the high gamma-ray readings and low velocity of the shales to the low and regular gamma-ray readings and higher velocity of the Hermod Formation sandstones (Fig 5.56) . Where the Hermod Formation rests directly on the Heimdal Formation the boundary may be indistinct, but the log response changes from an erratic pattern in the Heimdal Formation to a smoother one, reflecting the more homogeneous sandstones of the Hermod Formation.

Characteristics of the upper boundary

The Hermod Formation is overlain by the time-equivalent Sele Formation , and the boundary is an abrupt change from sandstones to dark shales. The gamma-ray response changes from low readings in the sandstones to significantly higher ones in the Sele Formation , and the velocity is lower in the Sele Formation (Fig 5.56) .

Distribution

The Hermod Formation is found in the South Viking Graben, in the northwestern part of quadrant 25. It may also be found in other parts of the South Viking Graben. The main distribution area is outlined in (Fig 5.47) .

Age

Late Paleocene.

Depositional environment

The Hermod Formation was deposited in submarine fan systems connected with the deltaic Moray Group in the west.

Source

Hidra Formation

Name

Named by Deegan & Scull (1977) after the Hidra High in Norwegian blocks 1/3 and 2/1. The name Hidra is after the island of Hidra on the southern coast of Norway.

Well type section

Norwegian well 1/3-1 from 4441 m to 4371 m, coordinates N 56°51'21.00", E 02°51'05.00" (Fig 5.24) . No cores.

Well reference sections

UK well 22/1-2A from 3783 m to 3738 m, coordinates N 57°56'12.20", E 01°02'55.80" (Fig 5.25) . No cores. UK well 29/25-1 from 2258.5 m to 2228 m, coordinates N 56°18'10.00", E 01°51'48.80" (Fig 5.26) . No cores. Danish well BO-1 from 2275.5 m to 2220 m, coordinates N 55°48'8.22", E 04°34'18.66" (Fig 5.27) . Cored through the upper 35 m.

Thickness

The formation is 70 m thick in the type well, 45 m in 22/1-2 A, 30.5 m in 29/25-1 and 55.5 m in BO-1. Seismic interpretation suggests that the formation reaches a maximum thickness of about 150 m in the northwestern part of the Central Trough in the Norwegian sector.

Lithology

In the type well the formation consists of white to light grey, hard chalks with thin interbeds of grey to black shale in the lower part of the formation. Locally the formation is more marly with interbedded marly chalk and marl. The chalks are occasionally softer with abundant glauconite and pyrite. The colour may be white, grey, green, brown or pink. At the base of the formation in UK well 22/1-2 A, hard, black, carbonaceous and argillaceous limestones are present. Traces of pink waxy tuff occur in places. The formation is generally highly bioturbated.

Basal stratotype

The formation usually shows a gamma-ray response that has constant low values and high velocities. These contrast sharply at the lower boundary with the higher gamma-ray response and lower velocity of the Åsgard and Sola formations. The lower boundary is more gradational when the carbonate-rich facies of the Rødby Formation is present beneath the Hidra Formation.

Characteristics of the upper boundary

The upper boundary is defined by the stratotype of the Blodøks Formation . The boundary is characterised by a change from the chalk lithology to mainly mudstone. This is seen as an abrupt change to higher gamma-ray response and a decrease in velocity in the Blodøks Formation . The boundary shows as a glauconitised hardground in the core from Danish well BO-1.

Distribution

The formation is found in the central and southern North Sea. In the Norwegian sector, it is missing above highs such as the Sørvestlandet, Mandal, Jæren, Utsira and Sele highs, the Grensen Ridge, as well as many of the salt diapirs.

Age

Cenomanian.

Depositional environment

Open marine with a perioditic or turbiditic origin for the sediments.

Source

Hod Formation

Name

Named by Deegan & Scull (1977) from the Hod Field in Norwegian block 2/11. The name Hod derives from one of the twelve principal gods in Norse mythology. Hod was a son of Odin.

Well type section

Norwegian well 1/3-1 from 4343 m to 3828 m, coordinates N 56°51'21.00", E 01°51'05.00" (Fig 5.24) . No cores.

Well reference sections

UK well 29/25-1 from 2225 m to 2012 m, coordinates N 56°18'10.00", E 01°51'48.80" (Fig 5.26) . No cores.
Norwegian well 2/8-8 from 2601 m to 2494 m, coordinates N 56°16'50.28", E 03°24'15.93" (Fig 5.28) . 36 m of cores discontinuously through the upper 78 m and lowermost 6 m of the formation.

Thickness

The formation is 515 m thick in the type well, 213 m in UK well 29/25-1 and 107 m in Norwegian well 2/8-8 . In the Norwegian sector, seismic interpretation indicates that the formation may reach a thickness of more than 700 m in the northwestern part of the Central Trough.

Lithology

In the type well the formation consists of hard, white to light grey, crypto- to microcrystalline limestones which may become argillaceous or chalky in places. White, light grey to light brown, soft to hard chalk facies may dominate the formation or alternate with limestones. The limestones may be pink or pale orange. Thin, silty, white, light grey to green or brown, and soft, grey to black, calcareous clay/shale laminae are occasionally present. Pyrite and glauconite may occur throughout the formation and the latter may be common in the lower part.

Basal stratotype

The lower boundary is usually marked by a distinct log break to a lower gamma-ray response and higher velocity from the Blodøks Formation to the Hod Formation (Fig 5.24) . The boundary may be less distinct when the Blodøks Formation is more calcareous (Fig 5.31) .

Characteristics of the upper boundary

The upper boundary towards the Tor Formation is generally marked by a change in gamma-ray readings to a more constant and slightly lower level, and also by higher velocity, (Fig 5.24, 5.31) . The upper boundary may represent an unconformity in the Ekofisk area (e.g. Norwegian well 2/8-8 , (Fig 5.28) .

Distribution

The formation is widely distributed in central and eastern parts of the central North Sea, passing laterally into sediments of the Herring and Flounder Formations to the west and the Tryggvason and Kyrre formations to the northwest.

Age

Turonian to Campanian.

Depositional environment

Open marine with deposition of cyclic pelagic carbonates (periodites) and distal turbidites (Skovbro 1983 and d'Heur 1986).

Remarks

An informal, tripartite subdivision of the Hod Formation into lower, middle and upper members is often possible in the southern part of the Central Trough in the Norwegian sector. The subdivision is based on the frequent presence of a higher clay content in the middle of the Hod Formation (Fig 5.24, 5.28, 5.29).

Lower member of the HOD Formation

This unit constitutes the largest part of the Hod Formation and is a sequence of bioturbated laminated chalks with a low clay content. It occurs in Norwegian wells 1/3-1 from 4343 m to 4066 m, 2/8-8 from 2601 m to 2538 m and 1/9-1 from 3648 m to 3353 m.

Middle member of the HOD Formation

This is a sequence consisting mainly of periodites, which generally have a greyish colour reflecting a marked increase in terrigenous clay. It is shown on well logs as an increase in gamma-ray readings. It occurs in Norwegian wells 1/3-1 from 4066 m to 4009 m, 2/8-8 from 2538 m to 2518 m and 1/9-1 from 3353 m to 3344m.

Upper member of the HOD Formation

This unit constitutes another sequence dominated by periodites with minor allochthonous intercalations, but with a return to a low clay content. It occurs in Norwegian wells 1/3-1 from 4009 m to 3828 m, 2/8-8 from 2518 m to 2494 m and 1/9-1 from 3344 m to 3312 m.

The Herring Formation of Deegan & Scull (1977) includes a similar lithology and was deposited at the same time as the Hod Formation. It is regarded here as the lower part of the Hod Formation. The Hod Formation is also equivalent in age to the Tryggvason and Kyrre formations (Fig 5.6) .

Source

Footnotes

References

Hordaland Group

Name

The group was named by Deegan and Scull (1977) after the county of Hordaland in Norway. In the Norwegian Sea the group's local develoment corresponds to the lower part of the informal Sklinna Group (H6).

Type area

The type area is the North Sea Tertiary Basin. Typical sections through the group are shown in Norwegian wells 2/2-1 (Fig 5.60) . and 24/12-1 (Fig 5.61) . (Fig 5.69) , shows a seismic section through the group in the Central Trough area. The lithostratigraphy is shown in (Fig 5.40) .
In the Norwegian Sea, well 6407/1-3 (Statoil), coordinates 64°52'25.48"N, 07°53'47"E, from 2212.5 m to 1762.5 m, is used to illustrate the local development of the group (Fig 4.37) . No cores.

Thickness

The group has a thickness of 1060 m in well 2/2-1 and 1365 m in well 24/12-1 . Its average thickness is around 1100-1200 m in the central and southern part of the Viking Graben, but in the northern Viking Graben the group only reaches a thickness of a few hundred metres. Maximum thicknesses in the central and southern part of the Viking Graben are approximately 1300 m and 1400 m, respectively. The thickness decreases towards the basin margins. The group is 450 m thick in well 6407/1-3 .

Lithology

The group consists of marine claystones with minor sandstones. The claystones are normally light grey to brown, fissile and fossiliferous. Red and green claystones sometimes occur at the base. Thin limestones and streaks of dolomite are present. Sandstones are developed at various levels in the group. These are generally very fine to medium grained, and are often interbedded with claystones.
In Haltenbanken the Hordaland Group consists of claystones and minor sandstones, assigned to the Brygge Formation . The sandstone content increases to the east.

Basal stratotype

In the North Sea the lower boundary shows an increase in gamma-ray intensity and a decrease in velocity from the laminated tuffs of the Balder Formation into the claystones of the Hordaland Group (Fig 5.56) . Where the Frigg Formation is present at the base of the Hordaland Group the lower boundary normally shows a decrease in gamma-ray response and an increase in velocity from the Balder Formation into the Frigg Formation (Fig 5.62) .
In the Norwegian Sea the base is defined by a decrease in interval transit time shown on the sonic log and by an increase in the density log readings.

Characteristics of the upper boundary

In the North Sea the upper boundary is placed at the contact with undifferentiated grey to grey-brown claystones of the Nordland Group or sandstones of the Utsira Formation . It represents an unconformity of Early to Middle Miocene age, which may be difficult to identify in some wells.
In the Central Trough, a zone occurs which has high gamma-ray readings and usually a slightly lower velocity than the underlying and overlying claystones. The upper boundary of the Hordaland Group is placed at the base of this zone (Fig 5.60) . On seismic sections, the sediments below this horizon normally have a distorted signature whilst those above it have a smoother one. The boundary shows a very small angular unconformity; it is not clear whether a small hiatus is present. In the Viking Graben, the upper boundary is normally the base of the sandy Utsira Formation 1). The contact is then marked by an upward decrease in gamma-ray intensity (Fig 5.70) . Where the basal part of the Nordland Group is developed as claystone the boundary is placed at log breaks associated with a change in claystone colour.
In the Norwegian Sea the top of the group is defined by a decrease in interval transit time on the sonic log and an increase in the density log readings.

Distribution

The group is distributed over most of the North Sea Tertiary Basin. It is incomplete at the basin margins, owing to erosion or non-deposition.
The Hordaland Group occurs throughout Haltenbanken. It thins eastwards and is eroded on the Nordland Ridge. Close to the coastline the group comprises a sandy sequence informally termed the Røyrvik Formation (Askvik and Rokoengen, 1985).

Age

The group is of Eocene to Early Miocene age both in the North Sea2) and the Norwegian Sea. Datings in wells 2/2-1 , 2/2-2 and 2/2-3 indicate that the uppermost part of the group may be of Middle Miocene age in the Central Trough.

Depositional environment

The sediments are marine, mainly deposited in deep water.

Subdivision

In the North Sea the Frigg Formation was formally erected by Deegan & Scull (1977). Three additional sandstone formations are now recognised in the Hordaland Group, and are described here. Claystone intervals between the sandstones are not defined as formations and remain as unnamed units of the Hordaland Group. The Grid and Skade formations are widely distributed in the Viking Graben area, whereas the Vade Formation, which is found in the Central Trough, has a limited distribution. Other sandstones, which cannot be assigned to the formations described here, are found in the Norwegian part of the North Sea Basin. Subregional work and further information from wells may enable more units to be formally erected in the future.
On Haltenbanken the Hordaland Group comprises the Brygge Formation . Lateral facies changes and breaks in the sequence may form the basis for future subdivision.

Remarks

The boundary between the Hordaland and Nordland groups in the Central Trough area may be slightly different from the position selected in well 2/7-1 by Deegan & Scull (1977). The boundary is difficult to identify in that well, owing to the borehole being damaged directly below the casing which is placed at 1591m (5221 ft).

Compiled from

Footnotes

References

Hugin Formation

Name

One of Odin's two ravens in Norse mythology.

Well type section

Norwegian well 15/9-2 (Statoil) from 3483 m to 3657 m, coord N 58°25' 34.06" , E 01 °42'28.2" (Fig 3.26) .

Well reference section

Norwegian well 15/6-5 (Esso) from 3627 m to 3679 m, coord N 58°30'29.67", E 01 °45' 50.4" (Fig 3.28) .

Thickness

The formation is 174 m thick in the type well and 52 m thick in the reference well.

Lithogy

The formation consists of light brown to yellow, very fine to medium grained sandstones. Occasional coarse grained layers are found. The sandstones have fair sorting, and the grains are subangular to subrounded. Shale and siltstone partings are common. Carbonaceous material and coal fragments are abundant. Occasional thin coal beds can be observed. The sandstones are often bioturbated, but cross bedding can sometimes be observed. The sandstones are often calcareous and glauconitic.

Boundaries

The lower boundary represents the transition from the coaly Sleipner Formation . The upper boundary represents the transition into the shales of the Viking Group , giving clear breaks both on sonic and gamma ray logs.

Distribution

The formation is found in the southern Viking Graben, north of the Jæren High.

Age

Early Bathonian to Early Oxfordian.

Depositional environment

The formation represents near shore, shallow marine sandstones with the occasional influence of continental fluviodeltaic conditions.

Source

Ile Formation

Name

From the Norwegian word for a stone sinker or a drag. The unit corresponds to the lower part of the informal Tomma Formation (H1-4) or “Tomma III”.

Well type section

Well 6507/11-3 (Saga Petroleum), coordinates 65°01'59.8"N, 07°30'42.34"E, from 2536 m to 2471.5 m (Fig 4.16) . The entire formation is cored except for the basal 2 m.

Well reference section

Well 6407/1-3 (Statoil), coordinates 64°52'25.48"N, 07°02'53.47"E, from 3813 m to 3741 m (Fig 4.19) . One core (10 m recovery), from the upper part of the unit.

Thickness

64.5 m in the type well and 72 m in the reference well.

Lithology

Fine to medium and occasionally coarse-grained sandstones with varying sorting are interbedded with thinly laminated siltstones and shales. Mica-rich intervals are common. Thin carbonate-cemented stringers occur, particularly in the lower parts of the unit.

Basal Stratotype

The lower boundary is defined at the base of a generally upwards coarsening sequence visible on the gamma ray log. In wells where this coarsening trend is not evident on logs, the base of the formation may be picked where the overall lithology changes from siltstone to sandstone. Often this transition is associated of one or more carbonate-rich beds.

Age

Late Toarcian to Aalenian.

Depositional environment

The formation represents various tidal-influenced delta or coastline settings.

Lateral extent and variation

The Ile Formation generally varies from 50 m to 100 m over most of Haltenbanken, with a general thickening to the west and a marked thinning to the northeast. It is also encountered in wells on Trænabanken. Sandstone-dominated successions of similar age have been located by sea bottom sampling and shallow drilling on the eastern part of the Trøndelag Platform (Bugge et al. 1984).

Correlation

No distinct comparable time equivalent formations are known from the North Sea area. In the Hammerfest Basin the middle part of the Stø Formation may be correlated to the Ile Formation.

Source

Intra Balder Formation sandstones (informal)

Intra Balder Formation sandstones have been encountered in a number of wells in the Viking Graben and adjacent areas. They are described as part of the Balder Formation and illustrated in the type well 25/11-1 .

Intra Draupne Formation SS (informal)

Intra Draupne Formation sandstones have been encountered in a number of wells in the Tampen and Oseberg areas and along the flanks of the Utsira and Jæren highs. The sandstones are generally considered to be of turbiditic origin (De`Ath and Schuyleman, 1981; Harms et al., 1981) and are described as part of the Draupne Formation . Well 15/3-1 S illustrates these arenaceous intercalations.

Intra Heather Formation SS (informal)

Intra Heather Formation sandstones have been encountered in a number of wells in the Sleipner, Oseberg, Tampen and Måløy/Sogn areas as well as along the flanks of the Utsira High. On the Horda Platform, sandy interdigitations of the Heather Formation have been formalized as the Krossfjord , Fensfjord and Sognefjord formations.

Intra Melke Formation sandstones (informal)

Intra Melke Formation sandstones have been encountered in a number of wells in block 6608/10 on the Rødøy High. The sandstones are intercalated with the Melke Formation .

Intra Åsgard Formation sandstones (informal)

Intra Åsgard Formation sandstones have been encountered in a few wells on the Utsira High. The sandstones are intercalated with the Åsgard Formation .

Isbjørn Formation

Name

From the Norwegian name for the Polar Bear (Ursus maritimus).

Definition

The type section is defined as the interval from 1834.7 m to 1745.4 m in well 7128/6-1 on the Finnmark Platform (Fig 9.48) , apparently concurring with informal unit L-8 of Ehrenberg et al. (1998a). Cores cover the entire section (Fig 9.49) . In this well the base of the formation is marked by a decrease in gamma ray response, reflecting the transition from silty warm water carbonates to clean cool-water carbonates.

Reference sections

A reference section is defined as the intervals from 3700 m to 3625 m and 3586 m to 3502 m in well 7121/1-1 R on the Loppa High, these intervals interfingering with Ulv Formation lithofacies (Fig 9.43) . A small core, 2.7 m long, exists from the uppermost part of the formation, between 3513.7 m and 3511.0 m. An additional reference section is designated in 7229/11-1 (4046 m to 3970 m), where the Isbjørn Formation rests directly on carbonate buildups of the Polarrev Formation . The uppermost part of the formation has been drilled in the southernmost part of the Finnmark Platform (cores 7128/12-U-01 from 569.2 m to 557.5 m and 7129/10-U-01 from 475.3 m to 464 m) (Bugge et al. 1995). The base is not easy to pick on logs in areas where the formation rests directly on the Polarrev Formation (e.g. 7229/11-1 ), as the boundary there represents a limestone-limestone contact (Fig 9.39) . It is recognised by a slight decrease in interval transit time and neutron porosity log values accompanied by an increase in bulk density in wells 7121/1-1 R and 7229/11-1 . However, over the buildups, the base of the formation is easily picked on seismic data. In 7121/1-1 R the transition from the interfingering Ulv Formation up into the Isbjørn Formation shows a marked decrease in gamma ray response, reflecting the transition from silty wackestones into cleaner packstones and grainstones (Fig 9.43) .

Thickness

The formation is approximately 75-90 m thick in the central and northern parts of the Finnmark Platform, thinning to 10 m in the IKU cores. It is slightly thinner in 7124/3-1 and absent from the outer shelf environments represented by the Ulv Formation in 7226/11-1 on the Bjarmeland Platform (Fig 9.38) . The formation is thickest in 7121/1-1 R on the southeastern flank of the Loppa High, where its two intercalations have a composite thickness of approximately 160 m (Fig 9.43) .

Lithology

Bedded, white to light grey bioclastic limestones with a fauna of mainly crinoids and bryozoans dominate the formation. The dominant facies are grainstones and packstones (Fig 9.50) . The formation has a characteristic low gamma ray response throughout, except for thin intervals of dark grey silty wackestone that represent temporary deeper water deposition related to flooding events. Chert nodules occur sporadically throughout the section. The dark grey silty limestone intervals are lithologically similar to the Ulv Formation , but are regarded as part of the Isbjørn Formation if they are of subordinate importance and less than 15-20 m thick. Thicker intervals such as that from 3625 m to 3586 m in well 7121/1-1 R are included in the Ulv Formation .

Lateral extent and variation

The formation represents deposition in inner shelf environments. Following a major transgression in the early Artinskian the formation developed over earlier carbonate build-ups and submerged structural highs and platforms. The formation is not known from either outer shelf regimes or from basinal areas.

Age

The base of the formation is apparently diachronous. In the type section in 7128/6-1 fusulinids suggest that the base is of mid-Sakmarian age and the top is late Artinskian (Ehrenberg et al. 1998a). In well 7229/11-1 the top of the underlying Polarrev Formation is dated as late Sakmarian, indicating a late Sakmarian or younger age for the formation in this well (Davydov 1998). The formation is apparently of late Artinskian age in IKU cores (Bugge et al. 1995).

Depositional environments

The bioclastic crinoid- and bryozoan- dominated grainstones and packstones of the Isbjørn Formation represent deposition in inner shelf environments on cool-water carbonate platforms (Stemmerik 1997). The more silty wackestone facies represent temporary flooding and deposition in slightly deeper environments below storm wave base.

Correlation

The upper part of the Isbjørn Formation correlates age- and facies-wise with the Hambergfjellet Formation on Bjørnøya and with the Vøringen Member of the Kapp Starostin Formation on Spitsbergen.

Source

Johansen Formation

Name

Named after Hjalmar Johansen, member of the Amundsen South Pole expedition.

Well type section

Norwegian well 31/2-1 (Shell) from 2176 m to 2272.5 m, coord N 60°46'19.16'', E 03°33'15.87" (Fig 3.13) .

Well reference section

None at present.

Thickness

95.5 m in the type well.

Lithology

In the type well the formation consists of a sequence of sandstones with thin calcite cemented streaks throughout. The lower part is medium to fine-grained, micaceous, well sorted sandstone which grades downwards into light grey silty micaceous claystone. The main section of the formation is composed of medium grained, friable sandstones, with well sorted quartz grains which are angular to subrounded. The uppermost part is composed of medium to fine grained, micaceous sandstones, which are moderately sorted, silty and argillaceous.

Boundaries

In the type well area the Johansen Formation splits the Amundsen Formation . It is distinguished by a low response, crescentic gamma ray profile.

Distribution

The formation is restricted to an area extending from the eastern part of the Horda Platform northwards to the Måløy Fault Blocks.

Age

Sinemurian to Pliensbachian.

Depositional environment

The formation was probably deposited on a high energy, shallow marine shelf.

Source

Jorsalfare Formation

Name

Named after Sigurd "Jorsalfare" Magnusson, a Norwegian king (A.D. 1103-1130).

Well type section

Norwegian well 25/1-1 from 2997 m to 2711 m, coordinates N 59°53'17.40", E 02°04'42.70" (Fig 5.33) . One core (17 m) in the middle of the formation and another (4 m) at the base.

Well reference sections

Norwegian well 35/3-2 from 1665 m to 1520 m, coordinates N 61051'05.98", E 03°46'28.22" (Fig 5.34) . No cores. Norwegian well 24/9-1 from 3117 m to 2752 m, coordinates N 59°16'09.48", E 01°47'31.18" (Fig 5.35) . No cores.

Thickness

The formation is 286 m thick in the type well ( 25/1-1 ), 145 m in well 35/3-2 and 365 m in well 24/9-1 .

Lithology

The formation generally consists of mudstones interbedded with thin limestone beds. The mudstones are light to medium grey, often calcareous. The limestones are white to light grey, fine grained, occasionally sandy and dolomitic.

Basal stratotype

The lower boundary is defined by a decrease in gamma-ray intensity and an increase in velocity, reflecting an increase in calcareous content from the Kyrre Formation into the Jorsalfare Formation (Fig 5.33 , 5.34) . In the Tampen Spur area, however, the boundary may be difficult to identify due to small differences in calcareous content. The lower boundary may be unconformable above the Jurassic sequences (e.g. in the Gullfaks area).

Characteristics of the upper boundary

The upper boundary may be towards the Våle , Lista or Ty formations of the Rogaland Group . When the upper boundary is towards the shale of the Lista Formation it is usually characterised by an upward increase in gamma-ray intensity and a distinct drop in velocity (Fig 5.34) . When it is towards the Våle Formation it does not show the same distinct drop in velocity and increase in gamma-ray intensity, because the overlying lithology consists of limestones or calcareous mudstones (Fig 5.35) . Where the upper boundary is towards the Ty Formation it is identified as a change to sandstone (Fig 5.33) .

Distribution

The formation is present in the Viking Graben and on the Tampen Spur. Its boundaries towards the Jorsalfare Formation in the Viking Graben, the Hardråde Formation on the Horda Platform and the Tor Formation on the Utsira High are illustrated in (Fig 5.32ab) .
The main characteristics that can be used to distinguish the three formations are:

Separation of these three formations may be difficult in transitional areas.

Age

Late Campanian to Maastrichtian.

Depositiohal environment

Open marine.

Remarks

The Jorsalfare Formation is time-equivalent with the Hardråde and Tor formations of the Shetland Group and also with the informal ”formation E” of Deegan & Scull (1977) (Fig 5.6) .

Source

Kai Formation

Name

The Norwegian word for quay. The formation corresponds to the informal Korgen Formation (H6-2).

Well type section

Well 6407/1-2 (Statoil), coordinates 64°47'50.61"N, 07°02'23.76"E, from 1690 m to 1419 m (Fig 4.38) . No cores.

Thickness

271 m in the type well.

Lithology

Alternating claystone, siltstone and sandstone with limestone stringers. Glauconite, pyrite and shell fragments are common.

Basal Stratotype

The base is defined by a decrease in interval transit time on the sonic log and an increase in the density log readings.

Lateral extent and variation

The formation is present throughout the Haltenbanken area apart from the crest of the Nordland Ridge. The sand content varies locally.

Age

Early Miocene to Late Pliocene.1)

Depositional environment

The formation was deposited in marine environments with varying water depths.

Source

Footnotes

Kapp Toscana Group

Name

The group is named after a cape on the southern coast of Van Keulenfjorden.

Type area

Central eastern Spitsbergen.

Thickness

Up to >475 m in Svalbard, up to 2000 m (?) on the Barents Sea Shelf.

Lithology

The Kapp Toscana Group comprises shales, siltstones and sandstones of Late Triassic to Middle Jurassic (Bathonian) age in Svalbard and on the Barents Sea Shelf. The group starts with the grey shales of the Tschermakfjellet Formation which normally grade upward into the immature sandstones of the De Geerdalen Formation (Storfjorden Subgroup). This interval contains a number of coarsening-upward successions with increasing proportions of sandstone towards the SW, NE and E, while shales dominate in the central areas of Svalbard. On Svalbard, the upper part forms a condensed clastic sedimentary succession, only a few metres thick in southern and western Svalbard, which gradually is more completely developed towards the east (Wilhelmøya Subgroup). Equivalents on the Barents Sea Shelf show a comparatively thick development (Realgrunnen Subgroup; Worsley et al. 1988). Sandstones and shales also dominate the group in the Barents Sea.

Distribution

The group is exposed along the Tertiary fold-thrust belt on western Spitsbergen, in central and eastern Spitsbergen, as well as on Barentsøya, Edgeøya, Hopen, Kong Karls Land and Bjørnøya. It continues in the subsurface between these islands and southwards across the Barents Sea Shelf to the Bjarmeland Platform and the Hammerfest and Nordkapp basins.

Age

Ladinian to Bathonian.

Depositional environment

The Kapp Toscana Group has been deposited in a generally nearshore, deltaic environment and is characterised by shallow marine and coastal reworking of deltaic and fluviodeltaic sediments (Mørk et al. 1982).

Subdivision

Five formations are recognized within the group on the Barents Sea Shelf: the Snadd , Fruholmen , Tubåen , Nordmela and Stø formations.

Compiled from

Klappmyss Formation

Name

The formation's name is derived from the seal species Cystophora cristata. The formation corresponds to T1-2 and the lower parts of T1-3 (Andenes and lower Gimsøy formations) of earlier informal terminology.

Well type section

Well 7120/12-2 (Norsk Hydro), coordinates 71°7'30.03"N, 20°48'19.00"E, from 3552 m to 3095 m (Fig 4.42) .

Well reference section

Well 7120/9-2 (Norsk Hydro), coordinates 71°29'40.81"N, 20°42'05.38"E, from 4806 to 4245 m (Fig 4.43)

Thickness

457 m in the type well and 561 m in the reference well.

Lithology

Medium to dark grey shales pass upwards into interbedded shales, siltstones and sandstones in the type well. The reference well shows a similar trend, but with more shale throughout.

Basal Stratotype

The base is defined by clear log breaks, with increasing gamma ray, interval transit time and neutron porosity readings. This represents an important sequence boundary throughout the area, reflecting an early Smithian transgressive pulse.

Lateral extent and variation

The formation thickens and fines northwards from the southern margins of the Hammerfest Basin.

Age

Palynofloras suggest a Smithian to Spathian age.

Depositional environment

Marginal to open marine environments are indicated, with renewed northwards coastal progradation following the early Smithian transgression.

Correlation

The unit is equivalent to the Tvillingodden Formation of western Spitsbergen.

Source

Klippfisk Formation

Name

Norwegian for “stockfish”, “dried cod”.

Well type section

Norwegian well 7430/10-U-01, coordinates N 74°12'47.79", E 30°14'44.22"

Well reference section

Norwegian well 7231/01-U-01 coordinates N 72°45'12.45" , E 31°07'30.21"

Thickness

In the type well the gross thickness of the formation is 8.9 m and in the reference well 4.5 m. 15 m on Kong Karls Land.

Lithology

Limestone, marl and calcareous sandstone.

Description

The Klippfisk Formation represents a condensed carbonate succession occurring in platform areas ( Kutling Member ), where it consists of limestones and marls, and is often glauconitic. The limestones may have a nodular appearance. Fossil debris, dominated by Inoceramus prisms may be abundant. The formation is strongly bioturbated.
In Kong Karls Land the coeval Tordenskjoldberget Member has a similar lithology and is assigned to the Klippfisk Formation.
The Klippfisk Formation is the carbonate platform time-equivalent of the Knurr Formation (Hammerfest Basin) and the Rurikfjellet Formation (Svalbard). The unit has also been penetrated by shallow drilling and sampling in Hopendjupet and in the Olga Basin as well as farther south in the Barents Sea.

Lower boundary definition

The base is defined at the abrupt decrease in gamma ray intensity, where the dark mudstones of the underlying formation are replaced by marls.

Distribution

Offshore unit, known from the Bjarmeland Platform and Kong Karls Land.

Age

Late Berriasian to Hauterivian, based on palynology, nanofossils and bivalves.

Compiled from

Knurr Formation

Name

From the fish species Eutrigla gurnardus (grey gurnard). The formation corresponds to T4-1 or Slettnes Formation of earlier informal terminology.

Well type

Well 7119/12-1 (Statoil), coordinates 71°06'08.00"N, 19°47'40.29"E, from 2497 m to 2441 m (Fig 4.51) .

Well reference section

Well 7120/12-1 (Norsk Hydro), coordinates 71°06'48.7"N, 20°45'20.1"E, from 1660 to 1375 m. A core was taken between 1535 and 1546 m in this well (Fig 4.50) .

Thickness

56 m in the type well and 285 m in the reference well.

Lithology

In the type well the formation consists of dark grey to greyish brown claystone with thin limestone and dolomite interbeds. Thin sandstones are also seen in the unit's lower parts, but these disappear laterally into the Hammerfest Basin. Red to yellow brown claystone generally occurs in the upper parts of the formation.

Basal Stratotype

The base is defined by decreasing gamma ray response and by the sonic log showing decreasing interval transit time. In the type well the density log response shows a slight decrease in an otherwise increasing trend across the boundary. However, the general pattern in most wells is an increase in the density log response accompanying the sonic log response. The base is marked by a thin sandy limestone in the reference well.

Lateral extent and variation

Present data suggest similar lithologies in all wells which penetrate the formation. The sand content is somewhat higher close to the Troms-Finnmark Fault Complex (e.g. well 7119/12-1 ). Most complete sequences are seen along basin margins.

Age

Dinoflagellates and foraminifera suggest a Ryazanian/Valanginian to early Barremian age.

Depositional environment

The formation was deposited in open and generally distal marine environments with local restricted bottom conditions.

Correlation

The formation is a lateral equivalent of the dark shale dominated Rurikfjellet Member of the Janusfjellet Formation on Svalbard.

Source

Kobbe Formation

Name

From the Norwegian collective name for several seal species common in arctic waters. The unit corresponds to T1-3 or upper Grimsøy Formation of earlier usage.

Well type section

Well 7120/12-2 (Norsk Hydro), coordinates 71°7'30.03"N, 20°48'19.00"E, from 3095 to 2927 m (Fig 4.44) .

Well reference section

Well 7120/9-2 (Norsk Hydro), coordinates 71°29'40.81"N, 20°42'05.38"E, from 4245 m to 3962 m (Fig 4.45) .

Thickness

168 m in the type well and 283 m in the reference well.

Lithology

A basal 20 m thick shale unit passes up into interbedded shale, siltstone and carbonate cemented sandstone.

Basal Stratotype

The formation is defined by upwards increasing gamma ray, interval transit time and neutron porosity responses into the basal shale. Log responses show much more variation above this unit.

Lateral extent and variation

This unit shows a coarser proximal facies development, along the southern margin of the Hammerfest Basin and fines towards the basin axis. The formation thickens northwards from 140 m on the Troms - Finnmark Platform. Thicknesses vary more from platform to basin than in the underlying units.