NANSEN FM
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Generell informasjon
Generell informasjon Attributt Verdi Litostrat. enhet Den litostratigrafiske enhetens offisielle navn.NANSEN FMNPDID for litostrat. enhet Sokkeldirektoratets entydige nøkkel for litostratigrafiske enheter.108Nivå Angir den litostratigrafiske enhetens nivå. Lovlige verdier: GROUP, FORMATION, MEMBER.FORMATIONLitostrat. enhet, forelder Den litostratigrafiske enhetens offisielle navn, refererer til forelder. Se også nivå. Vil være tom hvis Nivå = GROUP. -
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Beskrivelse
Nansen Formation
NameAfter the Norwegian polar scientist, explorer and statesman, Fridtjof Nansen.Well type sectionWell reference sectionThickness46 m in the type well and 19 m in the reference well.LithologyIn the type well the formation consists of medium- to coarse-grained, fairly well-sorted, homogeneous, white sandstones with a calcite-cement and a subordinate kaolinite matrix. Occasional horizons with granules or pebbles are present. Carbonaceous material and mica are rare but glauconite is quite common. Thin shale beds containing marine fossils are present, most commonly in its upper part.
At the top of the formation, from 3112 m to 3135 m in the type well is a more prominent calcareous horizon defined informally as the calcareous sandstone bed. This may grade locally into a sandy limestone, for example in the Brent Field.
The Nansen Formation thins towards the east and passes laterally into calcareous siltstones and shales in the Norwegian sector (Fig 1.17-18) . Although these siltstones and shales are lateral equivalents of the Nansen Formation they are placed in the overlaying Dunlin Group on lithological grounds.BoundariesThe originally Nansen Member of the Statfjord Formation was elevated to formation level by Lervik, 2006. The lower boundary is marked by the change from the non-calcareous sandstones of the Eiriksson Formation to the cleaner, generally calcareous sandstones of the Nansen Formation. The upper boundary is marked by the change from calcareous sandstone to the argillaceous sediments of the Dunlin Group . Both boundaries are well marked on gamma ray and sonic logs particularly in the UK sector where the formation is best developed. The calcareous sandstone bed at the top of the formation is normally reflected by a distinctive sonic log response (Fig 1.17-18) . However, as this passes laterally into calcareous siltstones and shales the sonic log still responds to the calcareous nature of the sediments.DistributionThe formation is widespread in the northern North Sea. Deegan and Scull (1977) describes the formation as well developed in the Brent Field area. The lower part of the formation overlaps westwards to the Cormorant Field area where it is probably the only representative of the Statfjord Group . The calcareous sandstone bed at the top does not extend as far west as the lower part of the formation.AgeProbably Hettangian to Early Sinemurian.Depositional environmentSubaerial depositional conditions are suggested for parts of the fine-grained units by the presence of calcrete, coals and root-traces. In contrast, delicate lenticular and flaser lamination, wave-ripple laminae and abundant bioturbation suggest that the other parts of the fine-grained units were deposited in standing water (Røe and Steel, 1985). Based on the stacking pattern of facies associations and the presence of marine fossils, the upper part of the sequence is interpreted to have been deposited in a marine environment.
Although the Nansen Formation in the Gullfaks and Statfjord area has been described as marine (Deegan and Scull, 1977; Røe and Steel, 1985), Nystuen and Fält (1995) interpret this formation as being a massive, rapidly deposited and dewatered fluvial sandstone with evidence of glauconite only near its top at Statfjord. Evidence for marine incursions at the Snorre Field is equivocal, for they may be stratigraphically higher than the Nansen Formaion. Equally the sandstones interpreted as shallow marine in origin may be interpreted as the deposits of a fluvial environment, such as a laterally extensive braided stream or a sheetflood.Compiled from-
Deegan, C. E. and Scull, B. J. (compilers) 1977: A standard lithostratigraphic nomenclature for the Central and Northern North Sea. UK Institute of Geological Sciences, Report 77/25. The Norwegian Petroleum Directorate, NPD-Bulletin No. 1, 36 pp.
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Goldsmith, P. J., Hudson, G. and Van Veen, P. Triassic. 105 – 127 in: Evans, D., Graham, C., Armour, A. and Bathurst, P. (editors and coordinators) 2003: The Millennium Atlas: petroleum geology of the central and northern North Sea. The Geological Society of London, 389 pp.
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Lervik, K.-S. 2006: Triassic lithostratigraphy of the Northern North Sea Basin. Norwegian Journal of Geology, Vol. 86, pp. 93-116.
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