The BGS Lexicon of Named Rock Units — Result Details

Ekofisk Formation

Computer Code: EKOF Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Danian Age (GD) — Danian Age (GD)
Lithological Description: The Ekofisk Formation typically consists of variably cemented chalky limestone, composed largely of the remains of coccoliths and calcareous microfossils. Argillaceous chalky limestone units, indicated by higher gamma responses, are also present; these appear to consist of limestone with thin, discrete, clay layers. Fine to medium grained, moderately to well sorted sandstones are present locally; they are distinguished from the associated limestones by their higher gamma values and generally by lower velocity and resistivity. Units of reworked chalk occur locally within the Ekofisk Formation in southern parts of the Central Graben (Kennedy 1987).
Definition of Lower Boundary: The base of the Ekofisk Formation is normally marked by a downward change from the basal marly limestone to the relatively clean, low-gamma limestone of the Tor Formation. Thin marly limestone units are locally present in the underlying limestone (e.g. 16/23-3), and in some areas expansion of these marly units leads to uncertainty as to the correct pick. Thus the base of the Ekofisk Formation in well 21/1-5 may be stratigraphically slightly lower than in more typical sections.
Definition of Upper Boundary: The top of the Ekofisk Foration is generally marked by a rapid downward transition from chalky marl of the Maureen Formation to limestone. On wireline logs the boundary is taken at the top of a section of consistently low gamma limestone; a downhole increase in sonic velocity commonly occurs at the same level. Where the gamma and sonic markers do not coincide, the boundary is picked on the gamma log (e.g. 14/30-1). In some sections, the basal Maureen marl is replaced by sandstone or reworked limestone (e.g. 14/19-9; 22/11-2). Commonly, these coarser clastic units are distinguished from the underlying Ekofisk Formation by their more erratic gamma and sonic log character, reflecting greater lithological variability (e.g. 22/16-1). Where relatively pure reworked limestones rest directly on Ekofisk limestone (e.g. 22/11-2) the boundary may be revealed by a downhole change from scattered to consistent dips.
Thickness: Reaches over 100 m in the Central and Northern North Sea.
Geographical Limits: The Ekofisk Formation extends throughout the basinal areas of the Central Graben, Outer Moray Firth and South Viking Graben (Knox and Holloway 1992) and the Southern North Sea (Lott and Knox 1994). The limit of the Ekofisk Formation appears to be erosional throughout the UK sector.
Parent Unit: Chalk Group (CK)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Norwegian Sector North Sea well N2/4-5 (Deegan & Scull 1977, p.27, fig.31): 3037-3164 m (9964-10380 ft) below KB. The top of the Ekofisk Formation as defined below (for UK sections) would be taken at c. 3043 m. 
Reference Section  North Sea well 14/30-1: 1932-2009 m (6371-6591 ft). 
Reference Section  North Sea well 16/23-3: 2727-2776 m (8946-9107 ft). 
Reference Section  North Sea well 21/10-1: 2859.5-29445 m (9382-9660 ft). 
Reference Section  North Sea well 29/15-1: 3082.5-3148 m (10113-10328 ft). 
Reference(s):
Knox, R W O'B and Holloway, S. 1992. 1. Paleogene of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Deegan, C E and Scull, B J. 1977. A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Report of the Institute of Geological Sciences, 77/25; NPD Bulletin No.1. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable