Ekofisk Formation (Ekofiskformasjonen)

For earlier definition, see NPD Bulletin no. 5.

updated to follow: Stratigraphic Guide to the Cromer Knoll, Shetland and Chalk Groups of the North Sea and Norwegian Sea. Felix M. Gradstein & Colin C. Waters (editors), Mike Charnock, Dirk Munsterman,  Michelle Hollerbach, Harald Brunstad, Øyvind Hammer & Luis Vergara (contributors). Newsletter on Stratigraphy, vol 49/1 pp71-280, 2016

Chalk Group


The term Ekofisk Formation was introduced by Deegan & Scull (1977). The definition in Norwegian waters is updated from Fritsen & Riis (2000), with an earlier definition by Isaksen and Tonstad (1989; see NPD Bulletin no. 5). It was formally redefined in the UK Sector by Knox & Holloway (1992) (see Table).


From the Ekofisk Field in Norwegian block 2/4 (Deegan & Scull, 1977, p.27).


Norwegian Sector: In the area around the Ekofisk field, where the thickest and most complete section is found, reworked chalk dominates. The main stratigraphic breaks appear at the base and top of the formation. The most complete Ekofisk Formation is found in the Ekofisk field area in the Norwegian Sector.
The lowermost part is generally formed by argillaceous, pelagic chalk belonging to nannofossil biozone NNTp1 and the lower part of NNTp2, up to NNTp2D/E (2/4-A-8). In the upper part of this “Ekofisk tight zone” some reworked layers may occur. Upwards the section is dominated by thick, heavily reworked Maastrichtian fossils. Dating is problematic due to the extensive reworking, but the zone seems to correspond mainly to the upper part of NNTp2 and the lower part of NNTp4. The unit consists mainly of massive, homogenous chalk and pebble floatstone interpreted as slides, slumps and debris flows, occasionally with thin pelagic layers in between.
The upper part of the Ekofisk Formation is also variably reworked, but less extensively than the lower part, and generally consists of slumped, deformed chalk and debris flows with interbeds of pelagic chalk. Reworking took place especially within zone NNTp4D, but also in the uppermost part of zone NNTp4 and during NNTp5A. The uppermost part, belonging to zone NNTp5B is predominantly pelagic and grades into the overlying marl of the Våle Formation, with no sharp transition. The source area for the reworked zones is believed to be mainly the Lindesnes Ridge to the southwest, but also with some contribution from the east and northeast. Further away from the Lindesnes Ridge, in Block 2/5, the reworking seems to be less penetrative, and the Ekofisk Formation is dominated by interbedded pelagic chalk and thin turbidites with occasional thicker debris flows (Well 2/5-1).

In the Valhall Field on the Lindesnes Ridge, the Maastrichtian chalk was reworked through shallow-water shoaling and winnowing across the central crest, feeding coarser-grained debris flows down the flank (Elle Member; Bergen & Sikora, 1999). This reworked unit seems to correlate at least in part to the lower reworked Maastrichtian deposits at the Ekofisk Field. A major flooding event at about 63 Ma BP terminated the reworking at Valhall, and younger Danian chalk is a highly condensed, stratigraphically discontinous, deep-water autochthonous deposit (Sikora et al., 1999). On the flanks of the Norwegian part of the Central Graben, as well as in the wells from the UK Sector, the lower part of the Ekofisk Formation is generally absent. Pelagic chalk with thin turbidites dominate, with occasional thicker reworked zones (i.e. 30/7a-2).

Danish Sector: The environment of the Ekofisk Formation in the southern part of the Danish Sector is dominated by pelagic deposition of a laminated, bioturbated mudstone (MFB-7). The lower parts are often argillaceous and chert bearing. Only minor reworking of mainly late Maastrichtian species occurs. The pelagic strata are occasionally interrupted by debris flows, in several cases marking the contact between adjacent biozones. In the central part of the Danish Sector a similar environment is found (Roar-2). However, slumps are more common compared to the southern part. In addition, the youngest part of the formation is absent. The chalk is less argillaceous and chert is rare. In general, the lowermost part (nannofossil zones NNTp1 and NNTp2B/C) of the Ekofisk Formation is absent from the study wells in the Danish Sector. This may reflect the erosion on the Tor-Ekofisk boundary continuing into the Danian. Indications of occurrence of the oldest nannofossil zone (NNTp1) are reported from Lulu-1.

Continuing northwards, slumps and turbidites dominate the lower part of the Ekofisk Formation in Baron-2, whereas the upper part is dominantly pelagic laminated, bioturbated mudstone. Like in Roar-2, the youngest part of the formation is absent. Approaching the Norwegian Sector, the thickest and stratigraphically most complete Ekofisk succession in the Danish Sector is found in Lulu-1. The coring of the Ekofisk Formation is, however, incomplete in this well such that the cores cover the upper and lower part but the central part (approximately 34 m) is uncored. Lulu-1 is the only of the Danish study wells where the oldest nannofossil biozone (NNTp1) is observed. The lowest part of the section is dominated by an argillaceous massive to laminated chalk mudstone. The argillaceous content shows a cyclic variation but generally the density of clay seems to increase upward. The section is dominantly pelagic and slumps are rare, mainly occurring in the deepest part. The section above is dominated by massive pelagic mudstone. Clay is less common than below but stylolites are numerous. Slumps are rare and mainly present toward the top of the lower part of the cored sections. The section uncored in Lulu-1 is known from offset wells to be dominated by reworked chalk. The base of the upper part of the cored sections is dominated by pelagic massive chalk mudstones and wackestones. To the top of nannofossil biozone NNTp4D, slumps and debris flows dominate and reworked chalk thus forms a significant part of this section. In the uppermost part, white pelagic chalk becomes dominant, represented by nannofossil biozone NNTp5B. This late Danian biozone is only found in Lulu-1 and in MFB-7 in the southernmost part of the Danish Sector.

UK Sector: The Ekofisk Formation typically consists of variably cemented white, pale grey, tan and beige chalky limestone, composed largely of the remains of coccoliths and calcareous microfossils (Knox & Holloway, 1992; Gatliff et al., 1994). Grey, argillaceous chalky limestone, calcareous mudstone and shale 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 Central Graben (Hatton, 1986; Kennedy, 1987). The formation is typically chert-free, although chert may be present in southernmost sections of the Southern North Sea (Lott & Knox, 1994).



The thickness of the Ekofisk Formation varies throughout central North Sea. In the southern end of the Danish Sector, the thickness is some 30 to 40 m whereas in the central area (Roar-2, Bo-1), the upper part or all of the Ekofisk Formation is absent and the thickness may be reduced significantly, down to a few meters. Further north, the thickness increases again (61 m, Baron-2) to more than 100 m in Mona-1 and Lulu-1. Cores from the Ekofisk Formation in Lulu-1 show significant influence from turbidites and this may also be the case in Mona-1, which displays a similar log pattern.
In UK waters the formation reaches over 100 m in the Central and Northern North Sea, with up to 120 m preserved in the basinal depocentres of the Central Graben and between 10 and 60 m over its intra-basinal high (Gatliff et al., 1994). In the Moray Firth the formation is between 50 and 80 m thick (Andrews et al., 1990). Typically, the formation is between 10 and 30 m in the Southern North Sea, but is c. 55 m in the north-east in well 39/2-1 (Lott & Knox, 1994).

Geographical distribution

The depositional center for the Ekofisk Formation extends along the axis of the Central Graben from the Mona area past the Ekofisk area, with thicknesses reaching about 170 m (139 m in 2/4-A-8). Major reworking is observed in this depositional centre. On the east and northeast flank of the Central Graben the Ekofisk Formation thins and is locally absent, as for example in well 2/2-2. On the western flank of the Central Graben, in the border area between the Norwegian and UK Sectors, the thickness generally varies between 50-100 m (e.g. 31/26a-10 and 1/9-1), reflecting an environment dominated primarily by pelagic deposition and debris flows. Due to inversion along the Lindesnes Ridge the Ekofisk Formation thins rapidly onto the ridge and is locally absent in the Hod and Valhall fields.

UK Sector: The Ekofisk Formation extends throughout the basinal areas of the Central Graben, Outer Moray Firth and South Viking Graben (Knox & Holloway, 1992) and the Southern North Sea around the quadrant boundary of 44/49 and in the north of quadrant 53 (Lott & Knox, 1994). The limit of the Ekofisk Formation appears to be erosional throughout the UK Sector.

Type well

Well name: 2/4-5 (Norwegian Sector)

WGS84 coordinates: Lat. 56º 34’ 27.24”N           Long. 03º 12’ 11.82”E
UTM coordinates: 6270151.28 N 512490.31 E
UTM zone: 31
Drilling operator name: Phillips Petroleum
Completion date: 01.08.1970
Status: P & A
Interval of type section & thickness in type well: 3037-3164 m (9964-10380 ft) below KB (Deegan & Scull, 1977, p.27, fig.31). The top of the Ekofisk Formation as defined below (for UK sections) would be taken at c. 3043 m. No cores.

Norwegian Reference Wells

1/3-1, 3354-3257 m MD
2/5-1, 3132-3041 m
(This well was added by Isaksen & Tonstad, 1989)

Lat. 56°5'21.00"N
Lat. 56°38'19.95"N

Long. 02°51'05.00"E. No cores.
Long. 03°21'07.94"E. Cored through
the upper 78 m.

UK Reference Wells

Central and Northern North Sea (Knox & Holloway, 1992)

14/30-1: 1932-2009 m (6371-6591 ft)
16/23-3: 2727-2776 m (8946-9107 ft)
21/10-1: 2859.5-29445 m (9382-9660 ft)
22/1-2A, 2982.5-2935 m MD
29/15-1: 3082.5-3148 m (10113-10328 ft)

Lat. 58º 07’ 52.6”N
Lat. 58º 12’ 32.6”N
Lat. 57º 43’ 50.2”N
Lat. 57°56'12.20"N
Lat. 56є 34' 10.4"N
Long. 00º 10’ 45.4”W
Long. 01º 33’ 14.0”E
Long. 00º 58’ 29.5”E
Long. 01°02'55.80"E. No cores.
Long. 01º 58’ 33.3”E

Southern North Sea (Lott & Knox, 1994)

49/9-1: 810-820 m (2658-2690 ft) Lat. 53º 44’ 32.2”N Long. 02º 44’ 23.1”E

Upper and lower boundaries

Upper Boundary

Norwegian and Danish Sector: The upper contact is characterized by an upward distinct change in lithology from the pure chalk of the Ekofisk Formation to massive layered shale. In some cases, a marl-dominated layer is found in between. In some areas, both in the Danish and the Norwegian Sectors, there is a gradual transition into the overlying marl, which may make it difficult to pick the exact top (Baron-2). The upper contact is observed on several of the wireline logs. It is most distinct on the Gamma Ray log, where the contact often is expressed as a very sharp break, which, however, is more gradual when marl is present at the contact, (compare Baron-2 and MFB-7). The sonic log shows a similar response. The density log is also showing a clear change as a consequence of the porosity contrast between the adjacent layers. The change on the neutron log is not as distinct as it is modified by the presence/absence of hydrocarbons/water in the pore spaces, but there is usually a distinct break in the neutron-density separation across the boundary (2/4-A-8). The contact is always recognizable biostratigraphically.

UK Sector: The top of the Ekofisk Formation is generally marked by a rapid downward transition from chalky marl of the Maureen Formation to limestone. In some sections, the basal Maureen marl is replaced by sandstone or reworked limestone (e.g. 14/19-9; 22/11-2).

Lower Boundary

Norwegian and Danish Sector: The lower contact of the Ekofisk Formation is defined by the distinct stratigraphic unconformity at the Cretaceous-Tertiary boundary over most of the study area. Based on lithology, the recognition of the contact may, especially in the central parts of the Danish Sector, be difficult due to low contrast in physical properties between the adjacent formations, as seen for example in the Roar-2 well. In the southern and northern part of the Danish area, the lower section of the Ekofisk Formation is influenced by chert and clay in contrast to the pure chalk of the underlying Tor Formation. Most typical for the contact is, however, the erosional hardground at the top of the Tor Formation. Recognition from wireline logs is less clear than for the upper contact. In the wells in the Danish Sector south of the Rinkøbing-Fyn High, there is no distinct break on the Gamma Ray. The erosional hardground on the top of the Tor formation is reflected by an increase on the density log in most wells. In the Baron-2 well, where the Tor Formation is absent, the contact between Ekofisk and Hod is marked by a similar break on the density log. In the Norwegian Sector, in the axial part of the Central Graben the contact is conformable, or with a minor hiatus, such as in the Ekofisk field. The lower contact is often marked by an increase of the Gamma Ray signal into a zone comprising argillaceous chalk in the lowermost Ekofisk Formation, informally known as the Ekofisk tight zone, for example in well 2/4-A-8. In some areas this zone is less well developed, and in wells in the Danish Sector south of the Rinkøbing-Fyn high there is no distinct break on the Gamma Ray log. On the flanks of the Central Graben and on the Lindesnes Ridge the contact is marked by an unconformity, and the lower part of the Ekofisk Formation is generally absent. Here the lower contact is typically marked by a basal hard ground (well 2/2-3). In some places, the Ekofisk Formation is very thin or absent, such as in the Hod and Valhall fields, where the Tor Formation is locally overlain by Paleocene shale. The contact is in most cases easily recognized from biostratigraphy, although reworking of Maastrichtian fossils into younger deposits occasionally occurs.

UK Sector: The base of the Ekofisk Formation is normally marked by a downward change from the basal marly limestone to the relatively clean, limestone of the Tor Formation (Knox & Holloway, 1992). 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. In the Southern North Sea, the lower boundary is defined by a downward change from chert-free to chert-rich chalky limestone (Lott & Knox, 1994).

Well log characteristics

Norwegian and Danish Sector: On the type of seismic sections used in this study Top Chalk is recognised as a clear seismic trough resulting from an increase in acoustic impedance (SEG reverse polarity standard). In areas where there is a gradual transition from the Ekofisk Formation to the overlying marls, the top may be difficult to pick exactly. Presence of hydrocarbons or excessive high formation porosity may result in deviations from the normally expected seismic appearance with the top of the chalk formation show dimming or even phase reversal of the seismic signal. Where thick enough to be resolved, the internal seismic character of the Ekofisk Formation tends to be chaotic, and internal units can only be interpreted locally.

UK Sector: On wireline logs the upper boundary of the Ekofisk Formation 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). Commonly, coarser clastic units present in the Maureen Formation 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. The lower boundary is taken at a marked decrease in gamma-ray values.



Norwegian and Danish Sector: Nannoplankton zones NNTp1-NNTp5B. Wells with cores from the Ekofisk Formation have been analysed to identify diagnostic nannofossil assemblages. In the present study, some 17 nannofossil zones have been identified. In the wells in the Danish Sector up to seven of these were found in individual wells and four were not observed at all. As core coverage is incomplete, these are minimum numbers. However, the stratigraphic breaks seem to occur at some distinct levels, namely the lowermost zones NNTp1 to NNTp2C/D, the intermediate zones NNTp4A/B/C and the upper zones NNTp4E/F and NNTp5A/B. These zones are either sparsely represented or absent. In the Norwegian Sector, a fairly complete stratigraphic section is found in the axial part of the Central Graben, where all 17 nannofossil zones have been identified. Up to approximately ten zones have been found in individual wells. The stratigraphic breaks that were noted in the Danish wells mentioned above can also be observed in the Norwegian wells, although somewhat shifted in position. In the depocentre in the Ekofisk area, deposition took place resulting from erosion further south, associated with some of the above mentioned stratigraphic breaks, but also here biostratigraphic breaks can be observed in individual wells. [SEE: Extensive details in Fritsen & Riis, 2000.]

UK Sector: The top of the formation is poorly characterized biostratigraphically, although there is a downhole decrease in the abundance and diversity on benthonic microfossils at this level. The dinoflagellate cyst Senoniasphaera inornata biomarker occurs within the formation (Knox & Holloway, 1992).


Danian (see Table), with the base of the Ekofisk unit at 66 Ma


The Ekofisk Formation has not been identified in the East Shetland Basin, although it is possibly represented in condensed form at the base of the marl unit here assigned to the Maureen Formation. The Ekofisk is not represented onshore in southern England (Table 1).

Depositional environment

The chalky limestones of the Ekofisk Formation mostly represent pelagic nannofossil oozes, deposited in an outer shelf to basinal setting, probably in water depths of over 200 m. Units of resedimented chalk, are present as calcareous debris flows, turbidites and autochthonous periodites, locally in the Central Graben (Skovbro, 1983, d'Heur, 1986, Hatton, 1986; Kennedy, 1987). Bioturbation is common, indicating that the bottom waters were mostly well oxygenated (Knox & Holloway, 1992).


No formal subdivision, but the Ekofisk Formation displays a characteristic stratigraphical variation in gamma-ray values in the UK Sector (e.g. well 14/30-1). The lower unit of high-gamma, argillaceous chalky limestone or calcareous mudstone may also locally include sandstones (e.g. 21/1-5), whereas the upper unit comprises low-gamma, often high-velocity, clean chalky limestones (Knox & Holloway, 1992). Two zones of the formation are readily correlatable within the Central Trough area (Hatton, 1986, Figs. 24, 29, 30 and 31).


Updated from Fritsen, A. & Riis, F. 2000. A revised chalk lithostratigraphic nomenclature; NPD Report, unpublished)

Isaksen, D. and Tonstad, K. (eds.) 1989: A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. NPD-Bulletin No. 5, 59 pp.

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