Ekofisk Formation (Ekofiskformasjonen)
Updated from Fritsen, A. & Riis, F. 2000. A revised chalk lithostratigraphic nomenclature; NPD Report, unpublished)
For earlier definition, see NPD Bulletin no. 5.
Chalk Group
Name
Named after the Ekofisk Field in Norwegian block 2/4 (Deegan & Scull 1977).
Well type section
Norwegian well 2/4-5 from 3164 to 3037 m,
coordinates N 56°34'29.77l", E 03°12'13.03". No cores.
Well reference sections
Norwegian well 1/3-1, 3354-3257 m MD,
coordinates N 56°5'21.00", E 02°51'05.00". No cores.
UK well 22/1-2A, 2982.5 to 2935 m MD, coordinates N 57°56'12.20", E 01°02'55.80".
No cores.
Norwegian well 2/5-1 from 3132 to 3041 m,
coordinates N 56°38'19.95", E 03°21'07.94". Cored through the upper 78 m.
(This well was added by Isaksen and Tonstad, 1989).
Thickness
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 meters 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 meters 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.
Lithology
The Ekofisk Formation comprises a highly varied range of deposits, with pelagic chalk dominating in the southern
and western part of the central North Sea, but 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 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.
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 a 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 and 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).
Characteristics of the lower boundary
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.
Characteristics of the upper boundary
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 sector, 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 recognisable biostratigraphically.
Log pattern or seismic characteristics
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.
Geographical distribution
The depositional centre 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.
Occurrences of formation tops in wells
Isochore map TOR-EKOFISK
Biostratigraphy and Stage/Age
Danian, with the base of the Ekofisk unit at 66 Ma. 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 and Riis, 2000.]
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,
Figs. 24, 29,30 and 31).
References
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.
