A revised lithostratigraphic nomenclature

A REVISED CHALK LITHOSTRATIGRAPHIC NOMENCLATURE Astri Fritsen & Fridtjof Riis, Norwegian Petroleum Directorate

The petroleum activity in the Noth Sea over the years revealed a need to update and revise the stratigraphic framework for the chalk units. The presence of "time gaps" and redeposition in the chalk reservoirs especially challenged the the established formal lithostratigraphic nomenclature. Biostratigraphic analysis combined with chalk lithofacies, well log correlation and seismic studies were used to develop a revised complete lithostratigraphic nomenclature for the Upper and in the Central . The most important change is the subdivision of the original formal Hod Formation into three new formations; Narve, Thud and Magne. This subdivision is based on the major stratigraphic breaks that were observed in the Hod Formation. These breaks represent time gaps of variable lengths in the biostratigraphic data, and are often seen as hardgrounds in cores, and especially in wells located on structural highs. Also in the seismic data, the Hod Formation was seen to embrace two significant sequence boundaries that could be tracked over large distances. On basis of international stratigraphic rules (Hedberg 1976), such major sequence boundaries are not recommended within a formation. Consequently, the split of the Hod Formation into three new formations was performed. To avoid confusion with the existing informal lithological subdivision of the Hod into lower, middle and upper Hod, unique names for the new units were suggested, following the tradition with names from the Norse mythology. The boundary definition of the Tor Formation has been revised, and the Vidar Formation overlying the Ekofisk Formation has been better defined.

data. The resulting joint biostratigraphic INTRODUCTION and lithostratigraphic framework for the Extensive reservoir modelling and drilling chalks in the Central Graben is presented activity in the chalk areas of the in this publication, and a revision of the the last twenty years has resulted in many formal Hod Formation into three new different ways of subdividing the chalk formations; Narve, Thud and Magne is reservoirs. The oil companies developed proposed, see Figure 1. The detailed their own local stratigraphic frameworks or documentation of the basis for the nomenclatures in order to fit the well data stratigraphy, including biotrat analyses and from their license areas. This practice composite well log panels, can be found in complicated communication between oil the report (Fritsen et al 1999), available on companies and the regional understanding compact disc from the Norwegian of the chalk facies and created a need to Petroleum Directorate. update and revise the formal stratigraphic framework for the chalk units. The new stratigraphic nomenclature will improve the communication between the In the Joint Chalk Research (JCR) phase oil companies in both Norwegian and V, 1997 – 2000, the project “A Joint Chalk Danish areas, and ensure a standard basis Stratigraphic Framework” was initiated in for regional mapping and correlations order to establish a common stratigraphic between the various chalk fields and nomenclature for the Upper Cretaceous prospects. Since the presence of "time and Paleocene chalks in the Central gaps" and redeposition in the chalk Graben and Norwegian-Danish Basin. The reservoirs is especially acknowledged in work was performed by a joint work the proposed new lithostratigraphic including geologists from the companies nomenclature, improved understanding of and agencies participating in JCR Phase V. deposition, and redeposition of Biostratigraphic analyses comprised the chalk, as well as better control of seismic majority of the project work, and the interpretation on and between fields will results were interpreted together with well result. logs, chalk depositional facies and seismic

- 1 - A revised Chalk lithostratigraphic nomenclature

TIME Danish JCR JCR North Sea UK SCALE informal NANNO. proposed formal formal Gradstein AGE Chalk units ZONES FORMATIONS FORMATIONS FORMATIONS et al ., Isachsen and Lieberkind et.al., this study this study Lott and Knox, 1994 1995 Tonstad, 1989 1982




80.00 JUKES UC14 HOD

83.50 UC13 THUD UC12 85.80 UC11 UC10 LAMPLUGH 89.00 CHALK-2 UNIT 90.00 UC9 NARVE



98.90 UC1

Figure 1: A revised chalk lithostratigraphic nomenclature

quantitative or semi-quantitative METHOD nannofossil- and micropaleontological A number of 35 wells from Norwegian, analyses were used in the study. Regional Danish and UK areas were chosen for the seismic was interpreted to resolve study. Biostratigraphic data, well stratigraphic surfaces between fields and interpretations and core descriptions from key wells, and chronostratigraphic these wells were made available from the diagrams were made to identify companies. Cores and wireline logs were stratigraphic time gaps and sequence studied by the work group, and intervals boundaries in the study wells. representing possible stratigraphic sequence boundaries, time gaps or Selection of the study wells was based on redeposition were defined. In total, 1250 various criteria, including the well´s meters of core were described based on the position regarding basin and structural previous developed JCR description highs, and available core and system. With more than 700 new core biostratigraphic data. Existing stratigraphic samples collected during core descriptions, type wells, such as 1/3-1 was also a total of more than 4000 biostratigraphic included. Wells that had core coverage

- 2 - A revised Chalk lithostratigraphic nomenclature across a formation boundary were Figure 2 shows the location of the study considered especially important, as were wells and the chalk fields in the North Sea wells that appeared to have been drilled Central Graben. across major stratigraphic time breaks.

2 3 4 5

7/11 57 00 57 00 2/2-2

1/3-1 1/3-8 2/2-3 56 45 JUDY 56 45 2/5-7 30/7A-2 2/5-1 TOR

2/4-B-19T2 2/5-9 2/4-A-8 EKOFISK 56 30 56 30 TOMME- 2/7-4 2/7-30 LITEN EDDA 1/9-1 2/7-B-11 ELDFISK 2/7-8 LULU-1 VALHALL 2/8-A-1 MONA-1 56 15 56 15 2/7-2 HOD T-1 2/11-A-2 SVEND

31/26A-10 BARON-2 SYD-ARNE 56 00 56 00


DAN 55 30 55 30 MFB-7 M-9X

2 3 4 5 0 20 40 Km RDN9909002/6 Figure 2: Location of the study wells and chalk fields

tectonics and inversion tectonics added to STRUCTURAL AND TECTONIC the structural complexity. Inversion highs, FRAMEWORK AND SEISMIC formed by uplift and reverse movement INTERPRETATION along pre-existing faults, were active at different occasions during and after chalk The Central Graben was tectonically active deposition, and the detailed timing of their during deposition of the chalk sequence. activity may differ. They often represent a Graben subsidence, inversion structures change in tectonic setting from a pre- and salt tectonics created a complex and Campanian trough to a high. changing pattern of basins and highs in the Late Cretaceous and Danian. Figure 3 shows the Central Graben and its main structural elements based on During chalk deposition, the Central Anderson (1995), Britze et al. (1995) and Graben subsided along its major boundary data from the Chalk Exploration Project faults towards the stable platforms. There (CEP), which was made available for JCR were several episodes when the relatively by Phillips Petroleum Company. uplifted platforms were eroded, and acted as source areas for redeposited chalk. Salt

- 3 - A revised Chalk lithostratigraphic nomenclature


2 3 4 5

7/11 Late Cretaceous 57 00 57 00 boundary faults 2/2-2 Late Cretaceous 1/3-1 active inversion faults 1/3-8 Inverted highs 2/2-3 56 45 56 45 Salt induced 2/5-7 structures 30/7A-2 2/5-1 Stable platform thin chalks 2/4-B-19T2 2/5-9 Basin areas 2/4-A-8 56 30 56 30 2/7-4 M Deep basin 2/7-30 A N depocentre 2/7-B-11 D 1/9-1 A 2/7-8 L H Main pre-Cretaceous IG LULU-1 H fault structures L M I 2/8-A-1 O MONA-1 N N 56 15 2/7-2 D A 56 15 E R S IDGE N ES R T-1 2/11-A-2 ID GE

31/26A-10 BARON-2A R N 56 00 56 00 E R I ID NG G E H E IG H B O - J E N BO-1 ADDA-2 S

R ROAR-2 55 45 I 55 45 D G E - 4X


55 30 55 30 MFB-7 M-9X

Figure 2-1 2 3 4 5 0 20 40 Km RDN9909002/6 Figure 3: Central Graben main structural elements

support correlation based on logs and A major depocenter for the Cretaceous . Seven sequence chalk located in the northwestern part of boundaries were interpreted to determine the Central Graben is penetrated by wells the important seismo-stratigraphic events 1/3-1 and 1/3-8. These wells have a fairly in the area, these were Blodøks, Intra complete chalk section, and they are Middle Hod, Top Middle Hod, equivalent important reference wells. Additional to an important sequence boundary in the depocenters with a chalk thickness Campanian, Top Hod, a sequence exceeding 900 m were located in the boundary at or near to the of the deeper parts of the complex graben Maastrichtian, Top Tor at the top of the structure. There are few wells drilled in Cretaceous, Top Ekofisk, at the top these depocenters, and those drilled were Danian, and Balder, an important near Top generally aimed at targets other than the Palaeocene event. chalk. Consequently, core and biostratigraphy data are sparse. In terms of the proposed , Intra Middle Hod can be correlated with As part of the study, a set of regional 2D top Narve in the northern part of the seismic lines were tied to the wells and region. The reflectors Top Middle Hod and interpreted. On the Norwegian side, about Top Hod correlate with top Thud and Top 500 km of good quality 2D seismic data Magne respectively. was interpreted. On the Danish side, two long composite lines were provided from Top Narve (Intra middle Hod) existing 2D and 3D surveys, in order to tie This event was picked on a weak peak only all the studied wells together. Main seismic in the basinal areas in the north and west of sequences were identified and reflectors the area. It represents a sequence boundary tied between the studied wells in order to lying between the Blodøks and Top Middle

- 4 - A revised Chalk lithostratigraphic nomenclature

Hod reflectors (figure 4). There is marked Towards the east, and towards structural onlap onto this reflector at the basin highs, the unit pinches out. margins and on the flanks of salt domes.

Seismic profile SH9503-402 tied to well 1/3-8


Balder 100 ms

Vidar Ekofisk


Intra Tor



s øk od Narve Bl 5Km

Synthetic seismogram and gamma log are shown in the well

Figure 4: Seismic profile tied to reference well 1/3-1

Top Thud (Top middle Hod) Top Magne (Top Hod) This event was generally picked on the This event was generally picked as a lowest part of a poor quality peak doublet diffuse peak at the base of the reflector (figure 4). It did not have a consistent separating a lower amplitude zone above appearance throughout the area but from a zone below comprising stronger sometimes separated a zone with stronger parallel reflectors (figure 4). In the parallel reflectors above from a quieter structurally higher areas the reflector zone beneath. On the platform margins and separates an overlying sequence of higher adjoining salt highs and inversion highs, amplitude parallel reflectors from a lower marked onlap was visible onto the amplitude zone below. This would imply reflector. In more basinal areas, both onlap that the Lower Tor sequence thickens onto and subcrop below the reflector could considerably into the basinal areas be recognised, probably indicating more particularly to the north and west around localised tectonic activity, perhaps 1/3-1. The overlying Tor formation could associated with underlying salt be seen to onlap the reflector in places at movements. The overall surface of the the platform edges and on the flanks of salt Middle Hod was broadly undulating and induced highs (figure 5). However in not always parallel to events below or general the Tor sequence showed a greater above, even in flatter areas. areal extent than the Magne, extending up

- 5 - A revised Chalk lithostratigraphic nomenclature onto the platform and highs albeit as a much reduced sequence.

Seismic profile SGT8606-203 tied to well 2/5-9






0 1 Ekofisk




Blodøks 5 km

Figure 5: Seismic profile tied to reference well 2/5-9

Well data indicate that the Magne The present study represents the most Formation is developed as a hardground at thorough stratigraphic study ever some of the highs. The representative wells performed on the North Sea chalks. In the are all located within gas clouds, and the following section, the proposed new hardground has not been tied to seismic lithostratigraphic formations are discussed lines. on basis of biostratigraphy, seismic data and depositional facies.

NORTH SEA CENTRAL GRABEN The formation boundary picks in the CHALK LITHOSTRATIGRAPHIC individual study wells are documented in NOMENCLATURE the report The first formal lithostratigraphic nomenclature for the central and northern Figure 6 shows the JCR proposed North Sea was published by Deegan & formations in a sequence stratigraphic Scull in 1977. The Cretaceous and Tertiary framework in the type/reference well 1/3- for Norwegian areas were later revised by 1. Isaksen and Tonstad (1989). Lieberkind et al. published an informal nomenclature for the chalks in the Danish Central Graben in 1982. For UK North Sea, Knox and Cordey published a revised lithostratigraphy in 1993.

- 6 - A revised Chalk lithostratigraphic nomenclature

Sequence stratigraphic overview, well 1/3-1 between the Narve, Thud and Magne formations

Reworked chalk The criteria for defining their boundaries Sequence boundary, transgression on seismic, biostratigraphic and well log Formation boundary, time transgressive evidence are described below and Sequence boundary, small hiatus summarised in table 1.

Basin infill, increasing amount of reworked Boundary definition Formation/ Seismic Biostrat Logs boundary Formation boundary Tor Sequence condensed on highs, hardgrounds locally Intra UC Uphole Sequence boundary, major onlap Tor / Magne Poorly 16 gamma boundary imaged Sequence developed increase only in basinal setting Magne

Sequence boundary, onlap, hiatus Magne / Thud Onlap intraUC Sequence boundary, onlap boundary surface 14 Thud

Thud / Narve Onlap UC 11 boundary surface Figure 6: Well reference section 1/3-1 Narve Narve / UC 3 Downhole Base chalk to Campanian chalk Blodøks gamma reflector stratigraphic framework boundary increase The original Hod Formation (cf. Deegan Blodøks and Scull, 1977) was informally split into a Table 1: Summary of formation boundary Lower, Middle and Upper unit (Isaksen definitions according to the various and Tonstad, 1989). In this study, an criteria alternative subdivision of the Turonian-

Campanian section is presented, proposing Narve formation three new formation names, Narve, Thud and Magne, stratigraphically placed Name: Narve formation (proposed). After between the Blodøks Formation and the the Norse god Narve, who was the son of Tor Formation. The geometrical Loke and Sigyn (Sturluson, 1954). relationships between the units are shown in figure 7. Well Type Section: 1/3-8, 4520-4337 m

MD. TOR Well Reference section: 2/8-A-1, 2581,5 - 2501,5 m MD.

Lower Boundary Characteristics: Picked

KS DØ primarily on biostratigraphic and log LO B criteria. The biostratigraphic criterion is penetration of microzone FCS13 or nannozone UC3. The log criterion is a downhole gamma increase indicating Figure 7: Schematic presentation of the penetration of the shales of the Blodøks sequence stratigraphic relationships Formation.

- 7 - A revised Chalk lithostratigraphic nomenclature

Upper Boundary Characteristics: Picked Log pattern: The Narve formation has primarily on seismic and biostratigraphic been penetrated in quite many wells in the criteria. The seismic criterion is an onlap Valhall area (Lindesnes Ridge) and in the surface. The biostratigraphic criterion is Ål basin to the west of the ridge. The penetration of nannozone UC11. gamma log patterns of these wells correlate well in this area, and a detailed correlation Thickness and Distribution throughout of different subzones is possible. A zone of Study Area: The thickness of this clean chalk with consistently low gamma formation ranges from zero to a few values, overlain by a gamma spike, is hundred metres in the study wells. It is situated in the middle of the Narve absent in the 2/2 wells in the Norwegian formation. This zone (informally named Sector and in the Lulu-1 well in the Danish Hod4 by BPAmoco) constitutes an Sector. It is thin (typically less than 100m important reservoir in the Valhall Field, thick) in wells on structural highs such as and it can easily be recognised on the logs. Valhall-Hod and Tor in the Norwegian However, its biostratigraphic age is not Sector, and thickest in wells in basinal consistent between the wells, and it is lows such as that in the Roar area in the possible that the “Hod4” subzone is Danish Sector, and in the northern and diachronous. western depocenters in the Norwegian sector. Age: Latest -earliest Santonian (for practical purposes, Lithology, Main Depositional Facies and Turonian-Coniacian). Environments: Lithofacies associated with the crestal biofacies on Valhall are Remarks: Eroded on crests of structures. typically (textural) mudstones and wackestones. Those associated with the Thud formation “shallow water” pelagic biofacies are varied, though typically bioturbated to Name: Thud formation (proposed). After laminated chalks or interlaminated chalks an alternative name for the Norse god Odin and clays indicative of slow . (Sturluson, 1954). Thud means “thin one”. Those associated with the “deep water” pelagic biofacies are typically massive Well Type Section: 1/3-8, 4337-4125 m chalks. Recognition of allochthonous as MD. against autochthonous chalks is rendered difficult by the poor bathymetric resolution Lower Boundary Characteristics: Picked afforded by the rare benthonic primarily on seismic and biostratigraphic . Lithofacies in core from the criteria. The seismic criterion is an onlap 2/8-A-1 well include bioturbated chalks surface. The biostratigraphic criterion is with Chondrites, Planolites and penetration of nannozone UC11. Zoophycos, and also allochthonous debris flows and slumps with micritic matrices Upper Boundary Characteristics: Picked and polymict/non-chalk clasts, indicating primarily on a seismic criterion (onlap affinity with the “shallow water” and surface), corresponding to the original “deep water” pelagic biofacies of the informal middle Hod sequence boundary. Valhall structure respectively. Allochthonous chalks are also observed in Thickness and Distribution throughout core from Mona-1. Study Area: The thickness of this formation ranges from zero to a few Biostratigraphic Characterisation: hundred metres in the study wells. It is Microzones FCS14-FCS18pp; nannozones absent in the 2/2 wells and in wells on UC4-UC11. structural highs, such as Valhall-Hod and Eldfisk in the Norwegian Sector and in the

- 8 - A revised Chalk lithostratigraphic nomenclature

Lulu-1 well in the Danish Sector. It is Well Type Section: 1/3-8, 4125-3952 m thickest in wells in basinal lows, such as MD. those in the 1/3 area in the Norwegian Sector and the Roar area in the Danish Well Reference section: 2/11-A-2 T2, Sector. 3486 - 3427,4 m MD.

Lithology, Main Depositional Facies and Lower Boundary Characteristics: Picked Environments: Poorly constrained owing primarily on a seismic criterion (onlap to the lack of core coverage. Biofacies in surface). cuttings samples from some wells are characterised by moderately abundant and Upper Boundary Characteristics: Picked diverse planktonic foraminifera and on seismic, biostratigraphic and log radiolaria and generally rarer calc- criteria. The seismic criterion is a reflector agglutinated and benthonic (which may also locally be an onlap foraminifera, indicating an affinity with the surface) separating a lower amplitude open marine platform or “shallow water” interval below from a higher amplitude pelagic biofacies of the Valhall structure. interval above, at least in structurally higher areas. The biostratigraphic criterion Biostratigraphic Characterisation: is penetration of microzone FCS21. The Microzones FCS18pp-FCS20pp; log criterion is a downhole gamma nannozones UC12-UC14 pp. increase (the Magne is more clay-rich than the overlying Tor). Log pattern: A complete Thud formation has been penetrated in few wells, because Thickness and Distribution throughout of its basinal setting. In the northern part of Study Area: The thickness of this the region, the gamma values in 1/3-1 and formation ranges from zero to a few 1/3-8 indicate a clay content that is higher hundred metres in the study wells. It is than in the Magne Formation. The absent in the 31/26A-10 well in the United relatively argillaceous chalks seem to Kingdom Sector, thin (typically less than alternate with clean chalks, giving rise to a 100 m thick) on structural highs such as ”box-shaped” pattern of the gamma log. Valhall-Hod, Eldfisk and Tor in the The time equivalent to the Thud Formation Norwegian Sector, and thickest in wells in in the Roar-2 well in the Danish sector has basinal lows such as those in the 1/3 and, developed a different log and seismic locally, 2/5 areas in the Norwegian Sector. pattern. Lithology, Main Depositional Facies and Age: Late Early Santonian to earliest Environments: Lithofacies associated with Campanian (for practical purposes, the crestal biofacies on the Valhall Santonian). structure are typically (textural) mudstones and wackestones. Those associated with Remarks: Laps on to Narve formation on the open marine platform, high flanks of structures. productivity upper slope and basinal biofacies are typically bioturbated Magne formation argillaceous wackestones and chalks or interbedded chalks and claystones Name: Magne formation (proposed). After (sometimes referred to as periodites), the Norse god Magne, who was the son of indicative of slow sedimentation or, in the Tor, and who supported him after his great case of the basinal biofacies, incipient fight with the giant Hrungnir (Sturluson, hardgrounds, indicative of extremely slow 1954). sedimentation ( starvation). Those associated with the eutrophic sub-biofacies

- 9 - A revised Chalk lithostratigraphic nomenclature are bioturbated, those associated with the remains even after careful integration of all dysoxic sub-biofacies are laminated (i.e., data. UC17 and 18 are Early Maastrichtian. non-bioturbated) pyritic chalks. UC19 and 20 are Late Maastrichtian.

Biostratigraphic Characterisation: Lower Boundary Characteristics: In Microzones intraFCS20pp-intraFCS22; general the lower Tor boundary is difficult nannozones intraUC14pp-intraUC16pp. to pick on logs. A decrease in the GR level, small or large, is common, though Age: Late Early to early Late Campanian quite often no change of the GR level is (for practical purposes, Campanian). observed. In the stratigraphically more complete wells there seems to be a Remarks: Onlaps Thud and oversteps on regional element to the log characteristics. to Narve (?) on flanks of structures. In the northern basin wells the sonic log Generally absent or thin on crests shows an increase in velocity upon (characterised by hardground entering the Tor Formation from Magne development). below. This is associated with either no change or a slight increase in the density Tor Formation reading, i.e lower porosity. From the Lindesnes Ridge and southeastwards the Name: Named by Deegan & Scull (1977) sonic log shows a decrease in velocity from the Tor Field in Norwegian blocks crossing from Magne to Tor Formation. 2/4 and 2/5. Tor was a son of Odin and one This is associated with either no change or of the principal Gods of Norse mythology. a slight decrease in the density reading, i.e higher porosity. Where nannozones Well Type Section: Norwegian well 1/3-1 UC16-18 are thin or missing a change to from 3828 to 3354 m is the type well for lower porosity associated with an increase the Tor Formation (Tonstad and Isaksen in velocity is observed at the boundary. 1989). These are observations of a general nature and exceptions can be found. Well Reference Sections: The Mona-1 well is chosen as a one of two reference The resistivity logs to some degree mirror sections for the Tor Formation. The the sonic and density logs in water bearing stratigraphic sequence in Mona-1 is chalk. Where the velocity and density go relatively complete with individual zones up an increased resistivity is often seen, being represented by a significant reflecting a more dense rock, and vice thickness. Except for the very top the Tor versa. Exceptions are wells where the Formation is cored throughout this well. bottom part of the Tor Formation is porous Discovery well for the Tor Field, well 2/5- and oil-bearing. Here a decrease in 1, is chosen as a second reference section velocity and density is accompanied by an for the Tor Formation. Only the upper half increase in resistivity. of the Tor Formation is cored but the 2/5-1 well has all Tor zones well developed. The lower boundary either coincides with the boundary between biozones UC16 and Age: The age of the Tor Formation has in 18 (UC17 is only scarcely present) or is this study been decided to encompass all of placed midway in UC16.. the Maastrichtian, i.e. nannofossil zones UC17 through 20 and intra Upper Boundary Characteristics: zone FCS22 through FCS23. At the In general the Top Tor is of the same age detailed level the top of zone UC16, which over the study area. The duration of the is of late Campanian age, cannot in all Tor/Ekofisk hiatus mostly depends on how wells be brought to coincide with well much is missing from the Ekofisk defined log breaks. This is a problem that Formation. Locally all of the Ekofisk is

- 10 - A revised Chalk lithostratigraphic nomenclature missing due to later inversion as seen on reflectors in the upper half, while the lower the crestal parts of the Hod and Valhall half most often show less distinct Fields. Here the Tor Formation, or reflectors. This difference is most reworked Tor Formation, is overlain by pronounced in the northern part of the Paleocene shales (2/8-A-1). study area, where the formation is also the thickest. It is probable, that the lower half The GR generally shows an increase. The is simply better developed to the north and Top Tor pick is usually put right at the that these deeper layers have a very thin start of the increase. development southeastwards in the basin resulting in more pronounced reflectors. In accordance with a shift to lower Neither the Base nor the Top Tor boundary porosities in the bottom part of the Ekofisk are seismically distinct reflectors and Formation the velocity shows a clear would be difficult to pick without well-tie. increase at Top Tor. However, the Onto the Lindesnes Ridge a major part of uppermost part of the Tor Formation can the Tor package disappears. The 2/7 and be well cemented and as a result appear 2/8 wells show that the missing part is exactly like the Ekofisk on density and from top down.. velocity logs. In the absence of good biostratigraphic data the usual slight Lithology and Depositional Facies: increase in GR should enable an accurate 15 of the study wells are represented by log pick, though. core in the Tor Formation.

A relative decrease in resistivity across the The Tor Formation is in general very clean Tor/Ekofisk boundary is seen. This picture with a low content of insolubles (<5%). is consistent even though the resistivity in The clean nature of the chalk is reflected in the top part of the Tor Formation is the light colour and the evidence of affected by pore fluids (water, oil or gas). pressure solution in the form of dental stylolites. Dental stylolites are typical for Thickness and Distribution: clean chalk, while the pressure solution in Tor thicknesses in the study wells vary the more dirty chalks appears as horsetails from 0 m in Baron-2 to 472m in 1/3-8. The solution seams. thickness variations in the study wells are however not representative of the North The Tor formation is composed of mixed Sea Basin Chalk/ Tor Formation pelagic and allochthonous chalks. There is distribution as most wells are drilled on a gradual upward increase in the amount of structural highs. Regional thickness maps allochthonous material. based on seismic interpretation show a general thickening of the Upper The distribution of sedimentary facies is Cretaceous package from southeast to driven by paleotopography, so that what is northwest in the Central Graben system seen today is the combined result of (Britze et al, 1995, Ziegler, 1990, Japsen, primary deposition and secondary 1998). This general observation has an processes in the form of reworking and overprint of local east or west depocentres . Therefore, to understand the going from north to southeast along the depositional facies, the location of the Graben axis (This study). This pattern is individual wells with respect to regional confirmed by the study wells also for the structural features (basinal axis, margins Tor Formation alone. and inversion ridges) and more local halokinetic structures, is critical. Seismic character: The Tor Formation seismic package is Biostratigraphic Characterisation: characterised by well defined continous

- 11 - A revised Chalk lithostratigraphic nomenclature

Biostratigraphically, the Tor Formation encompasses the upper part of nannoplankton Zone UC16 through to Zone UC20 and the two foraminiferid Zones FCS22 and FCS23, both of which have been further divided into subzones as part of the present study.

There is considerable biostratigraphic evidence for intra-formational reworking within the Tor throughout the study region. Several wells contain sections where Late Campanian and Early Maastrichtian Tor sediments have been re-deposited during Figure 8: Schematic diagram of deposition the Late Maastrichtian. There is also and erosion of the Ekofisk Formation evidence of reworking within the Late Maastrichtian. Despite the frequent Name: From the Ekofisk field, Norwegian presence of stacked allochthonous chalk Block 2/4. units within the Tor, the ages of individual units can be ascertained on the basis of the Well Type Section: 2/4-5, 3164-3037 m characteristic nannofloral and microfaunal MD (Deegan and Scull, 1977). associations. Well Reference Sections: 1/3-1, 3354- Many of the well sections examined 3257 m MD. include very thin and condensed Late UK 22/1-2A, 2982.5-2935 m MD. Campanian to Early Maastrichtian 2/5-1, 3132-3041 m MD. (This well was intervals, suggesting active structural added by Isaksen and Tonstad, 1989). growth during this period. The maximum phase of allochthonous chalk deposition is Age: Danian. Nannoplankton zones during the Late Maastrichtian resulting in NNTp1-NNTp5B. considerable variation in formation thickness across the region. Lower contact characteristics: The lower contact of the Ekofisk Formation is defined Subdivisions: by the distinct stratigraphic It has been decided not to propose any at the Cretaceous-Tertiary boundary over formal subdivision of the Tor Formation. most of the study area. Based on lithology, the recognition of the contact may, Ekofisk Formation especially in the central parts of the Danish A schematic diagram illustrating the sector, be difficult due to low contrast in stratigraphic framework of the Ekofisk physical properties between the adjacent Formation is shown in figure 8. Note that formations, as seen for example in the the diagram is based on well data only, the Roar-2 well. In the southern and northern internal seismic resolution of the Ekofisk part of the Danish area, the lower section Formation does not permit regional of the Ekofisk Formation is influenced by subdivision of the unit, and the exact and clay in contrast to the pure chalk erosional/depositional relationship is of the underlying Tor Formation. Most uncertain. Also, regional variations exist, typical for the contact is, however, the and the diagram is not meant to illustrate erosional hardground at the top of the Tor the exact stratigraphy in a certain area, but Formation (see Chapter 5.4 and well MFB- just to give an approximate picture of the 7. regional variations from basinal settings to structural highs.

- 12 - A revised Chalk lithostratigraphic nomenclature

Recognition from wireline logs is less clear Norwegian sector, there is a gradual than for the upper contact. In the wells in transition into the overlying marl, which the Danish sector south of the Rinkøbing- may make it difficult to pick the exact top Fyn High, there is no distinct break on the (Baron-2). Gamma Ray. The erosional hardground on the top of the Tor formation is reflected by The upper contact is observed on several an increase on the density log in most of the wireline logs. It is most distinct on wells. In the Baron-2 well, where the Tor the Gamma Ray log, where the contact formation is absent, the contact between often is expressed as a very sharp break, Ekofisk and Hod is marked by a similar which, however, is more gradual when break on the density log. marl is present at the contact. (Compare Baron-2 and MFB-7) The sonic log shows In the Norwegian sector, in the axial part a similar response. of the Central Graben the contact is conformable, or with a minor hiatus, such The density log is also showing a clear as in the Ekofisk field. The lower contact change as a consequence of the porosity is often marked by an increase of the contrast between the adjacent layers. The Gamma Ray signal into a zone comprising change on the neutron log is not as distinct argillaceous chalk in the lowermost as it is modified by the presence/absence Ekofisk Formation, informally known as of hydrocarbons/water in the pore spaces, the Ekofisk tight zone, for example in well but there is usually a distinct break in the 2/4-A-8. In some areas this zone is less neutron-density separation across the well developed, and in wells in the Danish boundary (2/4-A-8). sector south of the Rinkøbing-Fyn high there is no distinct break on the Gamma The contact is always recognisable Ray log. biostratigraphically.

On the flanks of the Central Graben and on Formation thickness and distribution the Lindesnes Ridge the contact is marked across study area: The thickness of the by an unconformity, and the lower part of Ekofisk Formation varies throughout the the Ekofisk Formation is generally absent. study area. In the southern end of the Here the lower contact is typically marked Danish sector, the thickness is some 30 to by a basal hard ground (well 2/2-3). In 40 meters whereas in the central area some places, the Ekofisk Formation is very (Roar-2, Bo-1), the upper part or all of the thin or absent, such as in the Hod and Ekofisk Formation is absent and the Valhall fields, where the Tor Formation is thickness may be reduced significantly, locally overlain by Paleocene shale. down to a few meters.

The contact is in most cases easily Further north, the thickness increases again recognized from biostratigraphy, although (61 m, Baron-2) to more than 100 meters reworking of Maastrichtian into in Mona-1 and Lulu-1. Cores from the younger deposits occasionally occur. Ekofisk formation in Lulu-1 show significant influence from and this may also be the case in Mona-1, which Upper contact characteristics: The upper displays a similar log pattern. contact is characterized by an upward distinct change in lithology from the pure The depositional centre for the Ekofisk chalk of the Ekofisk Formation to massive Formation extends along the axis of the layered shale. In some cases, a marl- Central Graben from the Mona area past dominated layer is found in between. In the Ekofisk area, with thicknesses reaching some areas, both in the Danish and the about 170 m (139 m in 2/4-A-8). Major

- 13 - A revised Chalk lithostratigraphic nomenclature reworking is observed in this depositional Like in Roar-2, the youngest part of the centre. formation is absent.

On the east and northeast flank of the Approaching the Norwegian sector, the Central Graben the Ekofisk Formation thickest and stratigraphically most thins and is locally absent, as for example complete Ekofisk succession in the Danish in well 2/2-2. On the western flank of the sector is found in Lulu-1. The coring of the Central Graben, in the border area between Ekofisk Formation is, however, incomplete the Norwegian and UK sectors, the in this well such that the cores cover the thickness generally varies between 50 -100 upper and lower part but the central part m (e.g. 31/26a-10 and 1/9-1), reflecting an (approximately 34 m) is uncored. Lulu-1 is environment dominated primarily by the only of the Danish study wells where pelagic deposition and debris flows. the oldest nannofossil biozone (NNTp1) is observed. The lowest part of the section is Due to inversion along the Lindesnes dominated by an argillaceous massive to Ridge the Ekofisk Formation thins rapidly laminated chalk mudstone. The onto the ridge and is locally absent in the argillaceous content shows a cyclic Hod and Valhall fields. variation but generally the density of clay seams increase upward. The section is Lithology, main depositional facies and dominantly pelagic and slumps are rare, environment: In general, the lowermost mainly occurring in the deepest part. The part (Nannofossil zones NNTp1 and section above is dominated by massive NNTp2B/C) of the Ekofisk Formation is pelagic mudstone. Clay is less common absent from the study wells in the Danish than below but stylolites are numerous. sector. This may reflect the erosion on the Slumps are rare and mainly present toward Tor-Ekofisk boundary continuing into the the top of the lower part of the cored Danian. Indications of occurrence of the sections. The section uncored in Lulu-1 is oldest zone (NNTp1) are reported from known from offset wells to be dominated Lulu-1. The environment of the Ekofisk by reworked chalk. The base of the upper Formation in the southern part of the part of the cored sections is dominated by Danish sector is dominated by pelagic pelagic massive chalk mudstones and deposition of a laminated, bioturbated wackestones. To the top of nannofossil mudstone (MFB-7). The lower parts are biozone NNTp4D, slumps and debris flows often argillaceous and chert bearing. Only dominate and reworked chalk thus forms a minor reworking of mainly Late significant part of this section. In the Maastrichtian species occurs. The pelagic uppermost part, pelagic chalk strata are occasionally interrupted by becomes dominant, represented by debris flows, in several cases marking the nannofossil biozone NNTp5B. This late contact between adjacent biozones. Danian biozone is only found in Lulu-1 and in MFB-7 in the southernmost part of In the central part of the Danish sector a the Danish sector. similar environment is found (Roar-2). However, slumps are more common The most complete Ekofisk Formation is compared to the southern part. In addition, found in the Ekofisk field area in the the youngest part of the formation is Norwegian sector. The lowermost part is absent. The chalk is less argillaceous and generally formed by argillaceous, pelagic chert is rare. Continuing northwards, chalk belonging to nannofossil biozone slumps and turbidites dominate the lower NNTp1 and the lower part of NNTp2, up part of the Ekofisk Formation in Baron-2, to NNTp2D/E (2/4-A-8). In the upper part whereas the upper part is dominantly of this “Ekofisk tight zone” some reworked pelagic laminated, bioturbated mudstone. layers may occur. Upwards the section is dominated by a thick, heavily reworked

- 14 - A revised Chalk lithostratigraphic nomenclature unit comprising predominantly On the flanks of the Norwegian part of the Maastrichtian fossils. Dating is Central Graben, as well as in the wells problematic due to the extensive from the UK sector, the lower part of the reworking, but the zone seems to Ekofisk Formation is generally absent. correspond mainly to the upper part of Pelagic chalk with thin turbidites NNTp2 and the lower part of NNTp4. The dominate, with occasional thicker unit consists mainly of massive, reworked zones (i.e. 30/7a-2). homogenous chalk and pebble floatstone interpreted as slides, slumps and debris In summary, the Ekofisk Formation flows, occasionally with thin pelagic layers comprises a highly varied range of in between. The upper part of the Ekofisk deposits, with pelagic chalk dominating in Formation is also variably reworked, but the southern and western part of the study less extensively than the lower part, and area, but in the area around the Ekofisk generally consists of slumped, deformed field, where the thickest and most chalk and debris flows with interbeds of complete section is found, reworked chalk pelagic chalk. Reworking took place dominates. The main stratigraphic breaks especially within zone NNTp4D, but also appear at the base and top of the formation. in the uppermost part of zone NNTp4 and during NNTp5A. The uppermost part, Seismic characteristics: On the type of belonging to zone NNTp5B is seismic sections used in this study Top predominantly pelagic and grades into the Chalk is recognised as a clear seismic overlying marl of the Våle Formation, with trough resulting from an increase in no sharp transition. acoustic impedance (SEG reverse polarity standard). In areas where there is a gradual The source area for the reworked zones is transition from the Ekofisk Formation to believed to be mainly the Lindesnes Ridge the overlying the top may be to the southwest, but also with some difficult to pick exactly. Presence of contribution from the east and northeast. hydrocarbons or excessive high formation Further away from the Lindesnes Ridge, in porosity may result in deviations from the Block 2/5, the reworking seems to be less normally expected seismic appearance penetrative, and the Ekofisk Formation is with the top of the chalk formation show dominated by interbedded pelagic chalk dimming or even phase reversal of the and thin turbidites with occasional thicker seismic signal. debris flows (Well 2/5-1). Where thick enough to be resolved, the In the Valhall Field on the Lindesnes internal seismic character of the Ekofisk Ridge, the Maastrichtian chalk was Formation tends to be chaotic, and internal reworked through shallow-water shoaling units can only be interpreted locally. and winnowing across the central crest, feeding coarser-grained debris flows down Biostratigraphic characteristics: All study the flank (Elle Member; Bergen and wells with cores from the Ekofisk Sikora, 1999). This reworked unit seems to formation have been analysed to identify correlate at least in part to the lower diagnostic nannofossil assemblages. In the reworked Maastrichtian deposits at the present study, some 17 nannofossil zones Ekofisk Field. A major flooding event at have been identified. In the wells in the about 63 Ma BP terminated the reworking Danish sector up to seven of these were at Valhall, and younger Danian chalk is a found in individual wells and four were not highly condensed, stratigraphically observed at all. As core coverage is discontinous, deep-water autochthonous incomplete, these are minimum numbers. deposit (Sikora et al., 1999). However, the stratigraphic breaks seem to occur at some distinct levels, namely the

- 15 - A revised Chalk lithostratigraphic nomenclature lowermost zones NNTp1 to NNTp2C/D, youngest zone in the Ekofisk Formation the intermediate zones NNTp4A/B/C and found in these wells, which may be due to the upper zones NNTp4E/F and erosion of the younger zones. However, in NNTp5A/B. These zones are either the pelagic-dominated MFB-7, the zone is sparsely represented or absent. 12 m thick. The Lulu-1 deposits are therefore likely to represent a combination The wells in the southernmost part of the of in situ deposits and a significant volume Danish sector (MFB-7 and M-9X) are of redeposited (older) sediments. showing the largest diversity in terms of biozones present. This is in agreement with In the Norwegian sector, a fairly complete the dominantly pelagic depositional stratigraphic section is found in the axial environment as interpreted from the cores. part of the Central Graben, where all 17 Although the interval is cored, the oldest nannofossil zones have been identified. Up zones have not been identified in these to approximately ten zones have been wells. This may be due to absence or found in individual wells. The stratigraphic condensing of the zones. Also, in these breaks that were noted in the Danish wells wells, reworking of fossils of mentioned above can also be observed in Maastrichtian age is observed, even in the the Norwegian wells, although somewhat shallowest samples. The wells in the shifted in position. In the depocentre in the central part of the Danish sector (Roar-2 Ekofisk area, deposition took place and Baron-2) show a pattern similar to the resulting from erosion further south, southern part except that the upper associated with some of the above biozones are absent. This follows generally mentioned stratigraphic breaks, but also the observations from logs. here biostratigraphic breaks can be observed in individual wells. Although the number of biozones present is low, the formation is relatively thick Zone NNTp1 has been found in some wells (30-60 m). This is at least partly caused by in the axial part of the Central Graben. significant reworking of older strata Zones 2A-2C is sparsely represented in the especially in the lowest Ekofisk, where data, even in the depocentre. A significant Maastrichtian fossils dominate. Reworking erosional episode took place within zone is observed continuously upwards through NNTp2, eroding into the Maastrichtian the formation and fossils as old as deposits on the Lindesnes Ridge and Campanian are found. The occurrence of depositing a thick reworked unit in the such old fossils at this level is linked to the basin, particularly during zones 2E-F. A Tor formation being thin or absent in this thin Danian unit comprising mainly area. reworked Maastrichtian chalk has also been described at the Valhall Field (Bergen The thickest Ekofisk section in the Danish and Sikora, 1999). The datings correspond sector is found in the Lulu-1 and Mona-1 approximately to zones NNTp1 and at least wells. Nevertheless, reworking seems, in part of zone 2, and the unit probably the studied cores, to be insignificant. Some represents the erosional remains left reworking is found in the upper part of the behind on the ridge. Zones NNTp2G/3/4A formation and, as mentioned above, the are again less frequently represented in the uncored central section is known to be data, and the amount of erosional deposits dominated by reworked chalk. Note, is less marked. These zones, in particular however, that the same biozone (NNTp4D) zone 3, are more common in the more is observed immediately below and above pelagically-dominated Danish wells. A the uncored section. This biozone is more flooding event took place on the Lindesnes than 65 m thick in Lulu-1. The same zone Ridge at about this time, and the younger is respectively 0 and 20 m in the central Danian chalk is represented by a thin, wells Roar-2 and Baron-2. It is the condensed deep-water section (Sikora et

- 16 - A revised Chalk lithostratigraphic nomenclature al., 1999). A new significant formation has been named, the Vidar erosional/depositional pulse of Formation. Maastrichtian material seems to have taken place within zones NNTp4B-4C. In the Name: Vidar was a son of the Norse god wells from the areas flanking the Odin. depositional centre, both on the Norwegian and the UK sector, most of the section up Well type section: Norwegian well 2/1-4 through the lower part of zone NNTp4 is from 3138 to 3075 m. No cores. missing (2/2-3). Well reference section: Norwegian well A change in deposition can be seen in the 1/3-1 from 3147 to 3095 m. No cores. Ekofisk area in the middle part of zone NNTp4, approximately zone 4D, when Lower Boundary Characteristics: The there is a marked reduction in reworking, lower boundary represents a sharp and especially in influx of Maastrichtian transition from the claystone of the Lista material. This is associated with a more Formation or the marl of the Våle widespread deposition of the Ekofisk Formation to the overlying of Formation over the structurally high areas, the Vidar Formation. This is marked by a with less erosion of the underlying Tor distinct decrease in gamma-ray readings Formation. Significant slumping and and an increase in velocity. debris flow continued, however, although less than in the section below. Upper Boundary Characteristics: The upper boundary represents a transition to Higher up in the section, pelagic deposits the claystones of the Lista Formation, are more dominating, but a new episode characterised by a dramatic increase in with reworking occurred in the upper part gamma-ray readings and a decrease in of zone NNTp4 and lower part of zone velocity. The upper boundary can be NNTp5 (NNTp4E/F-5A), approximately picked seismically. coinciding with a stratigraphic break observed both in the Danish and Thickness and distribution throughout Norwegian/UK wells. Study Area: The thickness varies from less than a few m to about 70 m in the study Zone NNTp5B is widespread in the area. Where the Vidar Formation is well Norwegian study area, but has not been developed, it is easily identified on the identified in the UK wells. It is dominated seismic lines, as strong reflections with by pelagic deposits, with a gradational poor continuity, indicating an increase in transition into the overlying marls. acoustic impedance. Based on the seismic interpretation, the Vidar Formation is Subdivision: In this project it was decided restricted to the northern part of the that no subdivisiun of the Ekofisk Central Graben, and it is found in the Formation will be performed, implying following of the studied wells: 1/3-1, 1/3- that each company continues their own 8, 2/2-3 and 2/5-7. nomenclature for zonation. Lithology, main Depositional Facies and Late Paleocene chalk units: Vidar Environments: Homogeneous is Formation the dominant lithology. Thin shales occur Reworked chalks of Early Paleocene to within the chalk beds. The chalk beds of Late Cretaceous age are observed in the the Vidar Formation may occur at different late Paleocene section. Chalk beds varying levels in the Lista Formation. Presence of from only a few cm thick and up to beds of reworked chalk of early Palaeocene and 70 metres thick are common. Only one late Cretaceous age indicates that the Vidar

- 17 - A revised Chalk lithostratigraphic nomenclature

Formation Chalk originates from the ANDERSEN, C., 1995: Geological Group. Mass flows from each map of 1:200,000. The side of the Central Graben are the most Danish Central Graben, Base Chalk probable transport mechanism for this and the , Danm. Geol. reworked unit. Unders., Map Series, No. 48. BURNETT, J. A., GALLAGHER, L. T. & Biostratigraphic Characterisation: All HAMPTON, M. J. 1998. Upper chalks of the Vidar formation are reworked Cretaceous. In: Bown, P.R. (Ed.) Danian or Late Cretaceous of age. Calcareous Nannofossil Biostratigraphy, Chapman & Hall. Age: Late Paleocene, consisting of reworked chalk mainly of Danian and BURNETT, J.A., HANCOCK, J. M., Maastrichtian age. KENNEDY, W. J. & LORD, A. R. 1992. Macrofossil, planktonic foraminiferal and nannofossil DISCUSSION zonation at the Campanian/Maastrichtian boundary.

Newsletters in Stratigraphy, 26.

BURNETT, J.A. & WHITHAM, F. 1999. Correlation between the nannofossil and macrofossil biostratigraphies and the lithostratigraphy of the Upper Cretaceous of NE . Proc. REFERENCES Yorks. Geol. Soc., 52.

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Biostratigraphy of the Southern ords.). 1996. New results on Norwegian and Danish North Sea biostratigraphy, palaeomagnetism, Area. Abhandlungen der geochemistry and correlation from the Geologischen Bundesanstalt, 39. standard section for the Upper Cretaceous white chalk of northern MORTIMORE, R., & POMEROL, B. (Lagerdorf – Kronsmoor - 1996. Upper Cretaceous tectonic Hemmoor. Mitt. Geol-Paläont. Inst. phases and end Cretaceous inversion Univ. Hamburg, 77. in the Chalk of the Anglo-. Proc. Geol. Assoc., 108. SIKORA, P. J., BERGEN, J. A. & FARMER, C. L. 1999. Chalk MORTIMORE, R., WOOD, C. J., palaeoenvironments and depositional POMEROL, B. & ERNST, G. 1996. model: Valhall & Hod fields, southern Dating the phases of the Subhercynian Norwegian North Sea. In: Jones, R. tectonic epoch: Late Cretaceous W. & Simmons, M. D. (eds.) tectonics and eustatics in the Biostratigraphy in Production and Cretaceous basins of northern Development Geology. Geol. Soc. Germany compared with the Anglo- London, Spec. Publ., 152. Paris Basin. Zbl. Geol. Paläont. Teil I, H.11/12. SISSINGH, W. 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. NYGAARD. E., ANDRESEN, C., Geologie en Mijnbouw. 56. MØLLER, C., CLAUSEN, C.K. AND STOUGE, S.. 1990. Integrated STOVER, L. E. 1966. Cretaceous multidisciplinary stratigraphy of the and associated nannofossils Chalk Group: an example from the from and the Netherlands. Danish Central Trough, In: J.B. Micropaleontology, 12. Burland (Ed.), Chalk: proceedings of STURLUSON, S., 1954. The Prose Edda the 1989 International Chalk - Tales from Norse Mythology: Symposium: London, Imperial Translated from the Icelandic by J. College, p. 195-201. Young. University of California PERCH-NIELSEN, K. 1979. Calcareous Press. nannofossil zonation at the SVENDSEN, N. 1979. The Cretaceous/Tertiary boundary in Cretaceous/Tertiary chalk in the Dan Denmark. In: Birkelund, T. & field of the Danish North Sea. In: Bromley, R. G. (eds.) Proceedings of Birkelund, T. & Bromley, R. G. (eds.) the Cretaceous/Tertiary Boundary Proceedings of the Events Symposium, Copenhagen, I. Cretaceous/Tertiary Boundary Events RASMUSSEN, L. B. 1978. Geological Symposium, Copenhagen, II. aspects of the Danish North Sea THIERSTEIN, H. R. 1974. Calcareous sector. Danm. Geol. Unders., III, Nannoplankton – Leg 26, Deep Sea Nr.44. Drilling Project. In: Davies, T. A., SCHÖNFELD, J. & BURNETT, J. 1991 Luyendyk, B. P. et al., Initial Reports Biostratigraphical correlation of the of the Deep Sea Drilling Project, Campanian-Maastrichtian boundary: XXVI. Lagerdorf - Hemmoor (northwest TOUMARKINE, M. and Germany), DSDP Sites 548A, 549 and LUTERBACHER, H. 1985. 551 (eastern North Atlantic) with Paleocene and Planktic palaeobiogeographical and Foraminifera. In: palaeoceanographical implications. Stratigraphy. (Eds. Bolli, H.M., Geological Magazine, 128. Saunders, J.B. and Perch-Nielsen, K.), SCHÖNFELD, J. & SCHULZ, M.-G.(co- Cambridge University Press,

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Cambridge, pp 87-154. VAROL, O. 1989. Paleocene calcareous nannofossil biostratigraphy. In : Crux, J.A. & van Heck, S. E. (Eds.). Nannofossils and their Applications. Ellis Horwood Ltd., Chichester. VAROL, O. 1998. . In: Bown, P.R. (Ed.) Calcareous Nannofossil Biostratigraphy Chapman & Hall. VEJBÆK, O.V. AND ANDERSEN, C. 1987. Cretaceous – Early Tertiary inversion tectonism in the Danish Central Trough. Tectonophysics, 137, p. 221-238 ZIEGLER, P. A., 1990. Geological Atlas of Western and Central . Enclosure 41. Shell Internationale Petroleum Maatschappij.

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