lithostrat subdivsion of the Lower cretaceous of the southern North Sea UK sector. Wireline logs, lithology

1. LOWER CRETACEOUS LITHOSTRATIGRAPHY
NE OF THE SOLE PIT AREA IN THE
UK SOUTHERN NORTH SEA
S. Crittenden*
Seven boreholes from block 49, UK sector have been divided lithostratigraphically to illustrate
the degree of comparison between the Dutch and British lithostratigraphical schemes for the Lower
Cretaceous of the southern North Sea.
The two schemes differ in scope and finesse, with the Dutch scheme offering more subdivisions.
Both are applicable to boreholes in the British as well as the Dutch sectors of the southern North Sea.
Borehole 49/25-1 (Shell/Esso) is designated as the reference borehole in the British sector to
illustrate the Dutch scheme.
The Upper Holland Marl Member is considered to be equivalent to the Red Chalk formation while
the Speeton Formation is divided into the equivalent of the Middle Holland Shale Member, Lower
Holland Marl Member and the Vlieland Shale Member of the Dutch sector scheme.
INTRODUCTION
During the last decade, three major publications have proposed lithostratigraphical schemes for
the Lower Cretaceous strata penetrated by exploration boreholes in the southern, central and northern
North Sea (Rhys, compiler, 1974; NAM and RGD, 1980; Deegan and Scull, compilers, 1977). A
further publication by Burnhill and Ramsay (1981) has discussed Middle Cretaceous
lithostratigraphy, and the electric log characteristics of each unit, of the UK central North Sea (as
defined by Deegan and Scull). Rhys (1974) has subdivided the Lower Cretaceous strata of the British
Sector, while NAM and RGD (1980) have subdivided the Lower Cretaceous of the Dutch sector of
the southern North Sea. Both schemes utilized data from offshore and onshore boreholes, as well as
data from the available literature on the onshore outcrop stratigraphy of the Lower Cretaceous.
Of the two schemes, the lithostratigraphical subdivision of the Dutch sector is more detailed. This
is perhaps a result of the greater economic importance of the Lower Cretaceous strata and its
contained reservoirs of hydrocarbons in the Dutch sector. The subsequent detailed exploration activity
by oil companies to find and delineate the reservoirs has resulted in a large amount of available data.
As yet the Lower Cretaceous strata in the British sector of the southern North Sea has not been
Journal of Petroleum Geology, 5, 2, pp. 191-202, l982 191
*Department of Geology, Plymouth Polytechnic, Drake Circus, Plymouth, Devon PL4 8AA, England.

2. considered to be a prime economic target for hydrocarbon exploration, and hence a
lithostratigraphical subdivision has been considered less important.
The two independent schemes proposed for the southern North Sea Lower Cretaceous are the
result of an international boundary and not geology. Borehole data principally from Block 49 (Fig. l)
of the British sector is used to illustrate the compatibility of both the lithostratigraphical schemes and
the relative ease with which one borehole may be correlated with another. The relevant Dutch sector
boreholes are not illustrated but may be found in NAM and RGD (1980).
The scheme proposed by Rhys (1974) is useful and has proved to be a base upon which to build
and develop a lithostratigraphical scheme for the central and northern North Sea (Deegan and Scull,
1977). The Dutch scheme (NAM and RGD, 1980), however, is well-established and has been used as
a viable hydrocarbon exploration tool by the Nederlands Aardolie Maatschappij B.V. since 1949. The
British scheme of Rhys (1974) has not been used extensively as an exploration tool in the British
southern North Sea, hence its shortcomings and inadequacies have not been recognized, but neither
has the terminology become entrenched in the literature.
Lithostratigraphy NE of the Sole Pit area of the North Sea192

3. DISCUSSION OF THE TWO LITHOSTRATIGRAPHICAL SCHEMES
Rhys (1974) subdivided the Lower Cretaceous strata into three formations: Red Chalk Formation,
Speeton Clay Formation, and Spilsby Sandstone Formation, all within the Cromer Knoll Group.
Borehole 48/22-2 (Burmah) was designated as the type section, and borehole 49/24-1 was figured by
Rhys as an illustration of the Lower Cretaceous lithostratigraphy (see Fig. 2).
NAM and RGD (1980) subdivided the marine Lower Cretaceous strata of the Dutch sector into
two formations; the Vlieland Formation and the Holland Formation, within the Rijnland Group. A
further subdivision of the two formations into members distinguishes the Dutch scheme from the
British scheme. The British scheme is also amenable to subdivision; in fact the same subdivisions as
the Dutch scheme (see Fig. 2, borehole 49/24-1, and Figs 3, 4). The Vlieland Formation is divided
into two members; the Vlieland Shale Member and a basal Vlieland Sandstone Member.
However, in the West Netherlands Basin (southern part of the Dutch sector southern North Sea—
SE portion of the Anglo-Dutch Basin) a number of sandstone bodies are present in the Vlieland
Formation all of which are given member status. It is these sandstone members which are the
hydrocarbon reservoirs in the Netherlands. All of these sandstone bodies pinch-out into the Vlieland
Shale Member to the north and are in fact only present as east-west orientated lenses on the southern basin
S. Crittenden 193

4. margin and depict the gradual transgression of the Early Cretaceous Sea on to the London-Brabant
Massif High (Bodenhausen and Ott, 1981). The sandstone bodies present within the Vlieland
Formation are probably represented in the British sector scheme by thin sandstone intercalations
within the Speeton Clay while the Vlieland Sandstone is equivalent to the Spilsby Sandstone
Formation which has a limited occurrence in the SW of the sector adjacent to the Lincolnshire and
Norfolk coast (Glennie and Boegner, 1981). The Vlieland Formation in part is represented in the
British Sector by the Speeton Clay Formation.
The Holland Formation above the Vlieland Formation is subdivided into three distinct members in
the Dutch southern North Sea: Upper Holland Marl Member, Middle Holland Shale Member and
Lower Holland Marl Member. A fourth member, the Holland Greensand, is present between the
Lower Holland Marl and the Middle Holland Shale on the southern margin of the basin, but it rapidly
shales-out to the north and is not seen in block 49. The base of the Middle Holland Shale is usually
marked by a thin basal sand or conglomerate (see 49/24-1, 4,410 ft, and 49/ 24-12, 5,060 ft, Figs 2, 3
and 4).
The Holland Formation embraces part of the Speeton Clay Formation and all of the Red Chalk
Formation of the scheme of Rhys (1974). The Upper Holland Marl Member seems to be directly
equivalent to the Red Chalk Formation, while the Middle Holland Shale and Lower Holland Marl
Members are both in the upper part of the Speeton Clay Formation (as seen in Rhys, 1974, boreholes
49/24-1; see Fig. 2, and 48/22-2). Fig. 1 depicts some of the boreholes used by Rhys (1974) and
NAM and RGD (1980, boreholes: Vlieland Oost-l, L5-l, L12-2) in addition to the boreholes used by
the present author in this study. A comparison of the three boreholes illustrated by NAM and RGD
(enclosure: 24, 27, 28, 1980), with the boreholes correlated in this study shows the remarkable
similarity and ease of correlation of the electric log pattern and lithology across the southern North
Sea Basin (Figs 3, 4).
LITHOSTRATIGRAPHICAL METHOD
Electric logs of the boreholes, especially the Gamma Ray and Sonic-interval transit time,
accurately reflect the lithology of the strata penetrated by the drill bit. However, the Caliper-log is
used in borehole 49/24-12 and the F.D.C. Log in Borehole 48/20-2 (Figs 3 and 4) as the Gamma Ray
and Sonic logs respectively were not available. Additional lithological control has been provided by
cuttings and sidewall cores (supplied by Shell UK Exploration and Production Ltd). This control was
adequate for the bulk determination of lithology, but accurate placement of boundaries was provided
by the Gamma Ray log response changes.
The boundaries of each lithostratigraphical unit are easily recognized in the subsurface. These
boundaries do not imply isochronous surfaces, but are mappable and provide a method of rapid
correlation and interpretation of the Lower Cretaceous strata in the southern North sea (Figs 3, 4).
However, biostratigraphy, by Foraminiferida, does give further information and assists in a regional
correlation and in the subsequent erection of a chronostratigraphy. Units formally defined using
lithostratigraphical criteria by Rhys (1974) and NAM and RGD (1980) have been dated
biostratigraphically by NAM and RGD, and the sections in Block 49 have been dated
biostratigraphically by the author (using Foraminiferida), but age per se has not been used as a
criterion for defining any of them.
The extent of any unconformities is not recognized by the lithostratigraphy except for the Base
Cretaceous Unconformity (Late Cimmerian Unconformity). The recognition of other unconformities
is dependent on micropalaeontological and geophysical criteria and can rarely be recognized on
lithographical evidence. For this reason, no other unconformities are recognized in this
lithostratigraphical discussion. The thickest Lower Cretaceous sequences are in boreholes located in
Cretaceous depositional basins. Boreholes located on pre-Cretaceous structural
Lithostratigraphy NE of the Sole Pit area of the North Sea194

6. highs have attenuated Lower Cretaceous sections often with the basal sequence missing (see 49/24-3).
The pattern of Cretaceous sedimentation has been strongly influenced by the pre-Cretaceous highs
(horst blocks/fault blocks), and transgression during the Early Cretaceous has resulted in Lower
Cretaceous sediments onlapping the highs. By Late Aptian times, these structures had been covered.
This structural/stratigraphical relationship of the Basal Cretaceous Unconformity is recognized in
most of the boreholes drilled in the southern North Sea and the unconformity varies in stratigraphical
magnitude. Salt diapirism and synsedimentary tectonics have also affected the depositional
characteristics of the Lower Cretaceous sediments in the southern North Sea. This has been discussed by
Glennie and Boegner (1981) in their paper on Sole Pit Inversion tectonics where Early Cretaceous uplift has
resulted in varying degrees of erosion and sedimentation over the Sole Pit area (NE flank). All the subsurface
lithological units have been precisely defined by NAM and RGD (1980) and by Rhys (1974), and are practical
nomenclatures. To prevent unnecessary proliferation of localized names, it is felt that the sub-divisions
of the British scheme (corresponding directly to the Dutch subdivisions) illustrated by this paper
should be left open and not formally named until further cooperation of Oil Companies and
Government Institutions reaches an accepted agreement on terminology.
LITHOSTRATIGRAPHY AND CORRELATION OF THE
STUDIED BOREHOLES OF BLOCK 49
The Plenus Marl Formation is used as the datum level for the correlation of the boreholes used in
this study. In three of the boreholes illustrated (49/24-4, 49/24-1 and 49/24-12), the Plenus Marl
Formation is absent. This may be related to the Mid-Cretaceous disconformity as discussed by
Burnhill and Ramsay (1980).
Red Chalk Formation Upper Holland Marl Member
Rhys (1974) defines this lithological unit as “a mudstone, calcareous to slightly calcareous, red-
brown with consistent small amounts of white to light grey mottling and lesser dark grey mottling”.
The carbonate content gradually increases towards the top of the unit reflected by an upward decrease
in the Gamma Ray reading and an increase in Sonic velocity. The top is taken as a sudden sharp
decrease in Gamma Ray reading and a sharp decrease in Sonic velocity at the base of the overlying
chalk-marly limestones of the Chalk Group. The base of the unit is marked by increased Gamma Ray
and decreased Sonic velocity in the underlying unit. Microfauna data (Foraminiferida) suggests a
Middle-Late Albian age.
This unit is recognizable over the whole of the southern North Sea Basin (and into the northern
and central North Sea Basins) and is mappable.
Recently, Wood and Smith (1978) have published a lithostratigraphical classification of the chalk
in North Yorkshire, Humberside and Lincolnshire. These authors argue that from onshore evidence
the Red Chalk Formation should be treated as part of the Chalk Group (Ferriby Chalk Formation) and
be called the Hunstanton Chalk Member. However, Kent, Rawson and Jeans in the discussion of the
paper by Wood and Smith believed that the separate identity of the Red Chalk as a formation would
be better preserved since it is a mappable unit. The electric-log characteristics of the Red Chalk
Formation offshore show that this unit is mappable and should be retained, but whether as of member
status (NAM and RGD 1980) or formation status is open to debate. This unit is present in all of the
studied boreholes. Borehole 49/25-1 is designated the reference borehole for the Upper Holland Marl
in the British southern North Sea.
Middle Holland Shale Member Top part of Speeton Clay Formation
This unit is recognized by its lower carbonate content when compared to the upper and
lower members of the Dutch scheme and to the overlying Red Chalk Formation
Lithostratigraphy NE of the Sole Pit area of the North Sea196

8. of the British scheme. Rhys (1974) has not formally recognized this unit, though it is apparent on the
borehole Gamma Ray and Sonic logs of 49/24-1 and 48/22-2 (773 ft-810 ft).This is also a mappable
unit in the southern North Sea Basin. It is recognized by a distinctive electric log character; high
Gamma Ray response and low Sonic velocity response when compared to the units above and below.
The base commonly has a distinctive negative kick in the Gamma Ray response and a corresponding
increase in Sonic velocity response marking a sandstone or conglomerate basal bed. This marks an
unconformity in 49/24-1 and NAM and RGD use this to denote the “Albian transgression”. This
unconformity is not depicted on the borehole correlation figures, as it can only be delineated by
biostratigraphical evidence. Even though this unit is recognized in the British sector, it is not
proposed here formally to name the unit as a member within the Speeton Clay Formation. Borehole
49/25-1 is designated the reference borehole for the Middle Holland Shale in the British southern
North Sea. Foraminiferida data indicate an Early Albian age for this unit.
Lower Holland Marl Member Top part of Speeton Clay Formation
This is a grey/red-brown calcareous mudstone/muddy limestone, shale unit. Its electric log
character makes it easily recognizable; relatively low Gamma Ray response and a characteristic high
Sonic velocity log response. This unit is not present in borehole 49/24-1 but is well-developed in
boreholes 49/25-1, 49/25-2, 49/2412, 49/20-2 (see Fig. 3) and 48/22-2 (810 ft-845 ft). In the Dutch
sector, this unit marks the base of the Holland Formation and overlies the Vlieland Formation. The
basal contact is marked by the Gamma Ray log response indicating an increase in carbonate content
in the Lower Holland Marl Member as compared to the Vlieland Formation. According to NAM and
RGD this boundary marks a slight but distinctive regional unconformity. It is logical to place the
Lower Holland Marl Member, Middle Holland Shale Member and the Upper Holland Marl Member
into the same formation: Holland Formation. They are all characterized by their carbonate content as
distinct from the underlying argillaceous—sandy Vlieland Formation. The Middle Holland Shale
Member is interpreted as an argillaceous phase between the calcareous members. The Middle Holland
Shale is also characterized by a red/grey colouration. However this classification of the units would
have to be revised in the British sector if the existing scheme of Rhys were to be used. Retaining the
Red Chalk Formation, and placing the Middle Holland Shale and Lower Holland Marl as Members in
the Speeton Clay Formation would be a solution. Borehole 49/25-1 is designated as the reference
borehole for the Lower Marl Member in the British southern North Sea. Foraminiferida data suggest
an Early Aptian age.
Vlieland Shale Member Speeton Clay Formation (part)
This is a sequence of marine, grey/brownish, grey-black shales which are slightly calcareous at the
top and become greyish olive-green, olive-grey black with depth. The Vlieland Shale Member was
not present as such in borehole 49/24-1 if the Speeton Clay noted by Rhys is taken as the Middle
Holland Shale. The base of the unit is defined by either local basal sandstone beds (Spilsby
sandstone) or older formations (Triassic in 49/ 24-1 as dated by Shell UK). The top is formed by the
base of the Lower Holland Marl and is characterized on electric logs by a change to a lower Gamma
Ray response and an increased Sonic response. NAM and RGD regard the top as marking a
disconformity. None of the studied boreholes in Block 49 have a basal sandstone present but there are
minor siltstone and sandstone intercalations in some sections. No subdivision of the shale unit is
attempted by the present author, but a division into locally-correlatable informal log units could be
made. Borehole 49/25-1 is designated as the reference borehole for the Vlieland Shale Member in the
British southern North Sea. Foraminiferida and Ostracoda data suggest a Barremian and older age.
Lithostratigraphy NE of the Sole Pit area of the North Sea198

9. CORRELATION TO ONSHORE SECTIONS IN THE UK TO THE
CENTRAL AND NORTHERN NORTH SEA
No attempt has been made to correlate the offshore borehole lithostratigraphy with any onshore
lithostratigraphy (borehole or outcrop). It is agreed that the key to the offshore in terms of
lithostratigraphy, biostratigraphy, chronostratigraphy and palaeogeography is often onshore outcrop
data. However, extension of the onshore outcrop stratigraphy (lithostratigraphical) schemes to the
offshore is fraught with difficulty. The onshore schemes are usually very detailed, whereas the
inherent drawbacks of cuttings’ material and side-wall core material and lack of continuous cored
sections offshore severely limit the detail with which an offshore lithostratigraphy may be
constructed. Neither the Speeton (1960) shallow borehole or the West Heslerton (1960) ‘Shell’
shallow borehole were electrically logged, so no direct comparison of the Lower Cretaceous section
of Yorkshire (Speeton) may be made with the offshore. The lithology and biostratigraphy (e.g.
Fletcher, 1973) of the Speeton Clay section at Filey Bay, Yorkshire does tempt a direct correlation to
the offshore sequence, but caution is needed as lateral facies changes may be pitfalls for the unwary
trying to relate lithologies to a chronostratigraphy via a biostratigraphy derived from facies-controlled
benthonic Foraminiferida. The onshore stratigraphy of the Lincolnshire and Norfolk Lower
Cretaceous is a far coarser and varied lithological sequence characteristic of a basin margin, and is
analogous to the basin margin deposits of the SW Netherlands Basin.
The Cromer Knoll Group erected by Rhys (1974) to embrace the Lower Cretaceous sediments of
the Southern North Sea has been extended to the central and northern North Sea by Deegan and Scull
(1977). In the central North Sea it is a sequence of marine clays, shales, marls and sandstones. In the
northern North Sea it is less arenaceous. (See Fig. 5). Deegan and Scull have recognized that in the
Cromer Knoll Group in the northern North Sea there are other units of formation status not yet
defined due to data restrictions.
S. Crittenden 199

10. CONCLUSION
The British and Dutch lithostratigraphical schemes for the Lower Cretaceous strata of the southern
North Sea are both applicable to the boreholes studied in Block 49 of the British sector. Both of the
schemes enable correlation of the boreholes within the block and correlation with other boreholes
across the southern North Sea. However, at present the Dutch scheme offers more scope and finesse
for correlation purposes because of its more subtle lithological subdivisions. These subdivisions are
present in the type borehole log sections of the British scheme and in the boreholes from Block 49.
They are, however, left unnamed in the British sector boreholes. The preference of the author would
be to use in an informal manner the Dutch scheme for the Lower Cretaceous in the southern North
Sea until such time as the British lithostratigraphical scheme is revised.
APPENDIX
Borehole49/25-1: reference borehole in the British sector for the
Dutch Lithostratigraphical Units
Operator: SHELL / ESSO
Coordinates: 53° 13’ 10.l” N.
02° 49’ 59.2” E.
Spudded: 15 / 2 / 1969
Abandoned: 20 / 4 / 1969
K.B.E. (AMSL) 96 ft (29 m): water depth 104 ft (31 m)
Upper Holland Marl Member: 6,184 ft - 6,288 ft
Middle Holland Shale Member: 6,288 ft-6,440 ft
Lower Holland Marl Member: 6,440 ft-6,520 ft
Vlieland Shale Member: 6,520 ft-6,952 ft
ACKNOWLEDGEMENTS
I wish to thank Shell UK Exploration and Production Ltd and Esso Exploration and Production
UK Ltd for the provision of borehole material and for permission to publish this paper. This paper
represents part of the author’s research carried out at Plymouth Polytechnic under the auspices of a
CASE award (NERC/Shell UK Exploration and Production Ltd) which is gratefully acknowledged.
Dr Ian P. Tunbridge (Plymouth Polytechnic) kindly read and suggested improvements to the original
manuscript. I wish also to thank the palaeontology/stratigraphy staff members of Shell UK Expro,
London; Shell International Petroleum Maatschappij, Rijswijk, Den Haag; and the Nederlandsde
Aardolie Maatschappij, B.V. Assen, for helpful discussion.
REFERENCES
BODENHAUSEN, J. W. A. and OTT, W. F., 1981. Habitat of the Rijswijk Oil Province, Onshore the
Netherlands. In: Illing, L. V. and Hobson, G. D. (Eds), Petroleum Geology of the Continental Shelf of NW
Europe. Institute of Petroleum, London. pp. 301-309.
BURNHILL, T. J. and RAMSAY, W. V., 1981. Mid-Cretaceous palaeontology and stratigraphy, Central North
Sea. In: Illing, L. V. and Hobson, G. D. (Eds), Petroleum Geology of the Continental Shelf of NW Europe.
Institute of Petroleum, London. pp. 245-254.
DEEGAN, C. E. and SCULL, B. J. (Compilers), 1977. A proposed standard lithostratigraphic nomenclature for
the Central and Northern North Sea. Rep. Inst. Geol. Sci., No. 77/52: Bull. Norw. Petrol. Direct., No. 1,
36 pp.
Lithostratigraphy NE of the Sole Pit area of the North Sea200

11. FLETCHER, B. N., 1973. The distribution of Lower Cretaceous (Berriasian-Barremian) foraminifera in the
Speeton Clay of Yorkshire, England. In: The Boreal Lower Cretaceous. Eds: Casey, L. and Lawson, P. F.,
pp. 161-168.
GLENNIE, K. W. and BOEGNER, P. L. E.,1981. Sole Pit Inversion Tectonics. In: Illing, L. V. and Hobson, G.
D. (Eds), Petroleum Geology of the Continental Shelf of NW Europe. Institute of Petroleum, London,
pp. 110-120.
NEDERLANDSE AARDOLIE MAATSCHAPPIJ BV and RIJKS GEOLOGISCHE DIENST, Haarlem, 1980.
Stratigraphic nomenclature of the Netherlands. Verh. van het Koninklijk Ned. Geologische Mijnbouw
Kundig genootschaap. 77 pp. and 36 enclosures.
RHYS, G. H. (Compiler), 1974. A proposed standard lithostratigraphic nomenclature for the southern North sea
and an outline structural nomenclature for the whole of the (UK) North Sea. A report of the Joint Oil
Industry-lnstitute of Geological Sciences Committee on North Sea nomenclature. Rep. Inst. Geol. Sci.,
No. 74/8, 14 pp.
WOOD, C. J. and SMITH, E. G., 1978. Lithostratigraphical classification of the chalk in North Yorkshire,
Humberside and Lincolnshire. Proc. Yorks. Geol. Soc. 42, 2, 263-287, plates 11-12.
S. Crittenden 201