Journal of Petroleum Geology, 10(4), pp. 395-414, 1987 395

THE “ALBIAN TRANSGRESSION” IN THE SOUTHERN NORTH SEA BASIN

Stephen Crittenden”

The“A1bian transgression”may be recognised lithostmtigraphically in a number ofboreholes in the UK sector of the southern North Sea Basin. This event is also recognised onshore the United Kingdom. The importance of this event for hydrocarbon exploration is discussed, and comparisons are made with sections in West Germany. The ‘Ylbian transgression” is a regional event in N W Europe which is important for understanding the pattern of basin evolution during Early and “Mid” Cretaceous times. It is also important because the diachronous coarse clastics associated with the transgression may prove to be of important reservoirpotential in the southern North Sea area (UK sector).

INTRODUCTION

The lithostratigraphical subdivision of the Early Cretaceous strata of the southern North Sea Basin has been discussed previously by a number of authors (Rhys, compiler 1974,1975; NAM and RGD, 1980; Crittenden, 1982). Crittenden ( 1982) discussed the lithostratigraphical scheme of NAM and RGD ( 1980), and compared and contrasted it with the scheme of Rhys ( 1974, 1975), and presented a lithostratigraphical and wireline log correlation, using both schemes, of the Early Cretaceous strata of a number of boreholes in block 49 (UK sector) (see Fig. 1).

In a later publication, Crittenden (1984) discussed in detail the Aptian and Albian lithostratigraphy and foraminifera1 biostratigraphy of one of these boreholes, 49/24-1, and made some comparisons with onshore N W European Early Cretaceous borehole and outcrop sections.

This paper complements the previous publications (Crittenden, 1982, 1984) by discussing and comparing the Albian strata from a number of boreholes in the southern North Sea with other areas in N W Europe. The widespread occurrence and recognition of the “Albian transgression” in N W Europe is discussed.

Tectonic instability during Aptian and Albian times had a pronounced effect on sedimentation, especially in marginal areas and around palaeohighs (local salt movement structures) of the N W European sedimentary basins. The product is a profusion of non- sequences, discordances and erosion levels of regional and local importance, which are all associated with phases of shallowing and deepening of the sea. The resultant numerous sedimentary facies characteristic of the Aptian and Albian strata.in N W Europe may be mapped, and dated biostratigraphically, to provide an approximation of palaeogeography and palaeogeographical changes through time (Gallois and Morter, 1982).

* Gearhart Geo Consultants Ltd, Howe Moss Drive, Kirkhill Industrial Estate, Dyce, Aberdeen, AB2 OGL, Scotland.

396 The ‘illbian transgression”

However, correlation difficulties are encountered as a result of the facies variations which, as they are a result of palaeoenvironment, are concommitant with variations in the benthonic and planktonic foraminifera1 fauna. The dearth of planktonic foraminiferids during Early to Middle Albian times, which precludes their use as accurate biostratigraphic tools, and the abundance of benthonic foraminiferids - particularly arenaceous faunas - is undoubtedly facies related.

It is considered that an appreciation of the sedimentary facies variations - spatial and temporal - during Aptian and Albian times is of paramount importance for hydrocarbon exploration in the southern North Sea Basin.

It would not be incorrect to suggest that a major “gross” transgression phase commenced in Late Aptian times (nutfieeldensis - nolani - jacobi Biozones) and progressed through the tardfircata subzone, the regularis subzone and mammilatum Biozone of Early Albian through to Middle and Late Albian times. Minor stillstands, tectonic movements and minor regressions/transgressions have resulted in polyphase events throughout this interval, which make the precise geological history difficult to unravel. Basinward, a number of unconfonnities/ hiatuses may be recognised, while across high areas these unconformities/hiatuses coalesce into what appears to be a single major event (Fig. 9). By Middle Albian - Late Albian times, the major high areas in the region were inundated by the sea. In the United Kingdom, the Gault Clay is the earliest Mesozoic unit known to extend across the Palaeozoic London-Brabant platform (Owen 197 1 a,b). The recognition of this transgression and associated coarser sedimentation at its base is/will be of paramount importance for the delineation of possible stratigraphic hydrocarbon traps in the UK southern North Sea, the reservoir being the coarser, clastic sediments associated with the transgression.

THE SOUTHERN NORTH SEA BASIN

Boreholes 49/24-3, 49/24-4 and 49/24-I (all ShelVEsso) provide an excellent example of a lithostratigraphical and biostratigraphical correlation of Albian age strata in the southern North Sea Basin. These boreholes are located on Early Mesozoic structural highs/fault blocks, which during latest Early Cretaceous time underwent uplift and erosion. These boreholes illustrate the onlap of Early Cretaceous age sediments on to pre-Cretaceous structural highs (Permian- Triassic strata). The onlap is a result of the Early Cretaceous marine transgression (Day et aZ., 1981), salt movement, and local tectonic activity (Glennie and Boegner, 1981). This has resulted in the subsequent erosion, or non-deposition, of either pre-Late Aptian/Early Albian age strata or pre-Late Albian age strata (Glennie and Boegner, 1981) with, in general, an increasing hiatus towards the top of the highs. The concept of the “coalescence of unconformities” across paleostructural highs is thus demonstrated. The microfaunal evidence indicates that by comparison with other areas in NW Europe, only a relatively thin veneer of Late Aptian and Albian sediments is present in the three boreholes.

Lithostratigrap hy The lithostratigraphic subdivision and correlation of the three boreholes has been discussed

and illustrated previously by Crittenden (1982, 1984) - Fig. 2, The gamma-ray and sonic/ interval transit-time logs from boreholes in the UK sector of the southern North Sea Basin all display a similar feature at the base of the Middle Holland Shale Member (Fig. 2, from Crittenden, 1982, 1984). This siltstone/sandstone - phosphate nodule bed horizon can be correlated with the greensand intercalations present toward the base of the Middle Holland Shale Member in the Netherlands offshore (NAM and RGD, 1980). These greensand intercalations are thickest along the margins af the southern North Sea Basin (and on the flanks of palaeohighs), and rapidly shale out toward the depositional centres. In the West Netherlands Basin, thicker developments of green, glauconitic, fine-grained, basin-margin sandstones and siltstones are termed the Holland Greensand Member(NAM and RGD, 1980), and are of Late

Stephen Crittenden 397

Fig. 1. Location of the studied boreholes in the southern North Sea Basin Cfrorn Crittenden. 1982).

Aptian to Early Albian age in the offshore Netherlands area (C.A. Burton, pers. comm.). Boreholes VZieZund Oost-2 and L5-2 in the Dutch sector of the southern North Sea illustrate the “basal Albian Gault” transgression, as depicted by the sandy basal horizon of the Middle Holland Shale Member (Crittenden, 1982; 1984; 1987) (Fig. 3).

Biostratigraphy Careful microfauna (foraminiferal) analysis provides accurate age dating for the Early

Cretaceous strata of boreholes 49/24-2 and49/24-4 (Figs. 4 and 5), and reveals the presence of a basal Early Cretaceous unconformity. PermeTriassic sediments are overlain unconformably by a thin interval of Late Aptian age strata, which in turn is overlain unconformably by an interval of Early to Middle Albian age strata (Crittenden, 1984).

In borehole 49/24-3 (Fig. 6) , Late Albian age sediments (Upper Holland Marl Member) overlie unconformably the Bunter Sandstone. This suggests that the boundary between the Upper Holland Marl and Middle Holland Shale Members is a disconformity/unconformity.

The biostratigraphical details (foraminifera) of the Early Cretaceous strata of borehole 49/ 24-2 have been discussed by Crittenden ( 1984), and are not repeated here. However, boreholes 49/24-4 and 49/24-3 are discussed briefly below.

(0 Borehole 49/24-3 There is an incomplete coverage of the Early Cretaceous interval, by ditch cuttings samples,

of borehole 49/24-3. The three samples from this borehole each contain a light red-brown coloured foraminiferal fauna. This is a characteristic of the red-brown calcareous mudstones which comprise the upper Holland Marl Member (Red Chalk Formation) in block 49. A subtle change in the fauna is noted at 5,000 ft, where the foraminiferal fauna is a dark reddish-brown colour. This may be a colouration artifact derived from the Bunter Sandstone Formation beneath.

398 The "A/bian transgression"

Stephen Crittenden 399

VLIELAND OOST - I

L5 1 (NAMI

Fig. 3. Lithostratigraphy of the Albian/Aptian strata of borehole Vlieland Oost - I and L5 - I (firom NAM and RGD, 1980).

Planktonic Foram in ifera The abundant planktonic foraminiferal fauna is dominated by the Hedbergella delrioensis

(Carsey) - Hedbergella infracretacea (Glaessner) - Hedbergella brittonensis (Loeblich and Tappan) plexus. Members of this plexus become larger in size up the borehole section. This size increase has been noted by Price ( 1977b) and Carter and Hart ( 1977) in the Albian strata of NW Europe. Large, high-spired forms are attributable to H. brittonensis (= Whiteinella brittonensis of e.g. Robaszynski and Caron, 1979), which is a characteristic species of the Late Albian and Early Cenomanian (Carter and Hart, 1977). Small, high-spired forms are attributable to H. infracretacea. Price (1 977b) has discussed this plexus and its stratigraphical use in the Albian strata of N W Europe in some detail, and it is not repeated here.

H. simplex (Morrow) is a Late Albian-Cenomanian species in NW Europe (Robaszynski and Caron, 1 979). Globigerinelloides bentonensis (Morrow) is an important index planktonic species, as its occurrence in large numbers (flood horizons) is indicative of the Late Albian in NW Europe (Price 1977a,b). Floods of this species have been recorded from horizons in Late Albian sediments of southern England, the Central North Sea, northern France and Germany (Hart, 1973; Carter and Hart, 1977a; Price 1977a,b; Burnhill and Ramsay, 1981; Harris, 1982). It is encountered rarely in the Middle Albian and it ranges into the Cenomanian. The planktonic foraminiferal fauna indicates a Late Albian age for the calcareous mudstones of the Upper Holland Marl Member (Red Chalk Formation) of borehole 49/24-3.

$00 The "Albian transgression"

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. * ( : . . 1 . . . . Hedberpella delrioensis pleius Hedbergella planispira Globigerinelloides benronensir liedbergella simplex

Fig. 4. The biostratigraphy of the AlbianIAptian strata of borehole 49/24 - 1

(from Crittenden, 1984).

Stephen Crittenden 40 1

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402 The “Albian transgression”

Bethonic Foraminifera The benthonic foraminifera are diverse and numerous. Arenobulimina chapmani Cushman,

A.fiankei (Cushman), A. advena (Cushman), A. sabulosa, Eggerellina mariae (ten Dam) and Quinqueloculina antiqua (Franke) are, according to Bartenstein ( 1976a,b,c; 1977; 1978a,b; 1979), Carter and Hart (1977), Price (1977b) and Harris (1982), important members of the Late Albian benthonic foraminiferal fauna in NW Europe. This species association suggests a Late Albian age for the Upper Holland Marl Member (Red Chalk Formation). The diverse Arenobulimina fauna and the absence of the Middle Albian species A. macfadyeni supports a Late Albian age. Walters (unpublished PhD thesis, 1958) recognized that Textularia chapmani Laliker is a typically Late Albian species. This Late Albian age assignment is supported by the occurrence of a diverse gavelinellid fauna, including Gavelinella cenomanica (Brotzen), G. baltica Brotzen, and G. group intermedia (Berthelin) which are recorded together in the Late Albian of NW Europe by a number of authors (e.g. Price, 1977b; Magniez-Jannin, 1975). The Lingulogavelinellid species substantiate the Late Albian age; L. ciryi inflata Malapris-Bizouard, according to Magniez-Jannin ( 1975), indicates an uppermost Late Albian (Vraconian-dispar Biozone) age in the Paris Basin.

Species of the SpiroplectinatdGaudryina group were not present in sufficient numbers to enable a rigorous application of the phylogenetic scheme of Grabert (1959) to be made. However, S. complanata (Reuss) is recorded from the Late Albian strata of NW Germany. Gaudryina gradata (Berthelin) is also considered to be a Late Albian/Cenomanian age species.

The benthonic foraminiferal fauna recorded from borehole 49/24-3 indicates a Late Albian age for the Upper Holland Marl Member (Red Chalk Formation). The fauna shows an increase in arenaceous species downhole. The basal sample (5,000 ft) has a marked increase in the number of specimens of the Glomospira gaultina group. This is perhaps associated with the environment of deposition (shallow marine: see Crittenden, 1984).

(ii) Borehole 49/24-4 There is good sample coverage of the Early Cretaceous strata of this borehole from 4,730 ft to

4,900 ft (every 10 ft). However, from 4,900 ft to the base of the Early Cretaceous at c. 4,955 ft the sample coverage is poor (2 samples), which is unfortunate as this is the interval where biostratigraphic control is most important for age dating.

Planktonic Foram in $era The planktonic foraminiferal fauna is numerous and dominated by the H. delrioensis - H.

irlfracretacea - H. brittonensis plexus. This plexus dominates the fauna recorded from the Upper Holland Marl Member (Red Chalk Formation) in the interval 4,720-30 ft to 4,860 ft. The other species present are H. simplex G. bentonensis and H. planispira (Tappan). The latter species is not common, but is distinctive and present throughout the greater part of the late Early Cretaceous (Bartenstein, 1965; Carter and Hart, 1977; Price, 1977a; Harris, 1982). By comparison with boreholes 49/24-3 and 49/24-1 (Crittenden, 1984) the planktonic foraminiferal fauna indicates a Late Albian age for the Upper Holland Marl Member (to 4,830 ft). The marked decrease in the planktonic fauna from 4,830-40 ft downwards, coupled with the benthonic foraminiferal evidence, suggests a Middle to Early Albian age to 4,900 ft. The presence of “Globigerinelloides”? gyroidinaeformis Moullade at 4,830-4,840 ft is interesting. This species has been used by various authors to denote the Middle and Early Albian strata in the North Sea Basin. It’s presence (1 specimen) at 4,830-4,840 ft and the single specimen recorded at 4,890-4,900 ft confirms a Middle-Early Albian age at those depths.

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A m n o d i x u s c r e f x r u s Glornosoira gaulrina grouc Triraxiz p yramida ra Gaudry~na dividenr Marssonella rrochus Eggercllina niariae - Arenobulirnine chapmdni - Arenobulirnrna lrankei - Arenobu/i;nina adveria Spiroplecrinara tornnlarrara Haplophragrnordes charman; Trochammina sp. 0uinqueloculi: ia ~ r r 1 3 1 i a - Eog2nu:i:ia sorci?y fvocosari,gr - :inidenrrfr,rd Lenriculine snectes - urii3enir':ed Denralina species - cmidwr / f ied Lenriculina cressicmia crazsicoz:? Gavelinrlia irr?rrnedia g r o w - GavelinElla bal:rca - Gavelinella crnomar:ica - L rngu/ogaveltnel:a albiensis s . l . Lingu/ogavelinella c i ry i infla:a Valvulmeria species - unidenri'ied - Arenobulirnina sabulosa Amrnobaculires subcreraceus Cribra:ina sp. Hyperammind gaultina Rxroglandul i . ia niu rabilis Denralria drsrincra Thurarnniina albican: Lingiiiogavelinella c i ry i ciryi Faisogau dry inella so. Texnilaria chapinan; Gaudryina gradara Vaginulina recra - brok?n Amrnosphaeroidina minu ra

Hedbergells sinrplex Hedbergclla delrioensrs - plexus Hedbergella hrirronenris Globigerrnellordes benrorienris

Fig. 6. The biostratigraphy of the AlbiadAptiaa strata of borehole 49/24 - 3.

404 The “Albian transgression”

Ben thon ic Foram in ifera The benthonic foraminiferal fauna is both numerous and diverse, and compares well with the

faunas recorded from the Upper Holland Marl in boreholes 49/24-1 and 49/24-3. The interval 4,730 to 4,830-40 ft is interpreted as Late Albian in age, and is characterised by an assemblage of arenobuliminids and gavelinellids usually associated with sediments of a Late Albian age in NW Europe (Carter and Hart, 1977).

Other taxa indicative of a Late Albian age are E. manae, Marssonella ozawai, Flourensina intermedia, Q. antiqua. T. pyramidata, Arenobulimina advena, T chapmani A. sabulosa and Vaginulina mediocarinata (Carter and Hart, 1977).

The first downhole occurrence ofA. macfadyeni at 4,820-4,830 ft denotes the penetration of sediments of an earliest Late to Middle Albian age. This is associated with a dramatic increase in abundance of G. gr. intermedia and denotes a faunal facies change. The sample 4,840-4,850 ft contains the first downhole Occurrences of Reophax minuta, Conorboides lamplughi and Osangularia schloenbachi which are all considered index species for strata of Middle Albian age and older (Crittenden, 1982,1984; Price, I977a, b; Carter and Hart, 1977). An increase in abundance of Glomospira gr. gaultina is also recorded in samples 4,840-50 ft and 4,850-60 ft (compare with boreholes 49/24-3, 49/24-1, 49/25-11. The top downhole Occurrence of G. dividens and Spiroplectinata indicates sediments of an Early Albian age (Grabert, 1959; Bartenstein, op. cit. ). The top consistent and common Occurrence of G. diuidens. Patellina sp., together with the top occurence of Gaudryinella sherlocki and Conorotalites bartensteini aptiensis, associated with abundant Glomospira gr. gaultina, are taken to indicate sediments of a Late Aptian age.

To conclude the discussion on the biostratigraphic subdivision of borehole 49/24-3, the following breakdown can be made:

Sediments of a Late Albian are present from 4,720-30 ft to 4,820-30 ft. Middle Albian age sediments are present to 4,870-80 ft Beneath this depth, a pragmatic approach is used to arrive at an Early Albian/?Late Aptian age, undifferentiated, for the interval from 4,870-8Oft to 4,910-20 ft. A Late Aptian age is indicated for the interval 4,910-20 ft to the last sample examined at 4,940-50 ft. There is some degree of doubt involved in this age breakdown, for the data available does not permit a finer resolution. For instance, the interval 4,820-30 ft to 4,840- 50 ft could be assigned a Late/Middle (undifferentiated) Albian age.

(iii) Stratigraphical conclusions for Boreholes 49/24-3 and 49/24-4 The nature of the samples (ditch cuttings) and large sample interval precludes (as all

micropalaeontologists are aware) a detailed breakdown of the studied sections. The faunas recorded from the two boreholes are not referred to the biozonations of Hecht (1938), Hart ( 1973), Carter and Hart ( 1977) and Price (1 977b). However, an age is assigned based upon comparison with faunas from studied onshore sections adjacent to the southern North Sea Basin. Both the planktonic and benthonic foraminiferal faunas recovered from the two boreholes indicate that the Upper Holland Marl Members (Red Chalk Formation) is Late Albian in age, and Middle Albian in age at the base; and that the Middle Holland Shale Member is Middle to Early Albian in age and, as suggested by Crittenden (1984), Late Aptian in age at the base.

The planktonic foraminifera are dominant in the Upper Holland Marl Member, indicating deep water with good open-ocean connections; while the benthonic foraminifera are dominant in the Middle Holland Shale Member, suggesting more restricted oceanic circulation. This same relationship has been noted by Price ( 1977b) and Magniez-Jannin ( 1975) for the Albian strata of onshore NW Europe. An examination of Fig. 2 shows the diachroneity of the Early-Middle Albian Gault transgression and its effect upon the thickness of the Albian sediments across “high areas”. It is apparent that there are probably a number of non-sequences and unconformities present within the studied sequences, most of such small magnitude that they cannot be detected with the biostratigraphical data available.

Sfephen Crittenden 405

L a t 50’46.8dN

Fig. 7. The Albian stratigraphy of the Winterborne Kingston borehole grom Morter, 1982).

ONSHORE THE UNITED KINGDOM

Regional outcrop data illustrate the “Albian trangression”:

(A) South of the London-Brabant Platform, it is expressed lithologically as the Lower Greensand, Carstone and the nodule beds at the base of the Gault Clay. For example: (i) Glauconitic sands of Early Albian age(mammi1atum Biozone) at the base of the Gault

Clay in the western outcrop of Early Cretaceous strata in Wiltshire and Buckinghamshire overlap onto the Kimmeridge Clay (Late Jurassic).

(ii) The Carstone of the Isle of Wight (mammilatum Biozone), which has a gradational junction with the overlying Gault Clay.

To the best of the Author’s knowledge, there has been no study published which discusses the reservoir potential of this stratigraphic interval onshore or offshore southern UK.

The Winterborne Kingston borehole (Fig. 7) is located at the western margin of the Early Cretaceous depositional basin (Wessex Basin or Southern Basin of some other authors) in

406 The “A lbian transgression”

southern England, where the Gault Clay and its sandy/conglomeratic base transgresses westward over folded and eroded earlier Cretaceous and Jurassic strata. Morter (1982) regards the basement beds of the Lower Gault Clay in this borehole as belonging to the mammilatum Biozone (345.24m-345.55m), with earlier Albian (Kitchenii subzone) and perhaps Late Aptian greensands beneath. This subGault Clay transgressive episode may be a direct correlative of the “basal Albian” event seen in boreholes in the North Sea Basin and onshore NW Europe. This transgression is marked by “coastal onlap”, the effects of which are also apparent in basinal areas as phosphatic nodule beds - siltiedsandier horizons (see Fig. 8).

These horizons, indicative of erosive periods, are often correlative with a decrease in planktonic foraminifera abundance (Price 1977b Gallois and Morter, 1982).

(B) North of the London-Brabant Platform, this important transgression episode is represented: (i) In Yorkshire, by the Greensand Streak at Speeton (regularis subzone of the

tardefurcata Biozone: Dilley, 1969; Owen et al., 1968) and elsewhere by the Carstone Grit. In Central and Southern Lincolnshire, by the Cartone Grit or pebble base of the Red Chalk where the Grit is absent (Owen, 1972; Owen et al., 1968; Rawson et al., 1978).

(iii) In Norfolk the Carstone is diachronous, and is Early Albian in age (tardefurcata - regulan’s subzone and mammilatum Biozone). It is overlain by Gault Clay. Ammonites recorded from the base of the Norfolk Carstone are derived Aptian forms (Casey, 196 1) while in situ brachipods and ammonites at the top of the Carstone give a tardefircata subzone and mammilatum Biozone age (Casey, 1961; Casey and Gallois, 1973).

(iv) The Gault subcrop on the northern flank of the London-Brabant Platform is variously of the loneatus, Icutus or inflaturn Biozone, and rests directly on Palaeozoic rocks. The Gault Clay thins from west to east from its Cambridgeshire outcrop as it oversteps the Palaeozoic rocks of the platform (Fig. 8).

(ii)

GERMANY

There is a wealth of useful, highly-detailed and factual data on the Albian age strata of Germany (e.g. for references and discussion: Kemper, 1973,1979). This is a direct result of the economic significance of the Early Cretaceous strata for hydrocarbon exploitation in Germany. From a brief perusal of the literature, it is apparent that there has been a distinct polarisation of the foraminifera1 research effort on the Early Cretaceous strata of NW Europe. This polarisation, or separation into two camps, is a result of geography and economics. The two camps are: (a) United Kingdom - southern North Sea - West Netherlands Basin - London Paris Basin research; and (b) Germany - East Netherlands research.

Only relatively recently (195O’s), has there been greater communication and transfer of thoughts and ideas between these two camps. It has to be pointed out, though, that obviously some transfer took place throughout the 19th and 20th centuries, as most geologists are aware of the overail similarity of the Early Cretaceous sequence of northern England with the sequence in NW Germany (Owen, 1979).

The data available from Germany provide a valuable insight into the development of Albian age strata in NW Europe, and has a direct consequence (or should have) and bearing on hydrocarbon exploration in the whole of the North Sea area. More importantly, Early Cretaceous exploration plays and concepts developed and pursued in Germany may be applicable to the North Sea area - the southem basin in particular.

The “basal Albian” transgression is represented in Germany as a development of sands, silts, glauconitic sands and phosphatic pebble beds which lie at the base of the “Mid-Cretaceous” section (usually basal Gault Clay or equivalent - Kemper, 1979, Fig. 2).

Stephen Crittenden 407

Fig. 8. The Albian strata onshore the United Kingdom Gfrom Crittenden, 1984).

146 H- I

APTIAN "BELLY" MOTIF AS SEEN IN THE SOUTHERN NORTH SEA SASIN

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Fig. 9. Profile through the Scheerhont oilfield, about 35 km NNW of Bentheim, Germany Gfrom Kemper, 1979).

408 The “Albian transgression”

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Fig. 10. The Albian strata of Munsterland, Germany; cross-section profile locations (jmm Schuster and Wolburg, I962 and 1963).

The Scheerhom oilfield (Fig. 3; Kemper 1979) provides an excellent example of the “base Albian” trangression, and illustrates the onlap of strata onto structural high areas. The concept of unconfonnity coalescence is also illustrated, as the magnitude of the stratigraphic hiatus increases onto the high. Part of Kemper‘s illustration is reproduced here as Fig. 9. The wireline log pattern is remarkably similar to log patterns encountered over the same stratigraphic interval in the southern North Sea Basin and onshore southern UK (Winterborne Kingston borehole - Fig. 7) (Crittenden, 1984).

Wireline log correlation diagrams from the Albian strata of Munsterland (Fig. 10) (Schuster and Wolburg, 1962,1963) illustrate very dramatically the “Albian transgression” and the sand facies associated with the transgression (Osning Sandstone). This sandstone as a lithostrati- graphic unit according to Kemper (1979) is a basin margin deposit of Early Valanginian to Middle Albian age which flanks the northern margin of the Rhenanian Massif (see Owen, 1979). These sands pinch-out to the north toward the basin centre - a situation very similar to the sandstone tongues which flank the northern margin of the London-Brabant Platform in the

Stephen Crittenden 409

410 The “A Ibian transgression ”

Fig. 12. Neuenkirchen 2 to Jburg 2 (Schuster and Wolburg, 1962).

West Netherlands Basin Early Cretaceous strata (NAM and RGD, 1980). Various correlation profiles of boreholes in the Munsterland area are shown in Figs. 1 1- 14. The work performed by Schuster and Wolburg illustrates very well how the careful integration of micropalaeontology, lithostratigraphy, chronostratigraphy and palaeoenvironment analysis is an important technique to help unravel the geological history, and to develop a basin evolution model, for an area to aid hydrocarbon exploration.

The concepts, models and exploration plays developed in Germany for Early Cretaceous strata are important for North Sea exploration, a fact which is emphasised by NAM and RGD ( 1980). Their excellent stratigraphic compendium, particularly for the Early Cretaceous, stems from the unique position of the Netherlands, encompassing as it does two exploration areas - the West Netherlands Basin-North Sea Basin, and the Lower Saxony Basin, of which Munsterland is a part.

CONCLUSION

The “Albian transgression” is a regional event in N W Europe which is important for the understanding of the pattern of basin evolution during Early and “Mid” Cretaceous times. I t is important for hydrocarbon exploration in Germany and the Netherlands and is of exploration importance in the North Sea. Structural movement of post-Cretaceous age has meant that the arenaceous sediments associated with the transgression are not necessarily located over the crests of present-day “Jurassic structures” as delineated by seismic surveys. The development of the arenaceous sediments and their reservoirhrapping potential is a result of the subtle interplay of stratigraphy and structure. In consequence, a thorough palaeogeographic understanding ofthe Aptian and Albian age strata in N W Europe is essential. A borehole drilled crestally to test a Permo-Triassic or Jurassic structural play will in most cases penetrate an overlying thin Early Cretaceous succession. The palaeogeographic configuration of the Early

Stephen Crittenden 41 1

I Fig. 13 (above). Karl-Mahne 2 to Jburg 2 (Schuster and Wolburg, 1962).

Fig. 14 (below). Weseke 1008 to Donar 5 (Schuster and Wolbulg, 1962).

il

412 The “A lbbian transgression ”

Cretaceous strata, plus the effects of Post-Jurassic and Post-Cretaceous structural movement, mean that although a well may be ideally located to test the PermeTriassic - Jurassic strata, it will not necessarily be located in an optimum position to test Early Cretaceous strata (Hesjedal and Hamar, 1983).

The concept of exploring for purely Early Cretaceous hydrocarbon plays in the southern North Sea has been developed in recent years. The reservoir potential is present, as is the source rock potential (Cretaceous and Jurassic), but an important question for the majority of the southern North Sea is whether the Early Cretaceous or Jurassic source is thermally mature enough to have generated oil. However, encouragingly, the West Netherlands Basin is a proven “oil kitchen” where Jurassic claystones source Early Cretaceous sands. Oil is found in Early Cretaceous strata of the Dutch sector of the southern North Sea, and it is the present Author’s opinion that before long, if purely Early Cretaceous plays are “worked-up” and drilled, hydrocarbons may be discovered in Early Cretaceous reservoirs in the UK sector of the southern North Sea.

ACKNOWLEDGEMENTS

The initial research upon which this paper is based was carried out at the Plymouth Polytechnic, UK., from 1980 to 1982 under the auspices of a CASE award fundedjointly by the Natural Environment Research Council (UK) and Shell UK Exploration and Production Ltd/ Esso Exploration and Production (UK) Ltd. I am grateful for their support. I wish to thank my research supervisor at Plymouth, Prof. Malcolm B. Hart, for his encouragement and support. His critical comments, instruction, patience and direction have been invaluable. I thank Carla, my wife, for her tolerance and understanding.

The opinions expressed in this paper do not reflect those of any mentioned organization.

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