Southern North Sea Basin Research Articles

An Interpretation of Vitrinite Reflectance Data From the Southern North Sea Basin

Summary Vitrinite reflectance data for the Carboniferous in 68 Southern North Sea Basin wells have been used to determine a maturity-depth relationship for the basin and to provide an estimate of basement inversion. The derived maturity-depth relationship is comparable with maturity gradients in other Paleozoic coal and gas basins known to have had similar geothermal histories. The earliest onset of gas generation in the Southern North Sea was in the late Jurassic following the accumulation of ± 11700 ft (3500 m) of sediment above the Carboniferous source beds. Estimates of inversion of the Sole Pit area using vitrinite data are similar to those based on shale velocity methods (± 1500 m maximum). In some areas the amount of inversion of the Carboniferous basement apparently exceeds that of the post-Zechstein section which may be attributed to removal of significant amounts of late Carboniferous strata during late Hercynian times; these areas are also associated with the principal gas discoveries in the basin.

Aminostratigraphy of European marine interglacial deposits

Molluscan fossils collected from shallow water marine sediment across NW Europe and nearby Arctic regions have been analysed for the extent of isoleucine epimerization (DL ratio) in indigenous protein residues. The DL ratios confirm that essentially all ‘classical’ Eemian sites from NW Europe are of the same age, and are correlative with the type locality near Amersfoort in the Netherlands; shells from interglacial marine sediment beneath the type Weichselian till in Poland also correlate with the type Eemian site. DL ratios in Holsteinian marine shells (0.29) are substantially higher than in their Eemian counterparts (0.17); ‘Late Cromerian’ shells yield even higher ratios (0.46). DL ratios in late glacial shells (0.06) and Middle Weichselian shells (0.09) permit differentiation from modern (0.01) and last interglacial material. Based on the position of the Matuyama-Brunhes boundary and the differences in DL ratios, the Eemian must correlate with isotope substage 5e, whereas the Holsteinian is most likely substage 7c, possibly stage 9 but certainly younger than stage 11. Intra-Saalian warm periods may be terrestrial equivalents of the younger substages of stage 7. Extensive pre-Eemian marine sediments along the SW coast of Denmark previously correlated with the Holsteinian are shown to be of ‘Late Cromerian’ age. The underlying till there is the first widespread evidence of a pre-Elsterian till in NW Europe.DL ratios in molluscs from last interglacial sites along the Arctic coast of the USSR, the Arctic Islands and eastern Greenland are substantially lower than in their European counterparts due to their low thermal histories. The combined mid- and high-latitude data are used to develop a predictive model for the expected DL ratio in any of several moderate epimerization-rate taxa for last interglacial sites with mean temperatures between −20 and +15°C.Not all sites could be unambiguously assigned to an established interglacial. The Fjøsanger (Norway) and Margareteberg (Sweden) sites previously thought to be Eemian, yield DL ratios higher than in secure nearby Eemian material. It is yet unresolved whether these are aberrant sites or if they predate the last interglacial. In situ shoreline deposits encountered in borings in SW Belgium and in exposures on the Belgium coastal plain contain molluscs that yield DL ratios intermediate between secure Eemian and Late Weichselian ratios, raising the possibility that a late stage 5 high-sea-level event attained near-modern levels in the southern North Sea basin. Resolution of these uncertainties is the focus of future work.

Some Petroleum Engineering Considerations in the Changeover of the Rough Gas Field to the Storage Mode

This article presents a reservoir engineering case history of the Rough field development. The role of the field in the supply/demand match is described and the reliability and production requirements are given. The modeling of surface facilities in the development of an overall scheme for a gas storage field is outlined. Finiteelement reservoir modeling techniques are used. The main results of the reservoir simulation are presented and include the field production/injection profile.

CARBONIFEROUS OIL AND GAS PRODUCTION IN THE EASTERN HEMISPHERE ‐ 1

Almost all production from Carboniferous formations in the Eastern Hemisphere comes from the Soviet Union, with less importantproduction from the United Kingdom, the Southern North Sea basin, the N W German basin (from the northern Netherlands to Poland), Algeria, Ghana, the Gulf of Suez, China and Australia. However, Carboniferous coals extending from the eastern part of Great Britain to Poland were the source of the hugegas reserves that are found in Permian and Triassic reservoirs in the Southern North Sea‐N W German basin complex. Production of oil during 1978 was approximately 2,680,000 b/dt and of gas, about 20.8 Bcfldf. The USSR share was about 2.6 MM brl and 5.2 Bcf per day. Carboniferous production, which peaked in 1975, has since declined steadily.Most of the USSR's Carboniferous production is from marine carbonates andfrom marine to deltaic sandstones in the Volga‐ Urals, Timan‐Pechora and Dnepr‐Donets basins. The latter basin is mainly gas productive. Smaller amounts of hydrocarbons arepresent in marine beds of the North Caspian (Pricaspian) basin of the European USSR and in similar strata of the Chu‐Sarysu basin of Kazakhstan. Some small, but significant, discoveries have been made in carbonates of the southern part of the West Siberian basin. Although the deep Khuff gas of the Middle East is probably mainly of Permian origin, some Late Carboniferous source materials could have generated part of the gas, inasmuch as the base of the Khuff is probably Late Carboniferous.Hydrocarbons in terrigenous Carboniferous rocks in Egypt are believed to have originated in non‐Carboniferous beds; the petroleum in the Carboniferous sandstone reservoirs of Ghana, China and Australia is generally indigenous, and is non‐marine to deltaic. Oil and gas in the Carboniferous strata of Algeria are in marine and deltaic beds and are partly indigenous. The huge Carboniferous gas reserves in Permian, Triassic and, to a lesser extent, Carboniferous reservoirs of the Southern North Sea‐NW German basin are of non‐marine origin. The source materials were the Upper Carboniferous coals of the same area.The volume of produced Carboniferous oil is about 23 B brl, 21 B brl of it from the Volga‐Urals basin alone. About 63 Tcf of gas has been produced, 51 Tcf from the Southern North Sea‐NW German basin. This volume of gas includes Carboniferous gases in Permian and Triassic reservoirs. Proved and probable reserves of Carboniferous oil are about 13 to 14 B brland 85 Tcf, of which more than 50 Tcf remains in the Netherlands “giant Groningen Barents Sea extensions of the Timan‐Pechora basins, and ‐just possibly ‐ in several Chinese basins. The Arabian Gulf(Persian Gulf) also has a very largegas potential, but thegas, mainly in the Khuff, is of unknown origin and may wellnot be Carboniferous. (The Chu‐Sarysu basin, USSR, could also be important.)

Structural outline and development

The Central Graben is a broad, complex trough with a long history of differential subsidence. It was probably initiated in the Permian and was controlled by major rifting during the Mesozoic. To the south in the Dutch sector the trough is divided into two parts. From here it passes northwards and divides the southern North Sea Basin into the Anglo-Dutch Basin and the Northwest German Basin. It also separates the Mid North Sea High from the Ringkøbing-Fyn High. These highs form broad, east-west trending, relative stable ridges. The further continuation of the Central Graben is to the northwest, towards the centre of the North Sea, where it passes into the Viking Graben and the Moray Firth Basin at about 58° N. Where the Central Graben divides the two major highs, there is an elongate central narrow horst, the Dogger High, which is the southernmost of a row of mid-Graben highs. Both sides of the Graben are clearly defined by normal rotational faults that were intermittently active from Triassic to Early Cretaceous times.

Clay mineral diagenesis in Rotliegend aeolian sandstones of the southern North Sea

AbstractThe distribution of diagenetically-formed clay minerals in Rotliegend dune sandstones of the Southern North Sea Basin is closely related to the following factors: (i) the petrography of the sandstones, (ii) the paleoburial depth and tectonic setting of the area, (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes, and (iv) the facies distribution of the overlying Zechstein. The diagenetic clay minerals are mainly conversion products of feldspars and, to a lesser extent, of detrital clays and micaceous lithic fragments. Sandstones containing dominant drusy illitic and chloritic clay minerals have been buried to depths > 3000 m; if kaolinite is the dominant clay mineral, burial depths were less. Sandstones containing feldspars (detrital and authigenic) up to approximately 7% of bulk volume have permeabilities that are about four times higher than sandstones with similar amounts of kaolinite, and as much as 200 times higher than sandstones with similar amounts of illite and/or chlorite.

Lithostratigraphy and depositional history of the Danish Triassic

The Triassic deposits of the Danish territory are mapped, described and characterized by means of wire line log motifs. Three facies provinces are recognized: A southern and central Germano-type Facies Province, a Northern Marginal Facies Province fringing the basin center, and a Central Graben Facies Province with affinities to the Southern North Sea Basin. The traditional German lithostratigraphic nomenclature previously used in the Germano-type Facies Province is proposed replaced by a system composed of four groups each of two formations corresponding to four Triassic megaphases of sedimentation: Bacton Group including Bunter Shale Formation and Bunter Sandstone Formation, Lolland Group (new) including Ørslev Formation (new) and Falster Formation (new), Jylland Group (new) including Tønder Formation (new) and Oddesund Formation (new), and Mars Group (new) including Vinding Formation and Gassum Formation. In the other facies provinces the nomenclature previously proposed for the Central and Southern North Sea is adopted. A summary of the basin evolution is given for each formation description.

SUBSIDENCE AND UPLIFT IN EAST YORKSHIRE AND LINCOLNSHIRE: A DOUBLE INVERSION

Summary The land ward part of the Mesozoic southern North Sea Basin in eastern England includes two major structural units, the Cleveland Basin of north-east Yorkshire * and the East Midlands Shelf. The Market Weighton Block north of the Humber is the northernmost sector of the Shelf. Each of these units continues offshore, the Cleveland Basin linking with the Sole Pit Trough, a Jurassic and Lower Cretaceous basin which was inverted in the late Cretaceous, and the East Midlands Shelf continuing as far east as the Dowsing Fault belt. The Market Weighton Block terminates eastwards beyond the coast some 80 km east of the Jurassic outcrop. Differential subsidence of the Cleveland Basin relative to the East Midlands Shelf took place in the Lower and Middle Jurassic. During this time the Market Weighton Block was relatively more buoyant (frequently marked by shoal areas and deposition of thinner sediments) than the more southerly parts of the East Midlands Shelf, and true uplift with erosion of accumulated sediments took place at intervals, specifically in the late Pliensbachian, Aalenian/Lower Bajocian and Callovian. During the earlier Upper Jurassic the regional relationship changed and the Cleveland area functioned as a stable shelf, with attenuated and shallow water deposits, while the East Midlands Shelf (with the Market Weighton Block) continued to subside normally, with Oxford and Ampthill Clays deposited in deeper water than the equivalents in Cleveland. Major uplift and erosion preceded Albian (Red Chalk) deposition over the northern part of the East Midlands Shelf, with a culmination in the Market Weighton sector. In this and the earlier phases, southward tilting extended some 120 km southwards across the Market Weighton Block into Lincolnshire. Subsequently the East Midlands Shelf retained in part its Upper Cretaceous cover, while the Cleveland Basin was uplifted, gently folded and extensively eroded, a phase of inversion relative to the Shelf, similar on a smaller scale to that of the Sole Pit Trough. On land, deep boreholes show a comparable earlier history. The Cleveland area coincides with a Lower Carboniferous basin which was extensively eroded before the Permian, whereas the East Midlands Shelf shows block-type Lower Carboniferous with thin Upper Carboniferous but still retains its Coal Measures cover, an inversion analogous to that of the Mesozoic.

La formation d'Herzeele : un nouveau stratotype du Pléistocène moyen marin de la mer du Nord

The Herzeele Brickyard (Northern France) offers a permanently well exposed outcrop of continental and marine deposits which are reaching a total thickness of 6 m and are overlying the Ypresian clay of eocene age, occuring at about 8 m N.G.F. Three marine phases represented by tidal flat and brackish sediments may be readily distinguished. The tidal flat sediments have previously been recognized in the area of Izenberge (Belgium) from which locality the name "Izenberge Cardium Sands" has been derived. The marine sediments may be subdivised into units of different lithology : the lower sandy unit, the middle loamy-clayey unit and the upper sandy clayey unit which latter is characterized by the abundance of Cardium edule and Macoma balthica. The marine phases are furthermore separated by continental deposits amongst which peat layers are occurring. The cover sediments are represented by eolian sands and loams interfering with palaeosoils. The series of sediments underlying these cover deposits is named the "Herzeele Formation" which represents a hthostratigraphical unit in the southern North Sea basin. The analysis of the heavy mineral content points to a change in sediment origin occuring after the lower marine sedimentation. The middle and upper marine units contain an increasing content of green hornblende and epidote. Some volcanic minerals were observed at different levels. The clay analyses by means of X-ray diffraction indicate that the different lithostratigraphical units bear polymineralic clay assemblages within which the smectitic fraction is predominant. Greene-Kelly's (1953) Li-test yielded a further detailed analysis of the smectitic components : beidellite, montmorillonite-like minerals as well as random mixed-layers illite-smectites. The analysis indicates a stratigraphical - mineralogical subdivision of the section which coincides with the lithostratigraphical one. The pollenanalytical data show that the whole of the Herzeele Formation most probably belongs to the second half of the Holsteinian interglacial, except for the very base composed of glauconiferous sands. It may readily be seen that the forest evolved from a mesocratic phase, characterized by a Quercetum mixtum with Picea, towards a telocratic phase dunng which the forest became dominated by Abies. Within the peat which is resting upon the glauconiferous sands, the pollen grains of Taxus, are very abundant in the overlying clay however, this species declines gradually and disappears at the top of the upper marine unit. These evidences are corresponding with the first half of the so-called Abies- zone. Buxus and Vitis, both undergo the same evolution. The only Tertiary relict, Pterocarya, made a short appearance at the top of the upper manne unit, while Azolla filicubides, was only discovered in the Quercetum zone. The diatom analyses indicate the conditions of brackish and marine sedimentation in a tidal environment. Although the magnetostratigraphical approach of deep-sea and lake sediments has proven to be successful, its application to continental and especially coastal sediments is hampered by the very nature of these sediments (large variation of sedimentation rate, depositional environment, lithology, a.o.) The palaeomagnetical study of the Herzeele Formation reveals a striking difference in magnetic behaviour between the upper and lower beds separated by the lowermost peat layer. The upper beds are characterized by a strong dispersion of the magnetisation directions and a low intensity. Therefore identification of clear-cut magnetozone(s) is not possible for the moment being. As to the extension of the Herzeele Formation in Belgium, it was only found south of the river Yzer. The comparison with the area of Izenberge itself did not reveal any lithostratigraphical correlation with the Herzeele Formation. The situation of the stratum (former shoreline) at this type-locality gives prove of the importance of the palaeographical evolution since the Lower Pleistocene with the formation of the southern North Sea basin and probably the early opening of the Strait of Dover as well. Therefore this stratum is considered as a landmark and a witness of several interglacial marine transgressions which are attributed to the Holsteinian and to the upper part of the "Cromerian complex".

TheNorthSea-ANe\v Major Oil Province In a Changing World

Abstract Current exploration in the North Sea is described and latest estimates of the reserves of oil and gas are given. The field development program is reviewed and forecasts of production presented. The principal economic constraints are identified and examined against the recent position taken by tile UK Government on tax and participation. Events during the past few years have combined to make the oil developments in the North Sea, paticularly interesting. Technically, the deep and hostile water of this latest major new oil province have demanded new solutions for new problems. Economically, the incidence of runaway inflation on high-cost development at a time when massive new taxes have been introduced to take advantage of high oil prices has created unique financial stresses. Politically the impact of the development on industrial countries has bud considerable social and national repercussions within a short period. History SOME 6,000 exploration wells were drilled onshore in Western Europe during the period 1920–59, resulting in a maximum total production of only 250,000 bbls/ day and few important gas discoveries. There was little basis, therefore, for anticipating spectacular hydrocarbon accumulation. In 1959 however, the giant gas field of Slochteren was discovered in the Netherlands, in a geological setting which immediately suggested that similar prospects might lie under the North Sea. The Southern North Sea basin was the first to be explored, and has subsequently proved to contain large gas reserves in the Permian sandstones similar to those of Slochteren. Five commercial gas fields have been put on production and eighteen other gas strikes have been made in UK waters. Not all are of commercial proportions, but none can apparently be developed, largely due to the low prevailing gas price negotiated with the monopoly buyer, the British Gas Corporation. Several gas fields have been found off the Netherlands and one of these is currently being developed. The high continental prices will encourage further development here. So far, 379 exploration and appraisal wells have been drilled in the southern part of the North Sea, including Danish waters, with a success ratio of approximately 1 in 20. The Northern North Sea is predominantly an oil prospective region of younger geological age and now appears to be one of the most prolific areas, for its size, in the world. The basin covers both Norwegian and UK offshore acreage and so far exploration has been limited to the area south of latitude 62"N. There are three major types of prospect in this basin. In order of increasing age, the first and youngest prospect is in the Lower Tertiary sandstones, of which the Forties Field is an example. The second prospect (although the first discovered) is in the domed and fractured Upper Cretaceous and Danian limestone, typified by the Ekofisk Field. The third major prospect, and the most important, is in the. Jurassic sandstones in the northern part of the basin. as evidenced by the Brent group of oilfields.

Review of North Sea Basin development

The large amount of data now available on the North Sea stratigraphy, and on the history of the positive structures which form the sites of the oilfields, provides a basis for a review of basin development. The area was initially occupied by two inter-cratonic basins: the Northern Basin had a Devonian ancestor; the Southern Basin dates back at least to the Carboniferous. During the Permian and Trias broad intercratonic subsidence continued in the south, but rifting developed across the separating Mid North Sea High and in the Northern Basin. Rifting (taphrogenic) control of subsidence became widely dominant through the Jurassic and lower Cretaceous, dying away in the upper Cretaceous, with major subsidence in the Viking Graben in the Northern Basin and in a series of narrow troughs in the south. No opening of the crust was associated with the central rifting: deep penetrations have encountered Devonian or Precambdan in the rift floor. For the most part the upper Cretaceous sagged into the earlier depressions with minor faulting only, but in the south inversion of the troughs took place in late Cretaceous and early Tertiary times; this may have had a compressional (orogenic) cause. The Tertiary basin developed as a single, relatively simple synclinal subsidence of the whole North Sea area, centred on the main rift system but showing an absence of fault-control. Interruptions in deposition on the individual structural high areas show wide correspondence in the North Sea. Except for a major late Triassic movement limited to the Netherlands and adjoining areas they show also close relation to those in Britain. The most notable are early middle Jurassic, early upper Jurassic and end Jurassic/lower Cretaceous. These produced erosional phases even in the deeper parts of the basin. They were entirely of epeirogenic type. Halokinetic movements of Permian salt produced many local complications (especially in the Southern North Sea Basin) with consequent structural and stratigraphical anomalies from the middle Trias onwards.

The Rotliegendes sandstone formation of the United Kingdom sector of the southern North Sea basin : Blanche, JB N.C.B. (Exploration) LTD. London, GB 3F, 12R. Trans. Instn. Min. Metall. Sect. B, V82, N801, 1973, P85–89

The Rotliegendes sandstone formation of the United Kingdom sector of the southern North Sea basin : Blanche, JB N.C.B. (Exploration) LTD. London, GB 3F, 12R. Trans. Instn. Min. Metall. Sect. B, V82, N801, 1973, P85–89

Triassic stratigraphy of the southern North Sea Basin

Intensive drilling activity, particularly in the United Kingdom part of the North Sea, has greatly increased our knowledge of the subsurface geology of north-west Europe. The Triassic, perhaps more than any other system, has provided results of a special interest. In this paper a stratigraphical study of the Triassic System of the southern North Sea basin has been undertaken by lithological and palynological analysis. The type Germanic Triassic and the dated Alpine Triassic are briefly reviewed. The facies development of the Germanic Triassic is traced by means of subsurface data from north-west Germany and the northern Netherlands into the North Sea. Further studies of the onshore Triassic of England from subsurface well data and surface outcrop sections are discussed. The results, particularly those from wells in the North Sea, provide a lead to the understanding of the position of the English Triassic of Keuper and Bunter within the framework of the Germanic Triassic.

OUTLINE GEOLOGY OF THE SOUTHERN NORTH SEA BASIN

Summary The address outlines the geological results of exploration for natural gas in the southern North Sea Basin as derived from geophysical surveys and deep drilling operations. The greater part of the marine area is stratigraphically comparable with the landward continuation of the basin in northern Germany. Marine Tertiary is well developed; the Trias includes a representative of Muschelkalk and the Permian is bipartite, with a Zechstein including thick evaporites overlying the Rotliegendes. True folding is very gentle, deep-seated folds and faults trending predominantly north-west–south-east (in the west) and north–south (in the east). A major phase of movement occurred in pre-Aptian times, resulting in widespread transgression of late Cretaceous on to early Jurassic and Triassic rocks. The dominant tectonic feature is the widespread development of halokinetic structures due to movement of Zechstein salt, the forms varying from pillows to diapiric salt plugs. This movement began in the Triassic and continued through Jurassic, Mesozoic and Tertiary times.