Southern North Sea Basin Research Articles

The Indefatigable Field, Blocks 49/18, 49/19, 49/23, 49/24, UK North Sea

Abstract The Indefatigable Field was discovered in June 1966 by the Amoco Operated Group well 49/18-1. The field is located 55 miles northeast of Great Yarmouth in the central part of the Southern North Sea Basin in water depths of 100 ft. The field lies in Blocks 49/18, 19 and 49/23 and 24. The reservoir section is Lower Permian Rotliegendes sandstones comprising a mixed sequence of aeolian and fluvial sediments at an average depth of 8500 ft subsea. Maximum reservoir thickness is in excess of 420 ft; however, the average pay thickness is of the order of 150 ft. The main reservoir trap is a complexly faulted NW-SE-trending anticlinal horst with a maximum vertical closure of 1300 ft. The field is approximately 17 miles long and 6 miles wide and has an areal extent of some 60 square miles. Recoverable reserves in the Field are estimated to be 4418 BCF of gas. The field is not unitized and is jointly operated by Amoco and Shell.

Carboniferous stratigraphy, sedimentation and correlation of well 48/3-3 in the southern North Sea Basin: integrated use of palynology, natural gamma/sonic logs and carbon/sulphur geochemistry

A methodology for the correlation of offshore Silesian (Upper Carboniferous) sequences is developed using an integrative approach based upon biostratigraphy, downhole logging and geochemistry. We develop the combined use of miospore zonation, gamma ray spectrometry, formation velocities and carbon/sulphur ratios to determine the location of marine horizons and other lithofacies in a thick (>1 km) largely uncored Silesian sequence in the Southern North Sea Gas Basin. At least nine major marine horizons are recognized within a basin-fill sequence of Marsdenian, Yeadonian and Langsettian ages (Namurian B/C and Westphalian A) in the well. These horizons help to provide a detailed subdivision of the offshore sequence, although the correlations must remain tentative until further deep wells are drilled in the area. The implications of the correlations for offshore palaeogeography are that the area of northern Quadrant 48 was a deep, periodically anoxic basin until infill by successive submarine fan lobes in late Marsdenian times. Yeadonian delta front and coastal plain deposits pass upwards into fluvio-deltaic/wetland facies of the Langsettian.

A buried granite batholith beneath the East Midland Shelf of the Southern North Sea Basin

Published BGS gravity data of the UK Southern North Sea Basin show the presence of a gravity low trending to the ESE across the northern half of Quadrant 47. Two simplified geological cross-sections have been constructed across this feature with sedimentary structure based upon well information and reflection seismic data. Gravity modelling of these sections demonstrates that the gravity anomaly is unlikely to be produced by the presence of thick, low density sediments. Consequently, the presence of a large, buried granite batholith is postulated. The buoyant influence of such a granite may have exerted considerable tectonic control during the development of the Southern North Sea Basin. In particular, its presence may help to explain the development of the East Midland Shelf, the associated complex fault movements along the Flamborough Head and Dowsing Fault Zones, and the areal distribution and thickness of the Rotliegendes sand.

Carboniferous geology of the Southern North Sea Basin and controls on hydrocarbon prospectivity

Abstract The thick sequences of Upper Palaeozoic strata present in the Southern North Sea Basin were probably deposited on a basement of Caledonian low-grade metamorphic rocks intruded by numerous late-Caledonian granitoids which form part of the subsurface extension of the Mid-European Caledonides linking the English Lake District and Northern Pennines to the Ardennes. This basement, along with the better known onshore areas of the British Isles and Ireland was intensely fragmented during late Devonian to mid-Carboniferous times by a major phase of extensional tectonics. Active fault-bounded tilted blocks developed in the Dinantian with deposition of fluviatile redbeds, marine carbonate/deltaic clastic cycles and deltaic coastal plain facies. These are penetrated by a number of wells on the southern flanks of the Mid-North Sea High, but details of basin geometries and facies distributions remain unclear. Crustal stretching appears to have given way to thermal subsidence by early to mid-Namurian times. This was coincident with a major climatic change, probably forced by glacial expansion in Gondwanaland. The major drainage system that resulted dominated sedimentation in the SNSCB, punctuated by numerous marine transgressions of undoubted glacio-eustatic origins. Within this broad depositional framework, Namurian facies were closely controlled by basement tilt block topography, with good evidence for a gradual southerly migration of the active fairways of fluviodeltaic sandbodies. Westphalian facies were dominated by the interplay between glacio-eustatic and sedimentary processes on a vast low-gradient alluvial plain with much less pronounced tectonic control. By late-Westphalian B times a phase of basin inversion caused the gradual elevation of the area as a retro-arc foreland basin. Vadose diagenesis and pedogenesis occurred in the fluviatile facies of the so-called Barren Red Measures (BRM). With continued compressive deformation a series of over 50 NW-SE orientated folds, some associated with thrusts, developed. Significant erosion and truncation of these inversion anticlines was accompanied by the deposition of the BRM facies in characteristic growth fold basins, with marked onlap around the basin margins. The Carboniferous play in the SNSCB has been greatly influenced by the subsequent Mesozoic/Cenozoic geological history of the basin, including periods of subsidence and uplift. We demonstrate that variations in original detrital mineralogy and various diagenetic processes have exerted significant controls upon Carboniferous reservoir quality.

Late Ryazanian black shales in the Southern North Sea Basin and their lithostratigraphical significance

SUMMARY The basal beds of the Speeton Clay Formation in Yorkshire and BGS Offshore Borehole 81/43, Beds E and D8 of late Ryazanian age, are in poorly fossiliferous, carbonate-free, organic-rich Kimmeridge Clay type facies. Above Bed D8 there is a rapid transition to carbonate-rich sediments with a lower organic carbon content and increasingly diverse faunal and palynofloral assemblages. In the Northern and Central North Sea basins similar coeval facies changes are represented by the Kimmeridge Clay Formation/Valhall Formation boundary.

Borehole seismic surveys for fault delineation in the Dutch North Sea

A significant share of gas production in the Dutch sector of the southern North Sea Basin comes from Permian Rotliegend fault blocks. Precise knowledge of the positions of these faults is necessary for efficient exploitation of the reservoir structures and for future field development strategies. Two areas are presented where the lateral resolution of the surface seismic data was not sufficient to determine positions of major fault block boundaries accurately. Walkaway borehole seismic profiles were shot over each of these areas with the objective of illuminating the fault boundaries to obtain an image with a higher resolution. The images were generated using borehole seismic reflection‐point mapping and migration techniques. Large‐aperture migrations tend to produce unacceptable migration smiles, while reflection‐point mapping is a model‐dependent process. A hybrid approach to these processes was necessary to avoid problems associated with the limited angular illumination permitted by the field acquisition geometries. Reliable images of the fault boundaries were obtained using migration apertures of less than ±5° relative to the structural dips in the background model and by matching that model with the surface seismic and borehole seismic data. The stability of the process and, therefore, the accuracy of the lateral positioning were verified by testing the migration process using a range of apertures.

Layer cake depth conversion, Part I

In areas of uniform lithology which have not undergone tectonic inversion or severe structural deformation, depth conversion can often be performed according to some simple mathematical function (e.g., average velocity as a linear function of depth). In areas where there are dramatic changes of velocity, such as thick carbonate or evaporite units alternating with thick clastic units (as those found in the southern North Sea basin), then simple functions cannot adequately cope with the inherent velocity inversions and are inadequate for depth conversion.

Quantitative Analysis of North Sea Subsidence

Stratigraphic and geophysical maps of the northern and southern North Sea basin are used to study mechanisms of intracratonic basin formation. These maps include (1) tectonic subsidence for ten stratigraphic units, (2) predicted and observed sediment overpressure, (3) geographic variation of compaction constants, (4) net basement fault extension, (5) free-air gravity, (6) flexural rigidity, (7) heat flow, and (8) predicted hydrocarbon maturation. In northern areas of the North Sea, observed basement-fault extension, crustal thinning, present-day flexural rigidity, and tectonic subsidence are predicted reasonably well by a simple stretching model of basin formation with rifting phases during the Triassic and mid-Jurassic/Early Cretaceous. In southern areas of the North Sea, however, a similar analysis suggests that other non-extensional rift-related processes dominated basin evolution. The effects of anomalously active aesthenospheric convection, coupled with basalt-eclogite and deep-crustal dewatering reactions, are considered as possible additional mechanisms. The troughlike form of Quaternary tectonic subsidence suggests a new phase of rifting may be starting in the North Sea. The importance of such thermal rejuvenation is borne out by the association of areas of differential subsidence with high heat flow and low flexural rigidity. One component of tectonic reactivation during this period may be due to thermally induced isostatic reequilibration of older tectonic loads.

Sedimentation in an ice-contact subaqueous setting: The mid-Pleistocene ‘North Sea Drifts’ of Norfolk, U.K.

In East Anglia, the mid-Pleistocene ‘North Sea Drifts’ extend over some 1500 km 2 of the coastal zone of North Norfolk. They are the oldest Pleistocene glacial sequences exposed in Britain, deposited during the Anglian Glaciation (ca. 450 ka BP?). Laminated and stratified pebbly mud (diamict) facies of the North Sea Drifts are widely regarded as the product of subglacial deposition below a terrestrial ice margin. In this paper these facies are identified as the product of subaqueous deposition in front of the margin of an ice sheet terminating in a large water body. Laminated, stratified, and massive diamict facies were deposited by the ‘rain-out’ of fine-grained sediment from suspension and coarse debris from floating ice, combined with downslope sediment gravity flow. Intraformational folds are the product of episodic downslope slumping and identify a regional paleoslope to the east, away from a major ice marginal moraine ridge (The Cromer Ridge). Exposures at Cromer show bulldozed ice-contact subaqueous sediments and ice-thrust chalk rafts. A fall in water-level emplaced shelly shoreface sands and beach gravels which are loaded into the underlying diamicts as large pan-shaped ‘sag basins’ with diapiric structures at their margins. Comparable sedimentary sequences are exposed elsewhere around the southern North Sea Basin; a glaciomarine setting in response to glacio-isostatic depression around the Anglian ice sheet is suggested.

Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin

Summary Upper Carboniferous sediments from the central part of the southern North Sea Basin were deposited on a broad fluvio-deltaic plain. Potential reservoir rocks occur within distributary channel facies sandstones, but porosity and permeability have been reduced by authigenic mineral growth and improved by at least two separate phases of porosity enhancement: firstly by the infiltration of meteoric waters during post-Carboniferous uplift, and secondly by the action of acidic fluids generated during burial. The depth of invasion of meteoric fluids is highly variable, ranging from a few metres to hundreds of metres, and the best porosity is preserved in sediments which have undergone both phases of porosity enhancement. Virtually all the observed porosity in Carboniferous sandstones can be interpreted as having a secondary origin. Integrating the deduced diagenetic history with a burial and temperature history for specific wells allows the timing of diagenetic events to be estimated.

Structural evolution, timing and tectonic style of the Sole Pit inversion

Abstract The NW-SE oriented Sole Pit High, located in the western half of the Southern North Sea Basin, is the result of three different structural processes initiated during the Late Palaeozoic. Basin subsidence, probably related to Late Hercynian pull-apart movements, led to continuous sedimentation from the Permian to the Middle Jurassic. Mid-Jurassic doming of its eastern flank, followed by erosion, transtensional movements and differential subsidence until the Early Cretaceous, reflect the Mid and Late Kimmerian crustal distension and fragmentation of the North Sea Rift. Basinwide inversion occurred between the Turonian and the Campanian and was repeated in the Oligocene resulting in a total uplift of some 1500 m. The present structural style is represented by a complex network of transpressional and transtensional faults at Permian Rotliegend levels, which are thought to represent the upward termination of a series of wrench-induced flower structures with a right-lateral sense of displacement. These faults are detached by Zechstein and Triassic salt from more orderly transtensional faults systems at shallower levels. In areas where salt is absent Late Kimmerian normal faults became reactivated in a reverse sense during inversion. However, the relationship of these shallow mappable tectonic components to the deeper pre-Permian structure of the Sole Pit High and to the underlying crustal configuration remains speculative.

A review of the latest Triassic succession in the U.K. sector of the Southern North Sea Basin

SUMMARY Well-log criteria are used to redefine the base of the Late Triassic ‘Winterton Formation’ in the U.K. Southern North Sea Basin. In a cored offshore succession the redefined formation is lithostratigraphically and biostratigraphically correlatable with the Penarth Group of onshore U.K. sequences. It is proposed that the use of the term Penarth Group is extended to the offshore area and that ‘Winterton Formation’ is abandoned.

THE “ALBIAN TRANSGRESSION” IN THE SOUTHERN NORTH SEA BASIN

The “Albian transgression” may be recognised lithostmtigraphically in a number of boreholes 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 “Albian transgression” is a regional event in NW 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).

Structural evolution, timing and tectonic style of the Sole Pit inversion

The NW-SE oriented Sole Pit High, located in the western half of the Southern North Sea Basin, is the result of three different structural processes initiated during the Late Palaeozoic. Basin subsidence, probably related to Late Hercynian pull-apart movements, led to continuous sedimentation from the Permian to the Middle Jurassic. Mid-Jurassic doming of its eastern flank, followed by erosion, transtensional movements and differential subsidence until the Early Cretaceous, reflect the Mid and Late Kimmerian crustal distension and fragmentation of the North Sea Rift. Basinwide inversion occurred between the Turonian and the Campanian and was repeated in the Oligocene resulting in a total uplift of some 1500 m.The present structural style is represented by a complex network of transpressional and transtensional faults at Permian Rotliegend levels, which are thought to represent the upward termination of a series of wrench-induced flower structures with a right-lateral sense of displacement. These faults are detached by Zechstein and Triassic salt from more orderly transtensional faults systems at shallower levels. In areas where salt is absent Late Kimmerian normal faults became reactivated in a reverse sense during inversion. However, the relationship of these shallow mappable tectonic components to the deeper pre-Permian structure of the Sole Pit High and to the underlying crustal configuration remains speculative.

The depositional environment and petroleum geochemistry of the Marl Slate-Kupferschiefer

Summary The Marl Slate is the basal unit of the Upper Permian (Zechstein) in northeast England. A finely laminated, organic-rich, silty dolomite/dolomitic siltstone, it is equivalent to the Kupferschiefer of the Southern North Sea basin and northern Germany and Poland. Several lines of evidence indicate the Marl Slate/Kupferschiefer accumulated in the anoxic bottom waters of a shallow (< 200 m), stratified, epicontinental sea. The vertical succession of the first Zechstein cycle is explicable in terms of the effect on sedimentation of a stratified water column undergoing progressive evaporitic concentration. Total organic contents of the Marl Slate generally fall in the range 5–15%, decreasing upwards. The kerogen is largely amorphous and oil-prone. Original pyrolysis yields approach 100 kg hydrocarbon/tonne rock. It is immature in northeast England but even where mature in the Southern North Sea basin and Northern Europe the unit is unlikely to have sourced commercially significant amounts of oil because of its limited thickness (generally <2 m).

The stratigraphy of the Lower Cretaceous Speeton Clay Formation in a cored borehole off the coast of north-east England

SUMMARY A fully cored stratigraphical borehole in the Southern North Sea Basin at 54°39′N, 0°14′E encountered a succession of Lower Cretaceous clays and mudstones, representing part of the Speeton Clay Formation of the Cromer Knoll Group, resting on Upper Jurassic Kimmeridge Clay. The Lower Cretaceous beds comprise a basal phosphatic pebble-bed (equivalent to bed E, the Coprolite Bed, at Speeton) overlain by c.90m of marine clays ranging from Ryazanian to Barremian (equivalent to beds D8 – LB6 at Speeton) and include seven thin bentonite horizons. The three uppermost bentonite horizons lie within the Valanginian to Hauterivian, an interval from which no bentonites have previously been recorded in the European area. Six of the seven bentonite horizons recorded in the borehole have now been recognised in the type section at Speeton, Yorkshire. The borehole succession yielded rich microfaunal assemblages, which have enabled a detailed correlation to be made with the beds of the type locality at Speeton. The sequence cored in the borehole demonstrates a remarkable similarity to the Speeton sequence, despite an 80-kilometre separation of the localities.

Diagenetic carbonate and evaporite minerals in Rotliegend aeolian sandstones of the Southern North Sea: their nature and relationship to secondary porosity development

AbstractDeeply buried (> 3·5 km) Rotliegend aeolian sandstones in the Southern North Sea Basin display a number of interesting diagenetic features including (i) zoned iron-rich carbonate cements, (ii) anhydrite, halite and baryte cements, (iii) at least two generations of authigenic illite, and (iv) significant secondary porosity created by cement and framework-grain dissolution. The creation and destruction of secondary porosity is the result of changes in porewater chemistry during burial and subsequent uplift. Three pore-fluid regimes can be identified: (1) alkaline, oxidizing conditions during shallow to intermediate burial; (2) acid, reducing conditions during intermediate to deep burial; (3) alkaline, reducing conditions during deep burial and uplift. The transition from stage 1 to stage 2 was probably caused by expulsion of waters from the underlying Carboniferous shales. The transition to stage 3 probably began when faulting associated with uplift allowed invasion by alkaline fluids derived from Zechstein sediments.

Unusually large Late Campanian – Early Maastrichtian foraminifera from the Southern North Sea Basin

Abstract. A low diversity fauna of unusually large (individuals of two to three times the normal size) agglutinated and calcareous benthic foraminifera is recorded from the Late Campanian – Early Maastrichtian Chalk sequences of two boreholes (Shell 49/19.1 and Shell 49/20.2) in the Southern North Sea Basin. Associated planktonic species are extremely rare. This distinctive assemblage provides a useful local stratigraphical marker.

The nature and origin of fissures in the East Anglian Coralline and Red Crags

AbstractVertical or inclined planar fissures that may for the most part be classified as joints are commonly seen in exposures of both the Coralline and Red Crag in East Anglia. Measurements of fissure orientation reveal orthogonal patterns of alignment in both formations. The field relations of the fissures to the host Crag sediment suggest a tectonic rather than periglacial origin. It is proposed that the fissure system is the product of early Pleistocene tectonic flexuring in the area, on the western margin of the subsiding southern North Sea basin.

Zechstein 1 and 2 Anhydrites: facts and problems of sedimentation

Summary The distribution and lithology of the anhydrite rocks of Zechstein 1 and Zechstein 2 were significantly influenced by the contemporary geography of the basin. Thick ‘pletholitic’ shallow-water deposits with large-scale synsedimentary derangements lie near the margins and pass laterally, with steep boundaries, into thin laminated deep-water rocks on the basin floor. Five sub-cycles (I–V) have been recognized in cored boreholes in the Z1 Werraanhydrit throughout the German Zechstein basin and permit correlation with the Zechstein 1 sequence of the southern North Sea Basin; although not complete in every place, each sub-cycle generally comprises flaser anhydrite overlain by laminated anhydrite. The abrupt changes of facies at the contacts between the sub-cycles cannot be attributed to eustatic sea-level changes but are caused by changes of the climate, water currents and related effects; the periodicity must have resulted from factors originating outside the basin. The following new adjectives are proposed, primarily for varieties of anhydrite although they may prove to be equally applicable to other sedimentary rock types: ‘Pletholitic’—from the Greek ‘plethos’, an indefinable mass—applicable to thick anhydrite rock with a diffuse, cloudy or chicken-wire structure and fabric but with little overall stratification. ‘Stratobolic’—from the Greek ‘bolos’, lump—anhydrite rock with extremely irregular small-scale and medium-scale bedding but with a strong sense of stratification en masse . This term includes ‘flaser’, which is mainly applied to stratobolic rocks with bedding of an exceptionally anastomosing character. ‘Lamellitic’—from the German ‘Lamellenanhydrit’—unusually uniformly and evenly laminated anhydrite, including some varved anhydrite. This term is proposed because the word ‘laminated’ is used in a range of senses and does not adequately stress the uniformity and continuity of bedding in some thinly and evenly layered anhydrite rocks.