bluuebunny/crossref_metadata_2025 huggingface dataset:
The tectonic history of the East Irish Sea Basin with reference to the Morecambe Fields
Authors: KNIPE R. J. , COWAN G. , BALENDRAN V. S.
Published: April 2010

Regional seismic lines from the East Irish Sea area and a 2D radial seismic survey across the Morecambe Fields have been used to assess fault pattern evolution and the history of tilting in the East Irish Sea Basin. Extension and fault-controlled sedimentation began in Permian times and continued throughout the Permo-Triassic. Later inversion and erosion has removed the post-Triassic cover in the area. The main extension direction during basin evolution was ENE–WSW to NE–SW. Under this extension, NE–SW-trending basin-bounding faults in the north and south were dominated by strike-slip and oblique displacements. Offset of the pre-Permian basement suggests the amount of post-Carboniferous net extension varies from 10–16%. The basin can be divided into two discrete structural domains which controlled the geological evolution of the East Irish Sea Basin. Basin evolution in the northern part of the basin was controlled by easterly dipping faults which induced tilting towards the southwest or west. The southeastern part of the basin is dominated by two faults which dip to the west or southwest, inducing tilting towards the east. The complex structural and tilting history of the Morecambe Fields arises from their position at the boundary of these two domains. Detailed analysis of this area illustrates how the propagation of the Keys Fault (from the north) and the Crosh Vusta Fault (from the south) competed to dominate tilting history. The faults which now bound the Morecambe Fields show only minor influence on Permo-Triassic sedimentation patterns, implying that the main displacement on these faults post-dated the deposition of the Sherwood Sandstone. The development of these boundary faults is thought to be related to the accommodation of arching between the Keys Fault and Tynwald fault system. The differing tilting histories of the North and South Morecambe Fields are indicated by the differing orientations of a previously shared palaeo-hydrocarbon–water contact. This is now preserved as the top platy-illite surface and is interpreted as arising from the development of a tilt-transfer zone during the propagation and linking of the northerly and southerly propagating fault arrays. Analysis of the tilting history of the Morecambe area has allowed the top platy-illite surface to be mapped outside well control.
The geology of the North Morecambe Gas Field, East Irish Sea Basin
Authors: STUART I. A.
Published: April 2010

The North Morecambe Gas Field lies in the East Irish Sea Basin close to the producing South Morecambe Field, from which it is separated by a very deep WSW–ENE graben. It is a shallow structure, crest slightly above −3000 ft, with bounding faults to the south and east, but dip-closed to the north and west. The GWC is at −3950 ft. The Triassic Sherwood Sandstone Group reservoir was deposited under semi-arid continental conditions and comprises a complex interplay of channel-fill sandstones, non-channelized sheetflood sandstones and high-permeability aeolian and reworked aeolian sandstones. The vertical alternation of these facies forms the basis for differentiating nine facies-defined reservoir units. Palaeocurrent analysis indicates flow from NE to SW. A complex diagenetic history is recognized, with several phases of dolomite and quartz cementation. Differential compaction is also a significant control on the disposition of reservoir properties. The greatest control on permeability (but not porosity) is platy illite which precipitated beneath a palaeo-gas/water contact at an early stage in the growth of the structure. This gives rise to a diagenetic layering of the reservoir into a high-permeability illite-free layer and a deeper, low-permeability, illite-affected layer. The Mercia Mudstone Group which provides the seal, comprises four mudstone–halite cycles. Thickening of these cycles on the north flank of the structure indicates very early development of the northerly component of dip. The imposition of northerly dip entirely pre-dates the growth of platy illite, and this conclusion has a major impact on the manner in which the surface which separates the illite layers has been mapped on the north flank. The gas, sourced from Westphalian/Namurian sediments, is enriched in CO
2
compared with South Morecambe. This, together with contrasts in vertical cement distribution between the two fields, indicates that the breaching and loss of the first gas charge which is believed to have occurred on South Morecambe did not occur on North Morecambe. Furthermore, although separated from South Morecambe by only a narrow graben, the aquifer pressure is 140 psia less than that of South Morecambe, indicating that the major basin faults are full seals. Proved reserves are 1.08 TCF. The field is currently being developed for first gas in 1994.
Evolution of North Atlantic volcanic continental margins
Authors: HINZ K. , ELDHOLM O. , BLOCK M. , SKOGSEID J.
Published: April 2010

Structurally, the North Atlantic margins are characterized by Late Cretaceous–Early Tertiary extension, developing predominantly rifted segments south of the Greenland–Senja Fracture Zone, and by oblique extension and wrenching leading to large sheared margin segments further north. The rifted segments document an intense, transient volcanic phase during break-up, emplacing huge extrusive constructions on the outer margin. Commercial exploration has mostly been landward of the shelf edge, but lately there is increased interest in the deep water basins on the outer margins and in the perennially sea-ice-covered shelf off Greenland. Evaluation of these areas requires an understanding of the processes and events prior, during and after the Paleocene–Eocene opening of the North Atlantic. Although the margin is a Cenozoic feature, its segmentation and evolution are governed by the pre-opening history, particularly the Late Jurassic–Early Cretaceous tectonic episode which resulted in large Cretaceous sedimentary basins. Moreover, the first few multi-channel seismic profiles from the NE Greenland inner shelf north of 79°N show a deep Permo-Carboniferous rift basin. The similarity of tectono-magmatic features off NE Greenland and Norway shows the value that conjugate margin studies provide in understanding volcanic margin formation. The events at break-up affected both the subsidence and thermal histories of the marginal basins and were associated with crustal uplift and erosion of the rift region, intrusive activity within the adjacent Mesozoic basins and lava flows spilling onto large areas of the pre-opening crust. The transient magmatism/volcanism had important environmental implications. After break-up, the margin experienced thermal subsidence and little structural deformation, but the extrusive complexes controlled the Paleogene sediment deposition.
Late Mesozoic–Cenozoic evolution of the southwestern Barents Sea
Authors: FALEIDE J. I. , VÅGNES E. , GUDLAUGSSON S. T.
Published: April 2010

Three geological provinces are recognized, separated by major fault zones: the oceanic Lofoten Basin and the Vestbakken volcanic province in the west; the southwestern Barents Sea basin province; and the eastern region which has largely acted as a stable platform since Late Paleozoic times. Since Middle Jurassic times, two structural stages are recognized in the southwestern Barents Sea: Late Mesozoic rifting and basin formation; and Early Tertiary rifting and opening of the Norwegian–Greenland Sea. This evolution reflects the main plate tectonic episodes in the North Atlantic–Arctic break-up of Pangea. Middle–Late Jurassic and Early Cretaceous structuration were characterized by regional extension accompanied by strike-slip adjustments along old structural lineaments, which developed as the Bjørnøya, Tromsø and Harstad basins. Late Cretaceous development was more complex, with extension west of the Senja Ridge and the Veslemøy High, and halokinesis in the Tromsø Basin. Tertiary structuration was related to the two-stage opening of the Norwegian–Greenland Sea and the formation of the predominantly sheared western Barents Sea continental margin. Tectonic activity shifted towards the west in successive phases. The southwestern Barents Sea basin province developed within the De Geer Zone in a region of rift-shear interaction. Initially, oblique extension linked the Arctic and North Atlantic rift systems (Middle Jurassic–Early Cretaceous). Later, a continental megashear developed (Late Cretaceous–Paleocene), and finally a sheared-rifted margin formed during the opening of the Norwegian–Greenland Sea (Eocene–Recent).
Mesozoic to Cenozoic plate reconstructions of the North Atlantic and hydrocarbon plays of the Atlantic margins
Authors: KNOTT S. D. , BURCHELL M. T. , JOLLEY E. J. , FRASER A. J.
Published: April 2010

An integrated approach using plate tectonic analyses and detailed comparative stratigraphy of the North Atlantic has placed new constraints on the Mesozoic to Cenozoic geological history of the Atlantic margin of NW Europe. Key reconstructions from Mesozoic time to the present day have been plotted to show the evolution of the North Atlantic, and in particular the Rockall Trough. The reconstructions show Rockall Plateau attached to Greenland from Late Paleozoic time (380 Ma) to Late Cretaceous time (83 Ma) since when Rockall remained attached to Eurasia. The Rockall Trough probably initiated during end-Carboniferous to Early Permian time and underwent further stretching episodes in the Early Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, Early Cretaceous, mid-Cretaceous and Late Cretaceous to give the present-day Rockall Trough configuration. The Permo-Triassic rift was dominated by oblique opening with a left-lateral component of strike-slip. Jurassic through Early Cretaceous extension was characterized by predominantly left-lateral strike-slip with a minor dip-slip component in the Faeroe basin and north Rockall Trough, and mainly dip-slip extension in central and south Rockall Trough. In Early Cretaceous time (mid-Aptian) the majority of the United Kingdom Continental Shelf (UKCS) Atlantic margin underwent orthogonal opening followed by continued extension in Late Cretaceous to Paleocene time, culminating in the opening of the North Atlantic west Rockall Plateau. The main Late Jurassic and Early Cretaceous rift episodes conveniently divide the stratigraphy into pre-, syn- and post-rift megasequences which form gross play fairways along the North Atlantic margin. Analysis of these fairways permits integration of data from both mature (e.g. North Sea) and immature (e.g. North Atlantic margin) exploration provinces and helps provide a consistent, predictive approach to the assessment of future hydrocarbon potential of the frontier basins lying along the North Atlantic margin.
Subsidence patterns in western margin basins: evidence from the Faeroe–Shetland Basin
Authors: TURNER J. D. , SCRUTTON R. A.
Published: April 2010

Anomalously high rates of Paleocene tectonic subsidence have been documented from a number of basins on the NW European continental shelf. This period of accelerated subsidence occurs during what is conventionally regarded as the post-rift phase of basin development. It cannot, therefore, be explained by theoretical models of basin formation in which the post-rift subsidence is driven only by the decay of the thermal anomaly generated during active extension. The Faeroe-Shetland Basin developed as a major Cretaceous depocentre, with Late Cretaceous times in particular characterized by extremely high rates of tectonic subsidence. Subsidence rates on the flanks of the basin slowed dramatically into Paleocene times as a major mid-Paleocene unconformity developed across the eastern side of the basin. Wells from the basin centre, on the other hand, show extremely high and accelerating rates of Late Paleocene subsidence associated with normal faulting. These high rates of observed tectonic subsidence lasted for less than 5 Ma and cannot easily be explained in terms of fluctuations in sediment supply rates or eustatic sea-level. This simultaneous flank uplift and basin centre subsidence acceleration is the same age as both the anomalous Paleocene subsidence reported from other nearby basins, and the oldest magmatism associated with the proto-Icelandic plume. Subsidence behaviour returned to normal, across the entire basin, at the same time as continental break-up was achieved between Greenland and Northwest Europe. This suggests that there is a link between regional tectonics and the anomalous Paleocene subsidence patterns observed in this and other basins on the NW European continental shelf.
Elevated palaeotemperatures prior to Early Tertiary cooling throughout the UK region: implications for hydrocarbon generation
Authors: GREEN P. F. , DUDDY I. R. , BRAY R. J. , LEWIS C. L. E.
Published: April 2010

Elevated palaeotemperatures prior to Early Tertiary cooling, which affected wide areas of the UK region, have been revealed by Apatite Fission Track Analysis (AFTA™). All available evidence suggests that palaeogeothermal gradients were close to present values and that much of the observed heating was due to greater depth of burial, by 1 to 2 km or more of section that was subsequently removed by uplift and erosion. Uplift and erosion were not restricted to local inversion axes. The palaeotemperature data suggest a broad, regional warping, producing kilometre-scale Tertiary uplift and erosion across a wide area, within which recognized inversion axes represent local regions of maximum uplift and erosion. AFTA data show no thermal effects associated with Cimmerian unconformities, and any heating associated with Cimmerian events was of lesser magnitude than Late Cretaceous to Early Tertiary heating. Over much of the UK region, source rocks attained maximum temperatures and reached peak maturity during this later heating episode. The regional extent of heating at this time and its significance to hydrocarbon source rock maturation have not been fully recognized in the past. The timing of the events described here suggests a link to the development of the Atlantic margin, Laramidé inversion tectonics and the onset of Alpine tectonism. However, definitive answers to such questions must await further research, particularly involving integration of AFTA and other thermal indicators with structural and geophysical data
The effect of Late Caledonian and Variscan continental escape tectonics on basement structure, Paleozoic basin kinematics and subsequent Mesozoic basin development in NW Europe
Authors: COWARD M. P.
Published: April 2010

Caledonian structures in NW Europe formed by northwestward accretion of continental fragments and magma tic arcs onto the Proterozoic North Atlantic Craton. The main crustal fabrics were formed by Caledonian basin development, subsequent inversion, collision tectonics with thick-skinned and thin-skinned thrusts and large NW–SE strike-slip faults. These fabrics were modified and offset by late Caledonian displacement on NE–SW-trending strike-slip faults.

Late Caledonian (Devonian and Early Carboniferous) tectonics in NW Europe resulted from the lateral expulsion of the English–North Sea–Baltic Block away from an Acadian indentor. The block was approximately triangular in shape bounded by (i) the Ural Ocean to the east, (ii) the left-lateral Great Glen–Midland Valley–North Atlantic shear systems to the NW and (iii) the right-lateral English Channel–South Polish Trough shear systems (in Devonian times) and the South Wales–Southern North Sea–Polish Trough shear zone (in Early Carboniferous times) to the south. During the Early Carboniferous the block expanded NW–SE, as it was released from the indentor, into space created by back-arc extension related to Variscan subduction.

Pull-apart basins which developed along the shear systems include the following, (i) In the Northern North Sea, the West Orkney Basin–East Shetland Platform–Viking Graben formed as a large pull–apart basin in the left-lateral shear systems during Devonian/Early Carboniferous times, (ii) In the Southern North Sea and East Midlands, NW-trending Carboniferous basins developed in a right-lateral shear system, associated with clockwise rotation of Caledonian basement blocks.

The Devonian and Carboniferous basin-fill depended on the position of the block boundary shear systems in the Caledonian and Variscan orogens and their thickened crust. The northern shear boundary affected crust
c.
50 km thick following Caledonian collision and hence the syn-rift and post-rift sediment fill was dominantly intracontinental. In the south the basin developed in crust of average thickness and hence marine conditions were dominant during the extension. However, Devonian and Carboniferous basins in the Northern North Sea provided channels for sediment fill from the Caledonian mountains into the southern basins.

During the Late Carboniferous, closure of the Ural Ocean and Ural plate collision reversed the sense of shear along the block margins and the wedge-shaped block was pushed back between the Acadian collision zones. This shear reversal caused inversion of the pull-apart and expansion basins in northern Britain. Subsequent reactivation of these basins during the Mesozoic was responsible for the tectonic framework of the North Sea.
Regional structural evolution of the North Sea: oblique slip and the reactivation of basement lineaments
Authors: BARTHOLOMEW I. D. , PETERS J. M. , POWELL C. M.
Published: April 2010

Basement lineaments pre-dating the dominant Mesozoic basin formation of the North Sea are fundamental controlling features within the polyphase tectonic evolution of NW Europe. These inherent zones of weakness were repeatedly reactivated during the evolution of the North Sea rift system. The regional and geometrical constraints imposed by the presence of these pre-existing tectonic grains during the Mesozoic extensional deformation suggest that oblique-slip movement must have occurred along many of the basin-bounding faults in a non-preferential orientation for dip-slip reactivation.

While the North Sea can be considered in broad terms as a series of linked, elongated grabens developed in response to dominantly E–W extensional intra-plate stresses, the orientation, distribution and character of pre-existing shear zones ultimately controlled the geometry of many Mesozoic basins and their faulted margins.

Four types of inherent basement features are recognized as having influenced subsequent basin development: (a) wide, diffuse zones of NW–SE and NE–SW basement shear, e.g. in the Central Graben; (b) narrow shear zones oblique to primary Mesozoic extensional faults, e.g. in the East Shetland Basin; (c) E–W-trending discrete fault systems, e.g. on the Western Platform; and (d) massive palaeohighs, e.g. the Halibut Horst.

The distribution of these distinct structural features throughout the North Sea was controlled by Early Paleozoic plate tectonic evolution. Repeated Mesozoic and Cenozoic reactivation along fundamental zones of weakness, including oblique-slip displacements and basinal inversion, has led to areas of highly complex structuration. Hence, there is the potential for numerous structural and stratigraphic hydrocarbon trapping mechanisms associated with these underlying basement lineaments.

Although the total amount of horizontal crustal movement may be limited, understanding of the role of oblique-slip along pre-existing zones of weakness and the effect of long-lived palaeohighs within the basin is crucial to place Mesozoic and Tertiary hydrocarbon accumulations within a realistic North Sea tectono-stratigraphic framework.
Controls on the development and evolution of structural styles in the Inner Moray Firth Basin
Authors: THOMSON K. , UNDERHILL J. R.
Published: April 2010

Although it has generally been believed that structural styles in the Inner Moray Firth (IMF) have been largely controlled by strike-slip movements on the Great Glen Fault (GGF), integrated seismic and field studies suggest otherwise. Instead, most structural styles appear to have developed and evolved as a result of dip-slip extension and thermal subsidence consequent upon two phases of rifting during the Permo-Triassic and Late Jurassic and subsequent regional uplift and local inversion during the Tertiary. The integration of demonstrable thickening of Kimmeridgian–Portlandian intervals across the GGF with sedimentological information from onshore outcrops and cored wells suggests that the basin had a half-graben geometry during the Late Jurassic with a depocentre adjacent to the Helmsdale Fault, analogous to half-graben geometries which characterized other Late Jurassic sequences in Greenland and the South Viking Graben. Progressive marine onlap suggests that more gentle regional (thermal) subsidence took place in an underfilled basin, during subsequent Early Cretaceous deposition. However, the seismic data and subcrop information show that the geometries resulting from such classic rift- and thermally-driven phases of extension were modified by Cenozoic regional uplift and inversion in response to intraplate compression resulting from NE Atlantic (Thulean) and Alpine events. These events also appear to have effected minor strike-slip motion on the GGF, with the development of spectacular ‘flower structure’ and ‘helicoidal’ geometries, and caused limited oblique-slip reactivation of some extensional structures. The most notable modification of structural styles occurs in areas adjacent to the major basin-bounding faults. Particularly complex structural inversion geometries occur in the northwest corner of the basin adjacent to the Wick Fault while anomalously-trending folds developed in response to space problems in the Sutherland Terrace, between the Helmsdale Fault and the GGF, as a result of opposing senses of slip on these faults.
The structural evolution of the eastern end of the Halibut Horst, Block 15/21, Outer Moray Firth, UK North Sea
Authors: HIBBERT M. J. , MACKERTICH D. S.
Published: April 2010

The eastern termination of the Halibut Horst occurs in Block 15/21 of the Outer Moray Firth and shows evidence of a complex structural history. The acquisition of high-resolution 3D seismic datasets across the area of study has permitted mapping of key horizons, better imaging of complex faulting within and around the horst, and enhanced understanding of the structural and stratigraphic evolution.

A marked change in active fault trends from NE–SW during the early/mid-Jurassic to WNW–ESE in the late Jurassic/early Cretaceous is evident. Late Jurassic tectonism resulted in significant erosion of the Halibut Horst Spur and syn-tectonic deposition in basinal areas to the north and south. Continuing reactivation and uplift of the horst during the Cretaceous and Tertiary resulted in a complex interrelationship between tectonics and sedimentation.
Extension and salt tectonics in the East Central Graben
Authors: PENGE J. , TAYLOR B. , HUCKERBY J. A. , MUNNS J. W.
Published: April 2010

Mesozoic structures in the East Central Graben were formed in an extensional tectonic environment by the complex interaction of deposition, basin extension, halokinesis and subsequent erosion. Detailed analysis of seismic data on the eastern flank of the Forties-Montrose High indicates that the Triassic section exhibits rafting: large fault blocks were translated, rotated and distorted on a ductile Zechstein salt substrate. Rafts can be defined by an empirical structural relationship between the Base Cretaceous Unconformity and the Top Zechstein surfaces. The rafts exerted important controls on the distribution of Triassic reservoir rocks and younger reservoir and source rocks. Halokinetic movement was essentially a passive process related to eastward basin subsidence, with salt upwelling between rafted Triassic blocks. Subaerial exposure following uplift during the Early to Middle Jurassic and contemporaneous erosion and deposition led to the development of significant landscape topography. Onlap during the Middle to Upper Jurassic has preserved features such as cuestas with significant fault-line escarpments capped by Skagerrak sandstones. Adjacent to these escarpments deep erosion resulted in incised valleys, where significant angular unconformities occur. Physical modelling has been used to determine the processes and sequence of events that led to the present form of the East Central Graben. A structural model has been developed which relates the timing and rate of deposition and extension, salt movement and subsequent erosion. The interaction of regional extension, development of Triassic rafts and passive salt movement has not been described before. The model represents an alternative to the conventional interpretation that the observed structural features in the area were primarily the result of salt withdrawal.
Comparison of Permo-Triassic and deep structure of the Forties-Montrose and Jaeren highs, Central Graben, UK arid Norwegian North Sea
Authors: PLATT N. H. , PHILIP P.
Published: April 2010

Recent acquisition of high resolution, deep penetration seismic in the Central North Sea has permitted imaging of the basement structure. The results indicate strong similarities between the deep structures of the Jaeren and Forties–Montrose highs. The Triassic and Zechstein strata show a distinctive relationship on these graben margin and median highs. Interpretation of the pre-Zechstein structure may indicate the distribution of Devonian and Carboniferous intervals in an area where previous imaging of the deep section has not been successful.

Southern parts of both the Jaeren and Forties–Montrose highs show the presence of a sub-Permian reflective package; this (?possible Carboniferous) sequence is absent in the north, where reflectors may be recognized within the deep section. A prominent horizontal reflector may represent a Middle Devonian limestone marker or the Devonian basal unconformity. This reflector may alternatively mark the top of Upper Paleozoic volcanic rocks. A deeper, westward- or northwestward-dipping reflector may represent the top of the crystalline basement or a deeper fault or shear zone of Caledonide origin.

The potential occurrence of Carboniferous strata at depth over extensive areas of the Central Graben and surrounding highs would have important implications for exploration in the area. Carboniferous coal measures might provide a sub-Zechstein gas-prone hydrocarbon source.
The structure of the Norwegian Central Trough (Central Graben area)
Authors: GOWERS M. B. , HOLTAR E. , SWENSSON E.
Published: April 2010

The structural geology of the Norwegian Central Trough (also known as the Central Graben) is complex and poorly understood. Previous publicized attempts to explain the structural evolution have failed to account for the variety of observations made from the data by oversimplifying the problem and forcing a particular chosen model. Building from detailed data analysis, seven distinct tectonic phases which affect the area can be defined. Triassic salt tectonics significantly modifies the response to later tectonic phases. Early Jurassic uplift and erosion precede the intense block-fault subsidence of the third phase in the Oxfordian to Volgian. A marked change in the Mid-Volgian heralds the fourth tectonic phase typified by block rotation and related erosion. A distinct early Cretaceous phase of subsidence follows a different pattern from Jurassic subsidence, and is tectonically linked to the sixth phase of inversion. Inversion movements are identified in at least six separate pulses starting in Hauterivian times and continuing into the seventh tectonic phase, that of thermal subsidence initiated in mid-Cretaceous times. These seven phases are largely superimposed on one another, and affect different structural elements within the Central Trough to differing degrees.
Structural styles in the Danish Central Graben
Authors: SUNDSBØ G. O. , MEGSON J. B.
Published: April 2010

In recent years a substantial part of the Danish Central Graben has been covered by high-resolution 3D and 2D seismic data, and a detailed study of the various tectonic elements, their evolution through time and their importance for hydrocarbon trapping has been made possible. This has revealed that in many instances important structural trends affecting reservoir distribution or production characteristics can only be detected using high-resolution 3D seismic. The evidence is that the Triassic, Middle Jurassic, and basal Upper Jurassic graben trends were predominantly north-south. The area which is now the deepest part of the Tail End Graben was relatively high at these times. The NW trends of the Danish Central Graben are not, on the whole, an accentuation of an older (Triassic/Middle Jurassic) rift along the same direction, but are new trends coming into effect in the Kimmeridgian. These NW-SE trends controlled rifting, with peak activity in the Volgian. In conclusion, the structures of most Danish fields exhibit the superposition of several distinct structural episodes. A detailed understanding of the nature of the individual tectonic events is necessary for unravelling the deformation history, which is important knowledge both in exploration and production phases. The various examples from the Danish Central Graben demonstrate that this requires a degree of seismic resolution that has only recently become achievable.
Mesozoic structural evolution of the UK Southern North Sea: insights from analysis of fault systems
Authors: ARTHUR T. J.
Published: April 2010

An understanding of the Mesosoic fault systems in the UK Southern North Sea is important in modelling the burial history of source and reservoir rocks, in determining the time of formation of structural traps in the Permian and older section and in mapping seismic velocity fields in Mesozoic rocks for accurate time to depth conversion.

The evolution of Mesozoic fault systems in the Southern North Sea can be understood by examination of several faults with different amounts of movement across them, and by analysis of fault patterns. Structural analysis has been undertaken in selected areas where complications due to salt movement are minor. Local fault analysis is illustrated by balanced cross-sections and maps from Blocks 49/28,48/23 and 48/11. The faults studied are shown to have been active extensional faults during the Jurassic and early Cretaceous, but have in general undergone only limited movement during later Cretaceous and Tertiary phases of subsidence and uplift.

The Mesozoic faults are detached from basal Permian and Carboniferous rocks by decollement in the Zechstein evaporites and are linked by transfer zones to form a basin-wide system of faults. Mesozoic fault systems overlie zones of weakness in pre-Zechstein rocks, but are not directly connected to them, so their orientation is controlled by Mesozoic forces rather than pre-existing structural grain.
Overpressures in the Central North Sea: implications for trap integrity and drilling safety
Authors: GAARENSTROOM L. , TROMP R. A. J. , de JONG M. C. , BRANDENBURG A. M.
Published: April 2010

The Central North Sea Graben is, at pre-Cretaceous levels, a relatively under-explored province of the United Kingdom Continental Shelf. The hydrocarbon potential was recognized in the early 1970s, but both technology and economics prohibited any serious exploration efforts at that time. Exploration for deep targets increased significantly during the second half of the 1980s. This resulted in the discovery and appraisal of several fields, such as the moderately overpressured Marnock and Skua fields and the highly overpressured Erskine, Puffin, Franklin and Heron fields.

From a regional viewpoint, the overpressures observed in the pre-Cretaceous reservoirs in the Central North Sea Graben follow a well-defined trend, increasing with depth. Similarly, the minimum effective stresses derived from leak-off pressures measured in the overlying seals follow a comparable trend related to the present-day depth of burial. This indicates that the present distribution of overpressures is controlled by retention and that the understanding of overpressure generation is of second-order importance.

A more detailed analysis combining the areal distribution of overpressures with the structural framework suggests that the Central North Sea Graben can be subdivided into several pressure cells. The boundaries between the cells are controlled by faults which, in many instances, show signs of repeated reactivation. This reactivation has led in some cases to failure of otherwise well-defined structural traps, whereas in other cases it may have enhanced the trapping integrity. Examples of intermediate cases can also be recognized, which clearly show that the pressure cells form part of a dynamic system, controlled primarily by the structural evolution of the Central North Sea.

A sound understanding of the structural framework and its evolution is therefore of paramount importance in predicting trap integrity and overpressure regimes prior to drilling. The drilling of deep high-pressure/high-temperature wells and the acquisition of modern 3D seismic data permit a continuous improvement in the understanding of overpressures and their distribution.
Silicate mineral authigenesis in the Hutton and NW Hutton fields: implications for sub-surface porosity development
Authors: McAULAY G. E. , BURLEY S. D. , JOHNES L. H.
Published: April 2010

The Hutton and NW Hutton fields occur in highly faulted, rotated block structures in Block 211/27 of the East Shetland Basin, Northern North Sea. Fourteen wells that core Brent Group reservoir from these fields have been studied in order to characterize the relationship between clay mineral authigenesis and porosity modification in the sub-surface.

Reservoir sands are present in the depth interval 9000–13 000 ft; the shallowest sandstones are subarkoses, contrasting with the deepest sandstones which are mostly feldspar-deficient quartz arenites. The change in composition is a result of diagenetic feldspar dissolution that takes place over the depth interval of study. Authigenic kaolin group minerals and illite are present over the depth range studied, and both exhibit depth-related increases. Three habits of authigenic kaolin group minerals are observed: ‘expanded-mica’, vermiform kaolinite and blocky dickite. The occurrence of vermiform kaolinite is enhanced in Hutton and the adjacent water zone, and does not occur in abundance below 10 500 ft. This habit formed as a result of syn-depositional meteoric water ingress. Structural models suggest that neither field underwent subsequent meteoric water flushing. At the deeper levels of its occurrence range, vermiform kaolinite shows signs of alteration to blocky dickite. Dickite occurs in both intergranular and grain dissolution pores. A depth-related trend of increasing dickite cementation is strongly linked to increasing K-feldspar dissolution. Enhanced dickite cementation is found adjacent to bed contacts in sandstones within mudstone sequences and in regressive sandbodies close to thick mudstone units. Illite shows a more pronounced depth-related increase across the studied interval. Illite precipitation is also genetically linked with detrital feldspar dissolution and is favoured by diminished pore water flow during deep burial. Authigenic clay distribution is partly controlled by local features such as detrital clay coats, in addition to larger-scale parameters such as primary lithology, facies associations, present burial depth and inferred palaeofluid flow. Intergranular porosity declines with increasing burial depth, while grain dissolution porosity increases with depth from Hutton to the shallow and intermediate depth wells of NW Hutton. Generation of enhanced porosity is not related to the unconformities overlying the Brent Group, but to sub-surface fluid migration pathways. Compaction is not significantly operative over the depth interval 10 000–13 000 ft. By 10 000 ft burial depth, up to 50% of the intergranular volume has been lost by mechanical compaction.
Clair appraisal: the benefits of a co-operative approach
Authors: CONEY D. , FYFE T. B. , RETAIL P. , SMITH P. J.
Published: April 2010

The Clair discovery lies 75 km west of the Shetlands, and comprises an elongate ridge of Lewisian basement and an associated roll-over of variable quality Devonian–Carboniferous continental red beds.

Clair was discovered in 1977, when 206/8-1A tested 25° API oil at 1500 BOPD from the red beds at the crest of the roll-over. Well 206/7-1 followed, producing 960 BOPD from fractured basement on the ridge. Ten further wells drilled between 1977 and 1985 indicated an oil in place measurable in billions of barrels, but test results were disappointing. The success of the discovery well was never repeated and commercial test production rates were never achieved.

In 1990, the companies with licence interests over Clair agreed to pool their resources and expertise and collaborate in a joint appraisal programme. A series of reservoir studies quantified the importance of different fracture types to productivity and predicted that the right combination of fractures and matrix could yield much higher flow rates than had previously been achieved. A 3D seismic survey over the central area of the discovery led to a significant improvement in understanding and visualization of the structure.

Two joint appraisal wells were drilled in 1991. The first, a horizontal well in fractured basement, tested at 2100 BOPD after acid wash stimulation.

The second well tested the red beds on the flank of the roll-over and achieved sustained flow rates of over 3000 BOPD from two zones (one of which had been hydraulically fractured) with a significant contribution from previously unidentified open natural fractures. The orientation of these open fractures suggests they are related to a late Cretaceous episode of faulting.

Two further appraisal wells and an extension to the 3D seismic coverage are planned for 1992.

There are still many uncertainties associated with Clair, but the 1991 wells have finally demonstrated that significant and potentially commercial flow rates are achievable. The behavioural change associated with the joint approach has been the catalyst for the success of this new initiative on Clair.
The Embla Field
Authors: KNIGHT I. A. , ALLEN L. R. , COIPEL J. , JACOBS L. , SCANLAN M. J.
Published: April 2010

The Embla Field is the first development of a pre-Jurassic reservoir in the Norwegian sector of the Central Graben. The field is located 5 km south of the Eldfisk Alpha platform, near the Greater Ekofisk production complex, which provides an infrastructure where development of marginal fields becomes commercially viable.

Although the field was initially discovered in 1974 with the 2/7-09 well, inconclusive drill stem test (DST) and geological data left the field potential uncertain. In 1988 a second well, 2/7-20, successfully tested oil and gas from a thick sandstone of indeterminate age. A third well, 2/7-21S, drilled in 1989, gave even better test results from two separate sandstones. Core, log and DST data from these two wells indicate that the reservoir is a faulted and highly fractured sequence of conglomeratic and finer-grained sandstones deposited in a continental environment of probable Paleozoic age.

A 3D seismic programme in 1989 also demonstrated the structural complexity of the field. Situated as it is on the Grensen Nose and bounded on its eastern flank by the Skrubbe Fault/Lindesnes Ridge inversion, it was evident the reservoir had experienced a complex burial and tectonic history. Poor seismic definition of the pre-Cretaceous stratigraphy further dictated the need for a flexible approach to field development.

A phased development programme was initiated, justified by the reserves attributable to the 2/7-20 and 2/7-21S wells. Expendable delineation wells were avoided. Instead, deviated step-out drilling from a central template was favoured, with the intent to subsequently tie the wells back to a remotely controlled surface production facility. First production is scheduled for early 1993.
The evolution of a fractured chalk reservoir: Machar Oilfield, UK North Sea
Authors: FOSTER P. T. , RATTEY P. R.
Published: April 2010

Machar is the first of the ‘Diapir’ fields discovered in the eastern trough of the Central North Sea, in BP sole interest Block 23/26a. It comprises a fractured Cretaceous chalk and Paleocene sand reservoir overlying a steeply dipping salt structure. The oil column is greater than 1100 m and reserve estimates are in the region of 100 million barrels of oil equivalent.

The history of the Machar reservoir represents a complex interplay of salt movement, oil migration and burial diagenesis. Structural studies have shown that the Permian Zechstein salt has been in a state of buoyant equilibrium for most of its geological history, with only one phase of true diapirism identified in the mid-Miocene. The buoyancy resulted in the deposition of a condensed (<300 m) Triassic–Eocene section over the salt high. In the surrounding area, sediment loading and related salt withdrawal caused ‘downbuilding’ and the deposition of a thick (3000 m) sediment package. A 50–300 m section of chalk was thus deposited over the diapir with a mature Jurassic oil source in the adjacent depocentre. Paleocene sands are thin or absent over the crest of the structure, but thicken to about 100 m downflank.

Regional subsidence and oil migration commenced in the early Oligocene, the trap filling from the top down. Here, shallow burial depth (
c
. 100 m) and early oil emplacement ensured porosity was preserved in the chalk matrix. At the same time, on the flanks of the structure greater depth of burial and diagenesis were destroying chalk porosity from the bottom up. These competing influences resulted in a diagenetic front marking the limit of pay in the chalk. That front lies in the depth range 2000–2500 m.

Passive movement of the chalk during periods of buoyancy, plus regional tectonic compression in the mid-Miocene, created a widespread fracture system. Studies have identified a continuous process of fracture, stylolite and cement formation across a variety of brittle and ductile deformation styles. Average spacing of open fractures in 200 m of chalk core from Well 23/26a-13 is 7.5 cm. About a third of these are open.

The burial history, oil emplacement and fracturing resulted in a reservoir with a dual porosity system. The fine-grained pelagic chalk matrix has primary porosity in the range 12–35% with permeability generally less than 1 mD. Secondary fracture porosity, although difficult to measure, is probably less than 1%. Production test data over fractured intervals, however, show permeability in excess of 1000 mD. The fracture system is the key to developing the chalk reservoir as it provides effective drainage of the tight matrix. In a wider context the recognition of buoyancy, not piercement, as the dominant structural process provides a model for exploration elsewhere.
Estimation of recoverable reserves: the geologist’s job
Authors: CORRIGAN A. F.
Published: April 2010

The accurate prediction of recoverable reserves in the North Sea has proved to be a difficult task for many operators. This paper summarizes the performance of a number of fields by tracking the way in which reserves have changed with time, compared to the volume of reserves thought to exist at the time that development approval was given. In many fields reserves have changed by between 20 and 70%, with the areal distribution also exhibiting considerable variation from that in the field development plan. The changes have been both positive and negative. In the latter case, the economic consequences of a significant reduction in reserves are obvious. However, increases in reserves also bring financial penalties in poorly sited and sized facilities, additional well requirements and inefficient use of funds.

Although this work has shown that there are a number of reasons for poor estimation, the underlying cause is usually insufficient recognition of reservoir complexity and thus the use of simple or inappropriate models to predict reserves. It is also clear that there are geological factors that are common to fields that have exhibited large losses or increases in reserves. In the latter category are fields that are structurally simple, often deposited in submarine fan environments, where the volume of movable oil was underestimated. Fields suffering large drops in reserves are often structurally complex, highly compartmented and contain high permeability zones.

It is proposed that geologists should take a much more active role in reserve estimation than has generally been the case in the past. In particular, at the appraisal stage, early recognition of complexity is vital, and understanding of the complexity should become one of the major goals of the appraisal wells. In addition, the geologist needs to become closely involved in the construction and operation of the simulation models that are used by most reservoir engineering groups to predict reserves. This is to ensure that geological features which affect drainage are correctly modelled in the simulators.
Wytch Farm oilfield: deterministic reservoir description of the Triassic Sherwood Sandstone
Authors: BOWMAN M. B. J. , McCLURE N. M. , WILKINSON D. W.
Published: April 2010

The Triassic Sherwood Sandstone is the principal reservoir in the Wytch Farm Field, located in Dorset, southern England. Composed of predominantly arkosic alluvial sandstones, it has excellent reservoir qualities but includes significant mudstone, calcrete and calcrete conglomerate baffles which affect the vertical passage of fluids. The identification of ‘flow units’ within this sequence has helped to determine appropriate layers for use in the reservoir simulation model. These flow units were defined using core data from 24 of the 48 wells and by log correlation. The well information was supplemented by outcrop data to help elucidate the geometry of individual layers.
Meillon–Saint Faust gas field, Aquitaine basin: structural re-evaluation aids understanding of water invasion
Authors: HALLER D. , HAMON G.
Published: April 2010

The case history of the Meillon–Saint Faust gas field is an example of how a structural interpretation based on well data can help in the understanding of the dynamic behaviour of the reservoir.

This field was initially developed with clustered wells at the structural culminations of the trap, relying on good fluid communication in the field. After production of 50–65% of the initial gas-in-place, the productivity had drastically decreased, due to overall water breakthrough at the wells, and the field was apparently approaching the end of its economic life.

Detailed structural analysis and reservoir simulation have now led to an improved understanding of field behaviour. Since the southern border of the field is understood to be a major E–W normal fault, it is apparent that most of the production wells are located in a highly fractured zone and that good fluid circulation is restricted to this southern fringe.

This structural model is taken into account for numerical reservoir simulation. The pressure predictions of these simulations match the pressure history and thus validate the structural model. The main conclusion of the simulation is that water has only invaded the fracture network and that the total water volume entering the reservoirs is low.