bluuebunny/crossref_metadata_2025 huggingface dataset:
Mesozoic extension in the North Sea: constraints from flexural backstripping, forward modelling and fault populations
Authors: ROBERTS A. M. , YIELDING G. , KUSZNIR N. J. , WALKER I. , DORN-LOPEZ D.
Published: April 2010

The magnitude and distribuion of late Jurassic extension in the Northern Viking Graben has been investigated by (i) syn-rift forward modelling using the flexural-cantilever model of continental rifting; (ii) post-rift flexural backstripping of a series of cross-sections; and (iii) the analysis of fault-population statistics.

Application of these three techniques indicates that Jurassic extension on the tilted fault-block terrains of the East Shetland Basin, Tampen Spur and western Horda Platform is on average
c.
15% (
β
= 1.15). In the graben axis the regional value of
β
rises to
c.
1.3, perhaps locally rising to 1.4. On the eastern Horda Platform Jurassic extension is low,
β
=
c.
1.05.

Flexural backstripping of the post-rift part of a cross-section through the Central Graben yields similar estimates of extension. At the flanks of the Jurassic basin
β
is estimated to be 1.2, rising to a likely maximum of
c.
1.3 in the basin centre.

These estimates of extension lie at, or towards, the low end of the previously published range of estimates. The principal reasons for this are (i) the incorporation of the thick sequence of Triassic and Lower Jurassic sediments in the backstripping; and (ii) the use of flexural isostasy (rather than Airy isostasy) in both the backstripping and forward modelling.

The estimates of Jurassic extension obtained in this study do not account for the observed crustal thinning and, therefore, point to there having been a significant pre-Jurassic extensional event in the North Sea, of probable Triassic age. While a residual thermal anomaly from this event may have made a small contribution to post-Jurassic subsidence, compaction of the Triassic-Middle Jurassic sequence has been a significant contribution of the Triassic event to the thickness of the Cretaceous/Tertiary basin.
Tectonic evolution and structural styles of the East Shetland Basin
Authors: LEE M. J. , HWANG Y. J.
Published: April 2010

Geometric and stratigraphic evidence indicates that the East Shetland Basin evolved by a rift mechanism similar to that defined in the East African Rift. Arcuate half-grabens of probable Devonian extensional origin are the fundamental structural units. The stratigraphic data suggest episodic footwall movements during the mid- to late Jurassic as well as alternating regional subsidence patterns, i.e. ‘teeter-totter’, against the Horda Platform. The Northern North Sea rift system, which includes the East Shetland Basin, is believed to have opened primarily by orthogonal movement of facing half-grabens rather than by large-scale strike-slip motion between these half-grabens.

Both extensional and compressional (or transpressional) features occur in the East Shetland Basin as syn-rift structures. The extensional structures include the well-known tilted fault blocks and relatively undocumented listric normal faults that occur on the crestal and flanking areas of the larger blocks. The principal compressional stress was associated with oblique shearing at the ends of actively extending half-grabens, which created local reverse faults and anticlinal domes. Some rotated horst blocks, e.g. Murchison Field structure, were also created by the transpressional motion. A broad, four-fold E–W structural zonation has been made for the East Shetland Basin and North Viking Graben based on the degree of half-graben development and structural styles.
A fundamental reappraisal of the structure of the Cormorant Field and its impact on field development strategy
Authors: DEMYTTENAERE R. R. A. , SLUIJK A. H. , BENTLEY M. R.
Published: April 2010

Production from the Cormorant Field is hampered by complex in-field faulting. The large number of faults and the presence of fault seals combine to restrict fluid flow within the reservoir and locally compartmentalize the field. A reliable structural description of the field is, therefore, a prerequisite for optimal field management. A revised structural framework has been established for the field, based on new and reprocessed seismic data, and the availability of newly developed seismic attribute mapping tools. Previous structural models have emphasized E–W extension on N–S normal faults. The new model also describes E–W extension, but invokes predominantly dip-slip movement on a network of planar, oblique-slip faults, the geometry of which has been strongly influenced by the reactivation of NE–SW-oriented lineaments in the basements. This has led to a significant revision of both the reservoir sub-compartment geometry, and the connected oil volumes associated with existing producer–injector well pairs.
Mesozoic evolution of the Jæren High area, Norwegian Central North Sea
Authors: HØILAND O. , KRISTENSEN J. , MONSEN T.
Published: April 2010

The Jæren High area is located in a structurally complex area where the Central Graben, the Ling Depression, the South Viking Graben and the Witch Ground Graben converge. Wells drilled on the Jæren High indicate rapid changes both in facies and thickness of the Mesozoic sequence. The evolution of the region during Mesozoic times was considerably influenced by the thickness and the distribution of the Zechstein salt. Arches underlain by thick Triassic strata are separated by elongated grabens or troughs where mainly late Jurassic strata rest on ridges of Zechstein salt.

At least three models are possible to explain the distribution of Mesozoic strata in grabens or troughs on the Jæren High. (1) Thin-skinned extension: a relatively thin Zechstein sequence overlain by Triassic strata was faulted, mainly in mid-Jurassic times. The salt started to pierce the footwalls resulting in the development of grabens separating the footwall and hangingwall blocks. These grabens trapped late Jurassic sands and shales, while the Triassic highs suffered erosion and non-deposition. (2) Salt dissolution: this model is based on the dissolution of Zechstein salt near to the surface or in the subsurface. During early to middle Triassic times salt started to move due to the weight of its overburden. Triassic depocentres and salt ridges developed. Middle and late Jurassic salt dissolution took place due to exposure of the salt at the surface, creating accommodation space for late Jurassic deposition of sands and shales. (3) Combined tectonism and salt dissolution: this model combines both tectonism and salt dissolution. Triassic and Jurassic extension resulted in uplift and rotation of the high. Subsequent erosion resulted in exposure of the salt along elongated ridges and salt dissolution took place due to salt exposure at the surface creating accommodation space for late Jurassic deposition.

The third model, a combination of tectonism and salt dissolution, is the preferred model as it can explain the particular Mesozoic palaeogeography of the Jæren High.
Structural styles from the Central Graben in the UK and Norway
Authors: SEARS R. A. , HARBURY A. R. , PROTOY A. J. G. , STEWART D. J.
Published: April 2010

A selection of structural styles observed on seismic data from the UK and Norwegian sectors of the Central Graben is presented. The interpretation of these structures is based on a polyphase tectonic model of structural evolution of the North Sea Basin which has been facilitated by the improved quality of pre-Base Cretaceous reflections seen on the increasingly available 3D seismic surveys.

The structural evolution of the Central Graben can be represented by a model invoking two main phases of structural development. Much of the basement framework was created during the Caledonian orogeny which gave rise to NW–SE- and NE–SW-trending diffuse shear zones. Subsequent repeated reactivation along these existing basement lineaments occurred from Late Paleozoic to Cenozoic times. During the late Jurassic to early Cretaceous, the maximum principal stress axis was orientated approximately N–S to NNE–SSW and the minimum principal stress axis orientated E–W to ESE–WNW. Because this stress field was at an angle to the existing framework trends, most of the faults were reactivated with an oblique-slip component which resulted in the formation of transpressional and transtensional features of varying orders of magnitude. Fault movements also locally activated halokinesis which further complicated the structural development.

Seismic examples illustrate how the introduction of an element of oblique-slip can clarify the interpretation of many of the observed structures in the UK and Norwegian Central Graben. Although often subtle in appearance, oblique-slip movements and corresponding zones of transpression and transtension must be identified before a clear picture of localized structural and sedimentary histories can emerge.
Fault reactivation and oblique-slip in the Southern North Sea
Authors: OUDMAYER B. C. , de JAGER J.
Published: April 2010

The structural evolution of the Southern North Sea region is controlled by a number of important tectonic phases. The dominant NW–SE structural grain first became evident during dextral wrenching in the Stephanian/Autunian. A second fault direction trends NNE–SSW, and probably also originated in pre-Permian times. Extensional faulting in the Triassic preceded major Middle Jurassic to Early Cretaceous rifting. Subsequent cooling subsidence was interrupted by inversion in the Late Cretaceous, the mid-Paleocene, and again in the mid-Tertiary. During these periods Jurassic basins were uplifted and, locally, subjected to deep erosion.

Despite the multitude of tectonic events, a high degree of fault parallelism can be observed in the Southern North Sea. Fault patterns and directions are strikingly similar, whether viewed on a regional scale or at prospect level. The general structural model is, therefore, one of repeated reactivation of basement faults, which continue to control the structural grain despite changes in tectonic regime. Only rarely and by coincidence do the main structural features have orientations that conform to the regional stress. Consequently the predominant mechanism is considered to be one of oblique-slip.

This phenomenon is particularly well observed on structural highs, such as the Cleaver Bank High, straddling the median line between the Netherlands and the UK. Here, the fault trend at Base Zechstein level runs essentially NW–SE. Repeated fault reactivation has led to anomalously high length-to-throw ratios. Locally short N–S and E–W link-ups accommodate oblique-slip. At higher stratigraphic levels en-échelon faults illustrate the strike-slip nature of the latest movements along the pre-existing NW–SE structural grain.

The presence of salt in the overburden causes faulting at higher levels to be decoupled from basement faulting. Sandbox experiments show that reverse faults and extensional grabens can develop in the post-salt sequence in response to oblique-slip along the pre-salt structural grain. The location of such grabens is invariably offset towards the footwall of the basement fault, the amount of offset dependent on the thickness of the salt.

These sandbox experiments provide valuable analogues for the structural mechanisms that are believed to have operated in the Southern North Sea. Applied to both 2D and 3D seismic interpretation, they allow fault mapping with a greater degree of confidence. Recognition of fault reactivation and oblique-slip will lead to a better understanding of the tectonic history, timing of structuration and trap development.
The use of natural He, Ne and Ar isotopes as constraints on hydrocarbon transport
Authors: BALLENTINE C. J. , O’NIONS R. K.
Published: April 2010

Hydrocarbon accumulations contain rare gases derived from the atmosphere, the crust and, in some cases, the mantle. The distinctive isotopic structure of these different rare gas components allows them to be resolved. The relative abundances of the He, Ne and Ar in the crustal-, mantle- and atmosphere-derived components provides information on the physical processes which have operated in the sub-surface. When combined with mass balance considerations, the rare gases provide powerful constraints on fluid provenance and transport.

The results of case studies from the Pannonian Basin in Hungary, and the Vienna Basin, Austria are reviewed. Natural gas reservoirs in the Pannonian and Vienna basins provide samples from depths between 0.5 and 5.5 km. The atmosphere- and crustal-derived rare gas components show systematic patterns of He/Ar and Ne/Ar fractionation which are in the sense predicted by rare gas partitioning between liquid, such as oil and water, and a gas phase. This could occur either during transport or upon emplacement into the reservoir. The role of diffusive processes in fractionation of rare gases cannot be resolved and is assumed to play only a minor role in hydrocarbon transport in the accumulations investigated.

The rare gases also convey information on the extent of interaction between groundwater and hydrocarbons. The relationship between major species, such as CH
4
and rare gas tracers, provides estimates for the minimum volume of groundwater that has interacted with the hydrocarbon phase. In the case of the Pannonian Basin, the results are consistent with a model in which the natural gas has been transported in a CH
4
-saturated water phase, from which the gas has separated upon cooling and/or decompression. The mass balance demonstrates that the interaction between the hydrocarbon gas and groundwater has been on a scale much greater than often envisaged in models of hydrocarbon formation and migration.
Clyde: reappraisal of a producing field
Authors: TURNER P. J.
Published: April 2010

The complexities inherent in a field are seldom appreciated prior to production and their impact poorly quantified. In order to exploit a reservoir fully, continuous and focused reappraisal is required throughout field life. Evaluation of reservoir performance through the integration of effective surveillance programmes and the analysis of development well results is critical to reduce the uncertainty carried at the planning/early development stage.

Successful development of Clyde has resulted from the application of appropriate techniques within an integrated multidisciplinary team. The timely acquisition of sufficient relevant data also proved an essential part of this process.

The integration of seismic, sedimentological, biostratigraphic and dynamic data from Clyde has provided significant improvements in intra-reservoir resolution, reservoir description and the identification of flow units. This has permitted the discrimination between reservoir models and has reduced uncertainty in field and future infill well performance. Various advanced techniques, such as absolute acoustic impedance and dip and azimuth plots, have been used to enhance our understanding of the Clyde Field. Their application, together with techniques such as parasequence mapping and pressure data analysis, is described. Through the development of a ‘reactive’ reservoir simulator, improvements in the geoscience model are easily incorporated and the simulator updated. This approach permits rapid quantification of the value of future development wells, recompletions and other reservoir management options. This is part of a continuous process which allows the integrated asset team to optimize the development strategy.
The Eakring Dukeswood oil field: an unconventional technique to describe a field’s geology
Authors: STOREY M. W. , NASH D. F.
Published: April 2010

The Eakring Dukeswood oil field of Nottinghamshire was last produced in 1971. All wells have since been abandoned and the licence ML 1 expired on 22 April 1992. Cumulative production was small by North Sea standards, but during the Battle of the Atlantic the field produced the bulk of the UK’s indigenous crude, peaking at 1600 BOPD in 1941. Cumulative recovery was some 6.5 MMSTB from an estimated 25.6 MMSTB in place in the completed horizons. In 1986 a study was undertaken to evaluate the remaining reserve potential.

The primary tool for such work would normally be a detailed geological model, but since less than 25% of the 197 grid drilled wells have any wireline logs or cores only structural models existed. There is very little seismic. A further complication is that there were nine productive horizons and the majority of well completions were unsegregated over four of them. Only 20% of cumulative production can be assigned unequivocally to any one reservoir.

Reservoir models, and individual reservoir contributions to production, had to be determined using less conventional techniques. Individual well performance under recycled peripheral water flood has proved to be a powerful tool to determine reservoir connectivity. Mapping decline rate reversal and water breakthrough by month indicates the transmissibility fairways and no-flow barriers in the system. It has also proved in some circumstances a useful discriminator of reservoir contribution.

The reservoirs are mainly Westphalian A to Namurian B siliciclastics. All were undersaturated at initial conditions. Porosity is often secondary, rarely above 20% and absolute permeability ranges from less than 1 to several hundred mD. Irreducible water saturation (S
wi
) from mercury injection data ranges between 30 and 50% and water flood experiments indicated residual oil saturation (S
or
) at 30%. The oil is waxy, up to 16% by weight, and has a high viscosity with correspondingly unfavourable mobility. API gravities range from 31.9–37.6°. Most production was obtained from the Loxley Edge Rock, the Sub Alton and Crawshaw sandstones—all by depletion drive, and from the Ashover Grit—in part water driven. Post-war water flooding was directed mainly at the Sub Alton and Crawshaw units.

Regional studies and sedimentological examination of two recent adjacent wells established the likely broad reservoir anatomy. This was then refined for the field from the qualitative transmissibility maps constructed from flood front behaviour. Geological models could then be assembled with some confidence and bypassed oil and secondary accumulations were identified. Relative permeability experiments on the modern core indicate an additional field reservoir/accumulation, previously unrecognized.
Petroleum systems of the Atlantic margin of northwest Europe
Authors: SPENCER A. M. , BIRKELAND Ø. , KNAG G. Ø. , FREDSTED R.
Published: April 2010

The Atlantic margin region of northwest Europe includes, on its inboard side, five basins which have been explored since the 1970s and contain proven, Jurassic-sourced petroleum systems: Halten Terrace, the Northern North Sea, the Faeroe-Shetland Basin, the Slyne-Erris troughs and Porcupine Basin. The hydrocarbons in most of these basins are in Jurassic sandstone reservoirs within late Jurassic fault traps. To the northwest, on the outboard side of the Atlantic margin, lie four frontier areas in which exploration is mostly only just beginning: the Vøring, Møre and Rockall basins and the Faeroes Shelf. The basins often have Cretaceous strata which are many kilometres thick, whilst Tertiary strata are thick in some places but thin (starved) in others. Cretaceous and Tertiary extensional faulting phases created fault traps in some parts of the basins and mid Tertiary compressional domes provide important traps elsewhere. The least known aspect of the petroleum geology of these basins is the presence and maturity history of any source rocks. The outboard basins are bordered to the northwest by a zone of Paleocene-Eocene volcanic rocks, which are over 5 km thick at the Faeroe Islands. The lava sheets were dominantly subaerial and may conceal eroded but prospective Paleocene and Mesozoic strata. The outboard frontier areas may contain extensive petroleum systems. In the Vøring Basin two major discoveries prove the existence of petroleum systems there but both their geographical extent and source are unknown. Petroleum systems may be present in the main Møre Basin, beneath some part of the Faeroes Shelf and along both margins of the Rockall Trough.
Oil seepage onshore West Greenland: evidence of multiple source rocks and oil mixing
Authors: BOJESEN-KOEFOED J. A. , CHRISTIANSEN F. G. , NYTOFT H. PETER , PEDERSEN A. K.
Published: April 2010

Widespread oil seepage and staining are observed in lavas and hyaloclastites in the lower part of the volcanic succession on northwestern Disko and western Nuussuaq, central West Greenland. Chemical analyses suggest the existence of several petroleum systems in the underlying Cretaceous and Paleocene fluvio–deltaic to marine sediments. Seepage and staining commonly occur within vesicular lava flow tops, and are often associated with mineral veins (mostly carbonates) in major fracture systems.

Organic geochemical analyses suggest the existence of at least five distinct oil types: (1) a waxy oil which, on the basis of the presence of abundant angiosperm biological markers, is interpreted as generated from Paleocene mudstones (the ‘Marraat type’); (2) a waxy oil, probably generated from coals and shales of the Cretaceous Atane Formation (the ‘Kuugannguaq type’); (3) a low to moderately waxy oil containing 28,30-bisnorhopane, and abundant C
27
-diasteranes and regular steranes (the ‘Itilli type’), possibly generated from presently unknown Cenomanian–Turonian marine mudstones; (4) a low wax oil of marine, possibly lagoonal/saline lacustrine origin, containing ring-A methylated steranes and a previously unknown series of extended 28-norhopanes (the ‘Eqalulik type’); (5) a waxy oil with biological marker characteristics different from both the Kuugannguaq and Marraat oil types (the ‘Niaqornaarsuk type’), probably generated from Campanian mudstones.

The presence of widespread seepage and staining originating from several source rocks is encouraging for exploration in basins both on- and offshore western Greenland, where the existence of prolific source rocks has previously been the main exploration risk.
Cretaceous (post-Valanginian) sedimentation and rift events in NE Greenland (71–77°N)
Authors: WHITHAM A. G. , PRICE S. P. , KORAINI A. M. , KELLY S. R. A.
Published: April 2010

A tectono-sedimentary model is proposed for the Cretaceous (post-Valanginian) succession of NE Greenland which provides insights into the evolution of adjacent margins. Subsequent to a major reorganization of fault blocks during rifting in the Volgian–Valanginian a seafloor topography controlled by block faulting was created and was maintained by faulting until the end of rifting in the Middle Albian. This topography was infilled after the Middle Albian and a shelf break margin was created and maintained until at least the Campanian. After this time there is no onshore sedimentary record in NE Greenland to constrain interpretations. Post-Valanginian sedimentation was dominated by the accumulation of fine sediments. However, significant thicknesses of coarse clastics are recorded at several stratigraphic levels in East Greenland. The primary controls on sandstone occurrence are steps in the basin bounding fault. The greatest accumulation of sandstones is associated with the largest of these steps. Sandstone supply to basinal areas was controlled by periods of lowstand. These have been identified in the Barremian, Late Aptian and Coniacian. Sandstone deposition occurred in fiuvio–deltaic, tidally influenced shallow marine and deep marine environments. The degradation of sandy footwall slopes did not lead to the formation of sandy sequences in the adjacent hanging wall successions.
Sandstone wedges of the Cretaceous-Lower Tertiary Kangerlussuaq Basin, East Greenland – outcrop analogues to the offshore North Atlantic
Authors: LARSEN M. , HAMBERG L. , OLAUSSEN S. , PREUSS T. , STEMMERIK L.
Published: April 2010

The Kangerlussuaq Basin is situated in southern East Greenland (68°30′N). The exposed 1 km thick sedimentary succession records a mid-Cretaceous to Tertiary basin history of Late Aptian–Albian transgression, Late Cretaceous–Early Paleocene highstand and mid-Paleocene uplift and erosion. The sediments are overlain by Upper Paleocene volcaniclastics, pyroclastics and flood basalts associated with continental break-up. Tertiary uplift and erosion has exposed the basin fill and it now represents one of the few outcrop analogues to the offshore Cretaceous-Tertiary sedimentary basins along the deep water margins of the North Atlantic.

Three major sandstone-dominated intervals are identified in the succession: (1) mid-Cretaceous fluvio–estuarine sandstones; (2) Lower Paleocene submarine fan sandstones; and (3) mid-Paleocene fluvial and deltaic sandstones and conglomerates. The originally excellent reservoir properties of the sandstone bodies have been strongly modified by more than 4 km of burial and local high heat flux. Mechanical compaction, quartz precipitation, calcite cementation and illitization are the main porosity and permeability reducing agents in the rock sequence.

The three major episodes of sandstone deposition were related to initial transgression and later relative sea-level falls and may help prediction of stratigraphic intervals with possible reservoir units in the undrilled offshore basins of the northern North Atlantic. The occurrence of mid-Paleocene proximal fluvial facies, thus suggests sediment bypass and deposition of coarse-grained clastics in the offshore areas southeast of Greenland and around the Faeroe Islands.
The regional geology and exploration potential of the NE Rockall Basin
Authors: WADDAMS P. , CORDINGLEY T.
Published: April 2010

The Northeast Rockall Basin lies 130 km to the NW of the Outer Hebrides and to the immediate SW of the Faeroe–Shetland Basin. It is a frontier basin by virtue of minimal well and seismic control, combined with problems created by a thick Lower Tertiary basalt cover. A regional geological evaluation of the area has therefore proved critical in developing a good understanding of the basin. Gravity and magnetic modelling along key seismic lines suggest that over 6000 m of pre-Tertiary section may be accommodated in the NE Rockall Basin and has revealed a number of important structural elements. In particular, the presence of NNE–SSW trending structural elements infers a Jurassic age to some of the rifting on the grounds that other major rifts of this trend were active at this time. BGS shallow boreholes have encountered Mesozoic sediments (including Kimmeridge Clay facies) on the Outer Hebrides Platform to the east. It is likely that similar sediments will occur within the NE Rockall Basin.

Migrant oil in the study area has an affinity with a Kimmeridge Clay Formation source rock. The presence of a Lower Jurassic source rock is also possible. The Kimmeridge Clay Formation has been modelled to reach a thickness of over 250 m in the centre of the basin. It is predicted to lie currently within the oil generation window over a large portion of the basin, with the top of the oil window occurring at a depth of approximately 3750 m. Modelling predicts that approximately 75 billion barrels of oil equivalent have been generated during Paleocene to Recent times. Gas generation is predicted to be minimal and localized.

The NE Rockall Basin and the Faeroe–Shetland Basin have been affected by the same Tertiary tectonic, thermal and igneous events. A regional Tertiary sequence stratigraphic framework has been established and correlated with a scheme created for the Faeroe–Shetland Basin. Lower Tertiary basin-floor fans are anticipated to form an important exploration target. In addition, prospectivity generated by footwall-derived fan-deltas developed along the margin of the NE Rockall Basin has been recognized. The delineation of Mesozoic prospects in the NE Rockall Basin must await the acquisition of additional seismic data and the interpretation of early drilling results. However, all the play elements required to generate Mesozoic prospectivity are currently thought to be present.
The structural and stratigraphic framework of the Irish Rockall Trough
Authors: CORFIELD S. , MURPHY N. , PARKER S.
Published: April 2010

The Irish sector of the Rockall Trough is the largest of the sedimentary basins offshore Ireland and is virtually unexplored. This study establishes the first regional structural and stratigraphic framework of the Mesozoic and Tertiary fill of the Irish sector of the Trough with the aim of assessing its hydrocarbon potential. Major structural features of the Irish Rockall Trough and the adjacent part of the Irish Continental Shelf have been mapped using a combination of 9000 km of confidential and released regional seismic lines, satellite gravity data and a compilation of marine and aeromagnetic data. A sequence stratigraphic framework for the Mesozoic and Tertiary has been established using the seismic data tied to ten wells in adjacent basins (Erris and Slyne troughs and Porcupine Basin) and to one well in the southern part of the UK Rockall Trough. A major advance was the recognition of new Mesozoic perched basins and tilted fault blocks on both margins of the Trough in areas previously interpreted as shallow basement. A further 12000 km of seismic data acquired in the summer of 1996 indicate that a considerable thickness of sediment is present in these basins containing a fill analogous to the Slyne and Erris troughs. The configuration of the perched basins suggests that a major linked rift system, coincident with the site of the present-day Rockall Trough, existed during the early Mesozoic. Within the Trough, a number of sub-basins have been identified, characterized by syn-rift wedges of interpreted Jurassic to Early Cretaceous age.
Jurassic non-marine source rocks and oils of the Porcupine Basin and other North Atlantic margin basins
Authors: BUTTERWORTH P. , HOLBA A. , HERTIG S. , HUGHES W. , ATKINSON C.
Published: April 2010

A detailed geochemical study has shown that oils from the Porcupine and Jeanne d’Arc basins were not sourced from a typical, fully marine Upper Jurassic Kimmeridge Clay Formation equivalent. Rather, these oils are thought to be a mixture of hydrocarbons derived from an atypical Upper Jurassic shale and from a Middle Jurassic non-marine source interval. Standard geochemical techniques were used to analyse a suite of oil samples from the Atlantic margin, along with new proprietary geochemical parameters which can discriminate age and depositional environment. The results indicate that oils from the Porcupine and Jeanne d’Arc basins are very similar, and originated from a mixture of an unusual marine source rock and a lacustrine algal source rock. This conclusion is strengthened by considering the regional geology, as several marine and non-marine potential source rock intervals are present in both the Porcupine and Jeanne d’Arc basins.

The chemistry of the oils from the Jeanne d’Arc Basin suggests that the Egret Member of the Rankin Formation is the unusual Upper Jurassic marine source rock. The Egret Member is also an excellent analogue for the Upper Jurassic source contributor in the Porcupine Basin, and palaeogeographical reconstructions for this time suggest that similar source facies were deposited in both basins. The lacustrine source contribution recognized in the Porcupine Basin oils is thought to originate from Middle Jurassic algal shales, which are moderate to very good oil-prone source rocks (total organic carbon contents: 1.3–3.9%; hydrogen indices: 143–573).

This newly recognized non-marine oil component in North Atlantic margin basins reduces the charge risk of Atlantic margin plays, particularly where the Kimmeridge Clay Formation is known to be absent or immature.
Reservoirs of the Dinantian (Lower Carboniferous) play of the Southern North Sea
Authors: MAYNARD J. R. , DUNAY R. E.
Published: April 2010

Critical to the successful exploitation of the northern margin of the Southern Gas Basin is the development of the Lower Carboniferous (Dinantian, Viséan) play. Whilst the exploration for intra-Carboniferous traps has been unsuccessful, the presence of large Base Permian evaporite sealed closures still holds opportunities. The discovery of hydrocarbons within one of these closures on the edge of the traditional gas basin suggests that migration of gas from Westphalian and Namurian coals and shales has occurred. Thus in this area, the primary remaining risk is the presence and distribution of reservoir. Two potential reservoirs are examined in the subsurface, the Fell Sandstone Formation and the Whitby Member. Onshore analogue data has been combined with the sparse offshore data to explore the distribution of these sandstones. The Fell Sandstone Formation was deposited at a time of high sediment supply resulting in a thick (300 m) succession of fluvial sandstones whose distribution is closely linked to the accommodation space developed in the hanging walls of contemporaneous faults. The Whitby Member (thickness 30 m) was deposited at a time of lower sediment supply which enabled relative sea-level changes to control the vertical distribution of sandstone bodies. It demonstrates some of the features which suggest it could infill a palaeovalley. If active faulting continued throughout this time then a similar hanging wall concentration of sandbodies, as is seen in the Fell Sandstone Formation, could be expected.
Continental-scale sequence stratigraphy of the Namurian, Upper Carboniferous and its applications to reservoir prediction
Authors: DAVIES S. , HAMPSON G. , FLINT S. , ELLIOTT T.
Published: April 2010

The search for Upper Carboniferous reservoirs can be aided by the development of a chronostratigraphic framework combining detailed sedimentological information with a template using the diagnostic ammonoid-bearing marine bands from onshore analogues derived from European, American and Canadian basins. Analysing Namurian successions constrains key sand-prone intervals, which include the upper Kinderscoutian, lower Marsdenian and top Yeadonian, from a number of depositional settings. A controversial issue for the Upper Carboniferous is the relative importance of high frequency and high magnitude glacio–eustatic sea-level fluctuations as a driving mechanism in the development of basin fill over the controls exerted through the prevailing tectonic regime, climate and sediment supply. The recognition and characterization of time-equivalent sea-level rises and, with greater significance for hydrocarbon exploration, sea-level falls, from a number of European basins attests to their influence. Candidate reservoirs can be constrained at various temporal resolutions and may also be restricted geographically. Significant candidate reservoirs include multistorey fluvial incised valley fills and deepwater sand-rich successions. The most productive onshore reservoirs are coarse-grained/conglomeratic fluvial intervals located in the base of incised valleys where a significant proportion of the subsequent valley fill is fine-grained estuarine sediments. There is potential for large hydrocarbon reservoirs within sand-rich deep water successions but their occurrence in tectonically complex or deeply buried areas has thus far precluded their exploitation. A chronostratigraphic framework established for the UK and Ireland can be applied to European basins and, in a more limited sense, to North America, where marine horizons can be matched. The application of this framework to the subsurface lies in the recognition of key candidate reservoirs and their likely occurrence, both geographically and temporally.
Banff Field, UK Central Graben – evaluation of a steeply dipping, fractured chalk reservoir
Authors: EVANS N. , RORISON P. , SYKES G.
Published: April 2010

The Banff Field is a steeply dipping raft of fractured Late Cretaceous and Danian chalk on the flank of a salt diapir structure on the western margin of the Central Graben in the UK North Sea. Paleocene sands draped over the raft and a carbonate cap rock provide additional reservoir potential.

The unusual geometry of the reservoir is a result of a complex structural history. Generally positive relief and minor pulsed growth through the Cretaceous and Lower Tertiary resulted in a condensed sequence of Cretaceous and Tertiary sediments over the structure. Contractional rejuvenation and active diapirism in the mid-Miocene deformed and fractured the thinned overburden to produce a tilted raft with reservoir dips generally over 45°. A vertical oil column of over 950 m (3000 ft) is present. Hydrocarbon migration began during the Oligocene, preserving good porosity in the chalk matrix during subsequent Late Miocene burial.

Seismic imaging difficulties caused by the steep dips and uncertainties over reservoir performance and connectivity
led
to a phased development plan. Phase 1
involved
a six month early production
system
(EPS) via two imaginatively designed production wells, during which time 5 × 10
6
BBL oil were produced. The two wells were drilled from the crest of the raft down to the oil water contact. Each well encountered over 1000 m (3300 ft) of oil-bearing chalk reservoir and provided important geological and reservoir performance data. Seismic data have been reprocessed using 3D pre-stack depth migration to improve image quality and lateral positioning. Geophysical, geological and biostratigraphical data have been integrated with the dynamic performance from the EPS to assess the controls on reservoir quality.

The bulk of the reserves, and highest productivity occur in the Late Cretaceous Tor Formation. Chalk porosity ranges from 15–35% but matrix permeabilities are low, generally less than 5 mD. High productivities (up to 40 000 BOPD) achieved during the EPS are attributed to specific zones within the Tor chalks which have a favourable combination of chalk lithofacies (including local reworking to produce chalk debris flows), open fractures, depth of burial, diagenetic history and stratigraphy. Sand units within the Ekofisk chalk may have a significant impact on fluid flow.
In situ
stress is expected to exert a major (favourable) influence on waterflood performance.

An effective phased development programme has resulted in an improved understanding of the complex controls on chalk reservoir performance in preparation for Phase 2 development. First oil is planned for mid-1998 and ultimate reserves are expected to be in excess of 60 × 10
6
BBL oil.
Rock physics and quantitative wavelet estimation for seismic interpretation: Tertiary North Sea
Authors: SIMM R. W. , XU S. , WHITE R. E.
Published: April 2010

A case study from the Tertiary of the North Sea is presented in which a well tie is improved through the application of zero offset processing and well log velocity prediction. The well ties are quantified using the techniques of White (1980, Partial coherence matching of synthetic seismograms with seismic traces.
Geophysical Prospecting,
28,
333–358) and Walden & White (1984, On the errors of fit and accuracy in matching synthetic seismograms and seismic traces.
Geophysical Prospecting,
32,
871–891) and log conditioning employs the Xu–White sand/clay velocity model (Xu, S. & White, R. E. 1995. A new velocity model for clay–sand mixtures.
Geophysical Prospecting,
43,
91–118; Xu, S. & White, R. E. 1996. A physical model for shear-wave velocity prediction.
Geophysical Prospecting,
44,
687–717). A well tie of a migrated stack section to a zero offset synthetic, produced with minimal data conditioning, gave a poor tie and event identification was in doubt. Additional data conditioning improved the tie dramatically (from 43% to 72% energy predicted and a reduction in phase error on the wavelet estimation from >20° to <10°). These enhancements in the well tie are important in justifying decisions to perform wavelet deconvolution, zero phasing or seismic trace inversion to acoustic impedance as well as correctly identifying the top reservoir reflection. This study also demonstrates that detailed log conditioning is crucial in this area to predict a diagnostic AVO (amplitude variation with offset) response related to the presence of hydrocarbons.
Applications of the coherency cube in the UKCS
Authors: HUGHES M. J. , DHANANI S. , FRIMPONG R. K. , GAINSKI M. , HASKELL N. L. , HEATH R. P. , KANTOROWICZ J. D. , MAGUIRE P. M. , NISSEN S. E.
Published: April 2010

Seismic coherency is a measure of the similarity between seismic traces. Coherency data play an important role in the delineation of structural and stratigraphic features by enhancing the images seen on conventional 3D seismic data. Through integration of coherency data with other technologies and calibration to well data, new applications are emerging. This paper discusses three examples from the UKCS.

Partial permeability barriers in the Leman Field were known to exist from well pressure data. A coherency cube revealed several lincations that separated the wells. These lineations can be correlated to a previously unmapped fault trend with very small throws on the vertical seismic lines. The coherency cube therefore provided additional information to improve confidence in the interpretation and enabled significant time-savings in the fault pattern interpretation.

Coherency data were used to assist in the welt planning for the Arkwright Field development programme. Coherency slices through high pressure zones were characterized by polygonal faulting, whilst less faulting was observed in areas of lower pore pressure. These pressure zones were calibrated to pore pressures predicted from the original discovery well. The development trajectories were optimized with respect to the fault patterns interpreted from the coherency data. Model predictions were confirmed by drilling results.

A coherency cube was utilized to enable a quick interpretation of the structural framework of an area of the Central Graben. In particular the distribution of faulted Triassic rafts and Jurassic rifts is easily observed because the relatively coherent Jurassic reflectors contrast well with the relatively incoherent Triassic seismic reflectivity.

These three examples illustrate how coherency data can be used in production, development and exploration settings to improve the imaging of geological features ranging in scale from reservoir barriers to major faults.
Kingdom Oil and Gas Field, 25 Year Commemorative volume
Authors: COWAN G. , BURLEY S. D. , HOEY N. , HOLLOWAY P. , BERMINGHAM P. , BEVERIDGE N. , HAMBORG M. , SYLTA Ø.
Published: April 2010

The East Irish Sea Basin is a simple petroleum system with a complex geological history. The petroleum play comprises a Namurian source with a Sherwood Sandstone Group carrier and reservoir sequence, sealed by shales and salts of the Mercia Mudstone Group. The source underwent rapid burial during the Triassic and Jurassic, reaching a maximum burial depth of 5 km by the end-Cretaceous. Uplift by as much as 2 km took place between 60 and 20 Ma across the basin.

One-dimensional modelling predicts that oil generation took place over a temperature range of 70–120°C. Primary gas generation began contemporaneously with oil generation but continued to temperatures >180°C. Pseudo-3D ray tracing modelling of the volume and distribution of hydrocarbons in the Sherwood Sandstone Group, matched to known hydrocarbon discoveries in the East Irish Sea Basin, indicates that before 180 Ma, traps were mainly gas filled. Continuous burial to 144 Ma resulted in an oil leg developing in most traps, with significant oil accumulations present in the south of the basin. Thermogenic gas generation in the Keys Basin displaced oil in the northern traps during the period leading to maximum burial at 65 Ma, by which time the basin was characterized by a distinct northern gas province and a southern oil province. Subsequent uplift from 60 Ma caused trapped gas to expand and further displace oil from most of the remaining traps in the southern part of the basin. Hydrocarbon flow rates were greatest between 65–60 Ma during maximum burial. Subsequently, hydrocarbon flow rates decreased dramatically, although gas generation continued until 20 Ma.
The meaning of oil depletion and its consequences
Authors: CAMPBELL C. J.
Published: April 2010

When we call oil and gas fossil fuels, we admit that they were formed in the geological past and are therefore subject to depletion. Oil provides 40% of traded energy, 90% of transport fuel, and is critical for agriculture. The oil industry has great knowledge and experience. It has searched the world with sophisticated technology, always looking for the biggest and best prospects. Extrapolation of the discovery trend of the past, therefore, offers the best means of forecasting future discovery and production. Since oil has to be found before it can be produced, it follows that production has to mirror discovery after a time-lag. World discovery on this basis peaked in 1964, meaning that the corresponding peak of production is now imminent. The consensus of published reports of the total endowment of ‘conventional oil’ is just under 2000 × 10
9
barrels. To this has to be added bitumen, heavy oils, deepwater oil, polar oil and natural gas liquids, bringing the total to about 2700 × 10
9
bbl
: Experience to-date shows that the peak in any country comes at, or before, the midpoint of depletion, which respects both the physics of the reservoir and the fact that most of the oil in any country occurs in a few large fields. Accordingly, we can expect global peak to come around 2010, followed not long afterwards by the peak or plateau of gas. The world must now prepare for the transition to declining oil and gas supply. Market economics that inevitably encourage rapid depletion have done immense damage by persuading governments to ignore resource constraints. In the past, geologists had a mission to identify the resources of the world for the benefit of mankind, and indeed they have still more to do in identifying what remains to be found in ever smaller and more difficult fields. But a far more important new mission is to explain just how finite the resources are, so that governments may adopt policies to cut demand, for which there is great scope, and bring on renewable energies as fast as possible. Failure to recognize the consequences of depletion leads to international tensions with countries vying with each other for access to the remaining oil supply, half of which lies in just five Middle East countries.