bluuebunny/crossref_metadata_2025 huggingface dataset:
The evolution of the Central North Sea Rift
Authors: ERRATT D. , THOMAS G. M. , WALL G. R. T.
Published: April 2010

Data drawn from regional 3D seismic coverage in the UK North Sea and 2D surveys from adjacent areas illustrate the sequential development of Triassic, Jurassic and Cretaceous rifting across the Norwegian, Danish and UK North Sea. The North Sea Rift system is described as a failed rift of Jurassic-Triassic age formed on a basement transected by Palaeozoic lines of weakness developed principally along NE-SW ‘Caledonide’ and NW-SE ‘Trans-European Fault Zone’ trends. The principal effect of these Palaeozoic lineaments has been to offset the development of extensional sub-basins within the rift in an en echelon fashion. The Triassic–Early Cretaceous history of the North Sea records a westward migration of the rift axis. During the Late Jurassic, a significant rotation of kinematic vectors resulted in an overprint of the pre-existing basin fabric throughout the Central North Sea–Outer Moray Firth area. Fault, isopach and stratigraphic data illustrate the effects of kinematic axis rotation during Jurassic rifting on the Late Jurassic–Cretaceous basin fill. The Jurassic polyphase rifting history of the North Sea is explained by a variation on a ‘vector triangle’ model proposed by earlier workers. On all scales, structural geometries within the Central North Sea are primarily attributable to basement extension and salt tectonics. The role of the inherited Palaeozoic basement grain during extension has been to offset the extensional structures and sub-basins in an en-echelon, relay manner, forming the fundamental structural components of the basin.
Processes and products of footwall degradation, northern Brent Field, Northern North Sea
Authors: McLEOD A. E. , UNDERHILL J. R.
Published: April 2010

Analysis of seismic, electrical log and core data from the northern part of the Brent Field enables the syn-tectonic fault scarp degradation complex which is mounted on the footwall scarp of the main block bounding fault to be characterized. The seismic geometry of this package of structurally degraded and sedimentologically reworked sediments describes a hierarchy of thickness variations that occur not only in a N–S orientation in response to changes in the underlying footwall geology, but also on a smaller, sub-km scale, where the complex is characterized by amalgamated cuspate-shaped forms. Within the complex, primary sedimentological integrity is generally preserved and displaced pre-rift formations remain stratigraphically the right-way-up. These observations support a model of translational–rotational sliding as the dominant mechanism for mass wastage from the scarp. Integrating these descriptions with biostratigraphical information from hanging wall Well 211/29–8, a nine stage model for the development and evolution of fault scarp degradation in the northern part of the Brent block is proposed. This describes the initiation of normal faulting and degradation in the Late Bajocian, followed by systematic failure of the scarp until the cessation of significant tectonic activity in the Volgian. The scarp continued to be denuded, probably by surface sediment reworking (e.g. slumps and flows), until complete burial was achieved in the Campanian. The mechanism of failure and the topography of the scarp are a function of the environment (submarine) and different relative strengths of the footwall rheologies. Although this study concludes that slide blocks have a potential to add significant volumes to the field hydrocarbon reserves, it also serves to illustrate the problems that are associated not only with accurate reservoir characterization but also in constraining the degree of compartmentalization within the complex.
The origin and genesis of major Jurassic unconformities within the triple junction area of the North Sea, UK
Authors: DAVIES R. J. , O’DONNELL D. , BENTHAM P. N. , GIBSON J. P. C. , CURRY M. R. , DUNAY R. E. , MAYNARD J. R.
Published: April 2010

Three Jurassic unconformities of intra-Aallenian, intra-Callovian and intra-middle Oxfordian age are consistently identified within the literature for the North Sea region but despite their common recognition these unconformities remain poorly understood and they are essentially spatially and temporally unconstrained. The recent development of a sequence stratigraphic framework for the Jurassic of the North Sea allows for a thorough investigation of the spatial and temporal characteristics of the intra-Callovian and intra-Oxfordian unconformities and these data have important implications for Middle and Late Jurassic rifting within the North Sea region.

The intra-Aalenian unconformity is believed to have been generated as a result of an increase in geothermal gradient within the area and subsequent thermal uplift that preceded rifting. The stratigraphic gap associated with this unconformity increases towards the proposed location of a Middle Jurassic, triple junction centred dome. In contrast the stratigraphic break associated with the intra-Callovian and the intra-middle Oxfordian unconformities is found to increase onto what are now intra-rift highs and rift margin areas. These unconformities occur at the same time as distinct changes in the geometry of the fill of the triple junction and also occur at the same time as seismic scale extension. They do not appear to correlate to any supposed global fall in relative sea-level. Stratigraphic data indicate that three phases of extension occurred during the Callovian, middle and late Oxfordian and lastly during the Kimmeridgian and Volgian. Although the intra-Callovian and intra-middle Oxfordian unconformities can be confused with the effects of the Middle Jurassic North Sea dome unconformity, they are in fact the result of extension during the initial Callovian and Middle/late Oxfordian rift phases. The recognition of these regional unconformities suggests that the Middle and Late Jurassic rifting process was punctuated rather than being continuous.
Tectono-stratigraphic development of the southern part of UKCS Quadrant 15 (eastern Witch Ground Graben): implications for the Mesozoic–Tertiary evolution of the Central North Sea Basin
Authors: JONES G. , RORISON P. , FROST R. , KNIPE R. , COLLERAN J.
Published: April 2010

A detailed structural analysis of southern part of UKCS Quadrant 15 has been integrated with a regional tectonic evaluation of the eastern Witch Ground Graben to provide a geological framework for exploration. Seismic data show that the tectonic development of the Witch Ground Graben was controlled by multiple reactivation of W–E, NE–SW and WNW–ESE to NW–SE Palaeozoic basement structures of Caledonian and Variscan origin. These structures can clearly be seen on regional seismic attribute maps at Mesozoic reservoir levels. They were first reactivated during Middle Jurassic (Toarcian–Bathonian) plume-related uplift, extension and volcanism. Extension was accommodated along several major crustal detachment faults, which locally appear to underpin the southern Fladen Ground Spur. These detachments also acted as volcanic conduits on a range of scales during crustal thinning. Following a phase of ?Bathonian–Oxfordian weak rifting and subsidence, these fault systems were again reactivated during the early Kimmeridgian-Ryazanian transtensional opening of the Witch Ground Graben, which is interpreted to have occurred in a sinistral simple shear system. This new model explains observations which suggest there was extension on ~W–E faults, variable transtensional and transpressional activity on NW–SE and WNW–ESE faults, and reverse reactivation of N–S faults over this interval. In Quadrant 15, structures formed during this phase are clearly seen at the Base Cretaceous level, the Upper Jurassic sequences having behaved as a thin, mechanically weak cover to the deforming rigid basement. Further structural activity is evidenced during the Tertiary, which is significant to the understanding of overlying Palaeogene reservoirs. This structural model has implications for Jurassic and Lower Cretaceous sandbody generation and deposition, for structural trap formation and integrity and for the distribution of reservoir compartments.
‘Laramide’ events: Mid North Sea High (UK Quadrants 38 & 39)
Authors: JOHNSON H. , QUINN M. F. , BULAT J. , LONG D.
Published: April 2010

The Mid North Sea High is a broad, approximately east-trending structural high that extends from the coast of the Scottish Borders region to the UK North Sea median line. Recently acquired seismic reflection profiles were used to map isopach patterns of the Chalk Group (Upper Cretaceous and lowermost Paleocene), of the combined Montrose and Moray groups (Paleocene and lowermost Eocene), and of the lower Stronsay Group (lower Eocene) in UK Quadrants 38 and 39, on the eastern Mid North Sea High. In an anomalous and undrilled part of this area, the Chalk Group thins over an area broadly centred on a long lived, buried, NNW-trending structural high with a core largely comprised of Devono-Carboniferous sediments. In contrast, the combined Montrose and Moray groups and the lower Stronsay Group sediments thicken over the buried high. We informally name the buried high the
α
Ridge.

Signal processing techniques were applied to the original stack sections to help clarify the geometry of seismic reflections, especially from within the Chalk Group and Palaeogene successions of the anomalous zone, and to interpret deep erosion at the top of the Chalk Group that probably occurred in Danian times. The area of deeply eroded Chalk was subsequently buried beneath an anomalously thick Palaeogene succession. The area of eroded Chalk is more extensive than the
α
Ridge and extends northwards beyond the survey area. From a regional perspective, the processes that may have controlled the observed depositional pattern are discussed. It is considered whether rapidly changing tectonic stress may have caused local ‘Laramide’ (mid Paleocene–Danian) uplift and erosion of the
α
Ridge and its immediately surrounding area, swiftly followed by enhanced subsidence (inversion) and deposition in the mid Paleocene (Danian), or whether the stratal pattern is better explained by the infilling of a large erosional hollow; mechanisms involving the local halokinesis and/or dissolution of Zechstein salt are discounted. We conclude that additional seismic and palaeogeographical information is required to fully test the models proposed in this paper.
Rift–raft tectonics: examples of gravitational tectonics from the Zechstein basins of northwest Europe
Authors: PENGE J. , MUNNS J. W. , TAYLOR B. , WINDLE T. M. F.
Published: April 2010

The concept of rift–raft tectonics was developed by Penge
et al
(1993) to describe structures formed above a salt substrate along the shallow margins of the East Central Graben. These structures were formed initially during the late Triassic–early Jurassic basement extension and modified by later Jurassic and Cretaceous phases. Rift–raft tectonism is characterized by the presence of relatively undeformed blocks (rafts) separated by zones of intense deformation (rifts) associated with reactive, active and passive halokinesis. This tectonic style is part of a continuum of such styles produced by gravity sliding, which can be observed in the Zechstein basins of northwest Europe, where near continuous sheets of Triassic sediments have been deformed in a dominantly extensional regime. Down-dip movement of these sheets caused compression in the deeper parts of the basins, thereby triggering a number of diapirs. Examples from the Fisher Bank Basin, East Central Graben and South Central Graben are illustrated and compared to those of the Irish Sea. Variation in forms are described and empirically related to tectonic environments as a result of the interplay of the controls identified. These include salt thickness, lithology of the Mesozoic overburden and regional dip. Physical and finite element modelling have been used to demonstrate the validity of the interpretation and thus demonstrate the importance of gravity sliding in forming structures in Zechstein basins.
Structural elements and petroleum geology of the Norwegian sector of the northern Barents Sea
Authors: GROGAN P. , ØSTVEDT-GHAZI A.-M. , LARSSEN G. B. , FOTLAND B. , NYBERG K. , DAHLGREN S. , EIDVIN T.
Published: April 2010

The northern Barents Sea is divided into two principal prospective provinces: (1) the platform areas south and east of Svalbard contain possible hydrocarbon plays in Upper Palaeozoic clastics and carbonates, and in Mesozoic sandstones; (2) The Barents Sea margin, which includes the Yermak Plateau, with possible plays in late Mesozoic and Tertiary sandstones. The platform areas are underlain by a Palaeozoic rift system of highs and intervening basins, containing Devonian and Early Carboniferous rocks overlain by relatively flat-lying sequences of Late Palaeozoic and Mesozoic age. The Barents Sea margin is a complex basin province formed in response to phases of the Late Mesozoic and Cenozoic break-up of the Pangea supercontinent, and subsequent seafloor spreading west of Spitsbergen.

The northern Barents Sea is as yet not open for commercial exploration, and no deep wells have been drilled in the offshore areas. It is one of the largest remaining frontier areas in western Europe and as such affords a wide range of exploration opportunities. However, the area is considered to be of high risk for petroleum exploration, with generally low statistical probabilities of discovery, largely because of the limited amount of data available to evaluate potential hydrocarbon plays, and in particular the presence, distribution, quality and maturity of source rocks. Hydrocarbon resource estimates are therefore characterized by large uncertainties.

Potential reservoir rocks are identified, and seismic mapping has revealed many structural and stratigraphic leads on the Sentralbanken High, the Kong Karls Land Platform, the Gardarbanken High and in the Mesozoic and Tertiary sub-basins northwest of Bjørnøya. Future challenges will be focused on reducing uncertainties in mapping the distribution of source rocks of suitable quality. Limited basin modelling studies indicate that oil may have been generated and accumulated in many provinces, but better constrained models are required to evaluate Early Cretaceous thermal events and Late Cenozoic uplift processes that may have had negative effects on both oil generation and the retention of hydrocarbons in traps.
The prospectivity of the Vøring and Møre basins on the Norwegian Sea continental margin
Authors: BREKKE H. , DAHLGREN S. , NYLAND B. , MAGNUS C.
Published: April 2010

The focus of exploration activity on the Norwegian Sea Continental Shelf has shifted westwards from the shallow water areas over the Trøndelag Platform and Halten Terrace to the deep water areas over the Cretaceous Vøring and Møre basins. The basins, especially the Vøring Basin, have been involved in a complex tectonic development since the main extensional phase in the late Middle Jurassic to Early Cretaceous, including Cenomanian–Campanian and Maastrichtian–Eocene extension and thermal uplift and intermittent phases of compression/transpression. While the Jurassic is the prospective interval in the Trøndelag Platform and Halten Terrace, the potential in the Vøring and Møre basins is seen mainly in the Cretaceous and Paleocene – the Jurassic is too deeply buried. The two first deep water exploration wells in these basins, 6707/10-1 and 6305/5-1, have already proven two effective plays, with excellent reservoir sands in the Upper Cretaceous and lower Paleocene charged with thermogenic gas in structural traps in rotated fault blocks and compressional domes. Palaeogeographical reconstructions indicate that these reservoir rocks may be widespread within the basin areas. No good oil-prone source rock at a suitable depth of burial is so far proven, but the palaeogeographical reconstructions linked to surrounding well data and regional analyses indicate a chance for the existence of Upper Cretaceous oil source intervals. The thermal history of the basins is as complex as their tectonic history, and is further complicated by the possible effects of the Paleocene–Eocene magmatic activity, making any basin modelling results at the present stage unreliable.
Techniques used in the exploration of turbidite reservoirs in a frontier setting – Helland Hansen licence, Vøring Basin, offshore mid Norway
Authors: SANCHEZ-FERRER F. , JAMES S. D. , LAK B. , EVANS A. M.
Published: April 2010

Following the award of operatorship in the frontier Vøring Basin. Norske Shell embarked on the 3D seismic evaluation of a new play in the Upper Cretaceous. Integration of regional geology with various geophysical techniques is shedding light on the lithological and depositional model in this uncalibrated area. The current interpretation is that of a deepwater fan, with deposition of reservoirs in sheet layered geometries. The first exploration well is scheduled for drilling early in 1998.
Cenozoic development of the Norwegian margin 60–64°N: sequences and sedimentary response to variable basin physiography and tectonic setting
Authors: MARTINSEN O. J. , BØEN F. , CHARNOCK M. A. , MANGERUD G. , NØTTVEDT A.
Published: April 2010

The Cenozoic development of the Norwegian margin between 60 and 64°N can be described in terms of five, unconformity-bounded, megasequences: Paleocene–lowermost Eocene (65–51 Ma), Lower Eocene–lowermost Oligocene (51–31 Ma), Lower–upper Oligocene (31–24 Ma), Miocene–Pliocene (24–1.9 Ma), and Pleistocene (1.9 Ma–present). Each sequence is constructed of systems tracts, varying in type and number between sequences. In the Møre Basin (62–64°N), lowstand wedges dominate and form an offlapping, regressive pattern. In the Northern North Sea Basin (60–62°N), lowstand, transgressive and highstand systems tracts are present, showing a more variable framework.

The variability observed is interpreted to reflect the response of sediment supplied, both in time and space, into different basin physiographies related to the overall tectonic context of the two areas. The Northern North Sea Basin in the Cenozoic received sediments generated from the basin margins in periods of uplift and lowstand. Clastic systems probably prograded into a gradually-shallowing central basin area, formed as a response to decaying, post-rift subsidence, margin uplift and sediment fill. The Møre Basin, in contrast, acted as a margin to incipient and on-going North Atlantic rifting, and the basin physiography was that of a more steeply inclined slope facing the northwest. Larger water depths caused sediment wedges to be restricted to the basin margin rather than acting as fill of the central basin area such as occurs in the North Sea. The offlapping pattern probably records repeated uplift of the Norwegian mainland, shedding sediments into the basin.
Early Tertiary heat flow along the UK Atlantic margin and adjacent areas
Authors: GREEN P. F. , DUDDY I. R. , HEGARTY K. A. , BRAY R. J.
Published: April 2010

The sedimentary basins of the UK North Atlantic margin are characterized by extensive igneous activity, mostly of Early Tertiary age, related to continental rifting which led to the separation of Europe from Greenland. Given this setting, it might be expected that elevated basal heat flow during the Early Tertiary would have exerted a critical control on the thermal histories of potential hydrocarbon source rocks. Apatite fission track analysis and vitrinite reflectance data have been used to identify, characterize and quantify significant palaeothermal events which have affected the region. This approach allows reconstruction of thermal histories based on directly measured data, rather than relying on theoretical models.

Results from the West of Shetland region show significant Tertiary heating, but this is often characterized by low palaeogradients or non-linear palaeotemperature profiles, suggesting heating caused by fluid movements. No evidence has been observed of enhanced Early Tertiary heat flow, and little evidence has been found to support significantly deeper burial during the Early Tertiary. Results from Hebridean basins also argue against a significantly increased Early Tertiary heat flow, while showing strong evidence for Late Tertiary uplift and erosion.

Consistent evidence for increased Early Tertiary heat flow is seen to the south, in the Lake District and Solway basins. This may reflect the ability of extensive igneous activity to release heat from the base of the crust to shallower levels throughout the Atlantic margin and the Tertiary igneous province, whereas in the absence of igneous activity, the lack of a pathway for heat to escape results in elevated basal heat flow.

Models based on high Early Tertiary heat flow are likely to over-predict maturity levels in Jurassic source rocks, and predict an earlier timing for hydrocarbon generation. Both factors could lead to false assessment of regional hydrocarbon prospectivity.
The Northeast Rockall Basin and its significance in the evolution of the Rockall-Faeroes/East Greenland rift system
Authors: TATE M. P. , DODD C. D. , GRANT N. T.
Published: April 2010

The newly mapped Northeast Rockall Basin lies between the Wyvile–Thomson Ridge and an ESE lineament extending from the onshore Ness Shear Zone. It contains a window in the early Tertiary flood-basalts through which a syn-rift sedimentary succession is imaged on seismic data. This succession was mapped in detail, calibrated to wells and boreholes drilled on the basin margins, and integrated with results from gravity, magnetic and backstrip modelling. Modelling and seismic stratigraphic analysis show that the Wyville–Thomson Ridge is a late Eocene to Oligo–Miocene fault-propagation fold, comprising basalt at the seabed and folded low density sediments within its core. Together with the co-linear Munkegrunnur and Ymir Ridges, this fold developed from positive inversion above a composite, crustal-scale ramp-flat detachment during N–S compression. Prior to mid-Tertiary inversion of the Wyville–Thomson Ridge, the basin may have formed a link between the main part of the Rockall Trough to the south and the Faeroe–Shetland and East Greenland rift systems to the north. Triassic, Jurassic and Early Cretaceous syn-rift sediments have been penetrated in the adjoining West Lewis Basin, and the Northeast Rockall Basin may contain sediments with a similar age range beneath the Tertiary flood basalts. Estimates from modelling allude to a resurgence of rifting from mid- to latest Cretaceous times, perhaps coeval with voluminous axial volcanism in the adjacent Rockall Trough. This episode of syn-rift volcanism, underlying the Paleocene volcanics, prohibits deeper seismic resolution to the south of the lineament in the main part of the Rockall Trough. The present-day bathymetry has resulted from a complex interaction between the loci of Mesozoic stretching, renewed post-basalt subsidence and major late Eocene to Oligo–Miocene inversion.
Structural setting, geological development and basin modelling in the Rockall Trough
Authors: SHANNON P. M. , JACOB A. W. B. , O’REILLY B. M. , HAUSERR F. , READMAN P. W. , MAKRIS J.
Published: April 2010

Evidence from wide-angle seismic data in the Irish sector of the Rockall Trough suggests that the basin is underlain by thinned continental crust which has undergone differential stretching. The upper crust has been thinned by a stretching factor of 8–10 while the middle and lower crust (and probably also the lithospheric mantle) was stretched by a factor of 2–3. The latter figure is suggested as being representative of the overall lithospheric stretching. Crustal modelling fails to demonstrate any significant effect of the Iceland plume on the development of the basin. The Rockall Trough contains up to 6 km of sedimentary strata. Well and seismic data from adjacent basins indicate that the succession in the basin is likely to be of Late Palaeozoic to Recent age. The pre-Cretaccous faeies are suggested to be broadly similar to other basins of the Atlantic borderlands. Cretaceous and Tertiary strata show progressive clastic starvation as thermal subsidence outstripped sedimentation in the basin, Sandy facies are likely to be concentrated towards the faulted basin margins and the re-entrant regions where the basin margin changes orientation. Basin modelling, based on normal incidence and wide-angle seismic profiles from a number of areas in the Irish sector of the basin, demonstrates that the observed seismic geometries and subsidence patterns cannot be explained by a single rift episode in the Cretaceous, The best fit of possible models suggests that the basin developed in response to discrete rift episodes in the Triassic, Late Jurassic and Early Cretaceous.
Petroleum geology of the Irish Rockall Trough – a frontier challenge
Authors: WALSH A. , KNAG G. , MORRIS M. , QUINQUIS H. , TRICKER P. , BIRD C. , BOWER S.
Published: April 2010

The Irish Rockall Trough is a frontier area recently opened for hydrocarbon exploration. The Trough, a NE–SW trending Mesozoic–Cenozoic basin, lies in water depths ranging between 500 m and 3000 m. Basin development began as early as the Late Carboniferous and a series of rift episodes continued sporadically throughout the Mesozoic and Tertiary. The most important of these occurred between the Late Jurassic and Early Cretaceous. The post-rift thermal sag phase followed in the Late Cretaceous and Tertiary. The resultant basin is characterized by thinned continental crust, a series of tilted fault blocks and a number of smaller ‘perched’ basins adjacent to the main basin-bounding faults.

This paper presents an assessment of the petroleum prospectivity of the Rockall Trough by focusing on its stratigraphic and structural development, integrating the results from modelling work and providing concise play models. A suite of regional palaeogeographic maps are used to develop a full understanding of the stratigraphic history of the Trough. A structural modelling study is then discussed in which two regional seismic lines are interpreted and flexurally back-stripped. Finally, a source-rock modelling study is presented whereby the timing and extent of oil and gas explusion within the potential kitchen areas are examined. The conclusions of this work are illustrated on a series of play concept diagrams.
The structural evolution of the Erris Trough, offshore northwest Ireland, and implications for hydrocarbon generation
Authors: CHAPMAN T. J. , BROKS T. M. , CORCORAN D. V. , DUNCAN L. A. , DANCER P. N.
Published: April 2010

The Erris Trough is a narrow, elongate, Mesozoic basin lying adjacent to the eastern margin of the Rockall Trough to the northwest of Ireland. Limited drilling in the area has proven Carboniferous, Permo-Triassic, Lower–Middle Jurassic and Cretaceous sediments beneath a thin Tertiary cover. Considerable variation in structural style and preserved stratigraphy is observed along the basin. Based on interpreted fault polarity and observed pre-Cretaceous stratal dip, the Erris Trough can be subdivided into three structural sub-basins separated by diffuse, poorly defined, overlapping or divergent transfer zones.

Carboniferous basin development in the area of the Erris Trough was significant but is poorly constrained. Permo-Triassic rifting produced a series of half-graben, some of which were controlled by down-to-the-southeast faults. This extensional phase was followed by post-rift subsidence during the Early and Middle Jurassic. In addition, N–S and NW–SE faults locally influenced depositional patterns. In the southern part of the Erris Trough a Middle to Late Jurassic rifting event produced a reversal of the earlier basin geometry with the generation of northwest- and west-downthrowing normal faults. Along the western margin of the Erris Trough, footwall uplift associated with this event induced massive, kilometre scale, uplift and erosion implying Late Jurassic rifting within the Rockall Trough. Restricted basins may have been developed in the area during the Cretaceous as a result of ‘ponding’ of deposition to the east of this zone of footwall uplift. A minor extensional phase occurred during the Aptian–Albian resulting in local reactivation of the Late Jurassic faults concurrent with rifting in the Rockall Trough. A westerly tilt of the basin was established during the Late Cretaceous–Early Tertiary caused by downflexing associated with thermal subsidence of the Rockall Trough. Regional uplift and erosion occurred during the Oligocene–Miocene.

Basin modelling indicates that Lower Jurassic source rocks may have generated oil during the Early Eocene to Early Miocene, dependent on an elevated geothermal gradient during the Early Tertiary. Late Carboniferous sediments may have generated hydrocarbons at a number of times during the basin’s history.
Comparative Jurassic and Cretaceous tectono-stratigraphy and reservoir development in the Jeanne d’Arc and Porcupine basins
Authors: WILLIAMS B. P. J. , SHANNON P. M. , SINCLAIR I. K.
Published: April 2010

The Jeanne d’Arc Basin offshore Newfoundland and the Porcupine Basin to the west of Ireland, developed under similar tectono-sedimentary conditions in pre-Late Cretaceous times. Rifting events in Permo-Triassic, Late Jurassic–Early Cretaceous and mid Cretaceous times were each followed by a phase of thermal subsidence. While a broadly common lithofacies pattern occurs within and between the basins, the detailed response of the basins differs in terms of style of structuring, reservoir architecture and quality of source rock development. The Late Jurassic–Early Cretaceous (Tithonian to Berriasian) rift episode is the most significant, with good reservoir sandstones and rich source rocks forming part of this rift sequence.

A Late Oxfordian to Kimmeridgian onset warp phase resulted in the development of shallow marine limestones and deeper water organic-rich shales and marls in the Jeanne d’Arc Basin, with non-marine, shoreface and restricted marine sandstones and shales in the Porcupine Basin. The Late Jurassic–Early Cretaceous syn-rift episode resulted in the development of sandstone-prone successions which interfinger with good oil-prone source rocks in both basins. Reservoir architecture in both basins was influenced by this episode. Within the Porcupine Basin a range of lithofacies developed, ranging from fluvial channels and bars, to submarine fan turbidites. Within the Jeanne d’Arc Basin, periodic rejuvenation of fault footwalls produced conglomeratic pulses resulting in transverse alluvial fans which interfinger with axially-flowing braid channels and bars within the incised valley fills. Periods of increased subsidence resulted in the development of lacustrine facies and flood plain mudstones.
Sedimentary basins and petroleum systems offshore Newfoundland and Labrador
Authors: DeSILVA N. R.
Published: April 2010

The sedimentary basins offshore Newfoundland and Labrador include those related to the opening and closing of the Palaeo-Atlantic (Iapetus) Ocean and those formed during the opening of the present Atlantic Ocean. The oldest among these are the Lower Palaeozoic Anticosti Basin and the Carboniferous Maritimes successor basin offshore western Newfoundland. The Mesozoic sedimentary basins on the Grand Banks and on the Labrador Shelf record the progressively northward opening of the present Atlantic Ocean. These include the South Whale Sub-basin of the Scotian Basin, Whale Basin, Horseshoe Basin, Carson Basin, Jeanne d’Arc Basin, Flemish Pass Basin, East Newfoundland Basin, Hopedale Basin and Saglek Basin.

Petroleum systems occur in the Anticosti Basin, Maritimes Basin, South Whale Sub-basin, Jeanne d’Arc Basin, Flemish Pass Basin and the Hopedale Basin. In the Anticosti Basin, Lower Palaeozoic marine shales constitute source rocks and carbonates form the reservoir rocks. In the Maritimes Basin, Lower Carboniferous lacustrine rocks form the source rocks and Carboniferous sandstones form the major reservoir rocks. On the Grand Banks, Kimmeridgian shale represents the source and Upper Jurassic and Lower Cretaceous sandstones constitute the main reservoir units. Offshore Labrador, Tertiary shales are considered to be the source and Cretaceous sandstones form the principal reservoir rocks. All the basins with petroleum systems have a favourable combination of elements and processes needed for hydrocarbon accumulations.
Rifting and the development of the Faeroe-Shetland Basin
Authors: DEAN KEVIN , McLACHLAN KEVIN , CHAMBERS ALAN
Published: April 2010

This paper discusses the development of the Faeroe–Shetland Basin. The area has been affected by at least five rift events from the Permo-Triassic to the Paleocene. Numerous authors have invoked the reactivation of older structures as a major influence on subsequent tectonic events. In this paper the aim is to demonstrate that the locus of rifting shifted with time.
Palaeogene magmatism in the Faeroe–Shetland Basin: influences on uplift history and sedimentation
Authors: NAYLOR P. H. , BELL B. R. , JOLLEY D. W. , DURNALL P. , FREDSTED R.
Published: April 2010

Palaeogene sedimentary basin development along the NE Atlantic margin was strongly influenced by a major period of magmatism associated with the initiation of ocean-floor spreading between NW Europe and East Greenland. Five elements to the magmatism in the Faeroe–Shetland Basin can be identified and related to the Palaeogene depositional sequences:

extensive lava fields and lavas from central complexes (the Erlend Complex for example) erupted into subaerial and marginal marine environments and consequently influenced sediment distribution within the basin;

dyke swarms, which constitute the feeder system to the lava fields;

sill complexes, possibly related to the fissure systems, which affect reservoir quality on a local scale;

central igneous complexes overlying magma chambers which controlled clastic sedimentation patterns (Westray for example);

tuffs (for example the Balder Formation and Kettla Member) which aid correlation of reservoirs and seals.

The main development of Paleocene sandstone reservoirs along the axis of the Faeroe–Shetland Basin appears to have been synchronous with the phases of thermal uplift along the basin margin and pulsed volcanism at
c.
62 Ma, 58 Ma and 56.6–55 Ma. The major episodes of reservoir deposition may reflect the activity of the Iceland plume and provide independent evidence of the pulsed nature of the magmatism. This model integrates igneous, sedimentary and tectonic data with precise radiogenic ages and biostratigraphy. It allows detailed correlation of reservoirs and seals within the Paleocene play fairway and improves prediction of stratigraphic trapping styles common in this play.
Stratigraphic relationships of the Lower Tertiary of the Faeroe Basalt Plateau and the Faeroe–Shetland Basin
Authors: KIØRBOE L.
Published: April 2010

The Faeroe Basalt Plateau is located in the western part of the Faeroe–Shetland Channel. The Plateau consists of Lower Tertiary basalt subaerially extruded and accumulated both on land and under water. The basaltic succession is described and interpreted using seismic facies analysis and genetically related seismic units. The formation of each unit is related to eruptive pulses. The variation in the stacking pattern of the seismic units is related to: (1) the amount of available lava; and (2) the change of relative sea-level.

The Faeroe–Shetland Basin is a major NE–SW trending sedimentary basin located in the Faeroe–Shetland Channel. The Lower Tertiary sediments of the Faeroe–Shetland Basin are subdivided into sequences that are tentatively correlated to the lavas of the Faeroe Basalt Plateau and East Greenland.
Reconciling gravity and seismic data in the Faeroe–Shetland Basin, West of Shetland
Authors: ASHCROFT W. A. , HURST A. , MORGAN C. J.
Published: April 2010

Two-dimensional gravity modelling across the Faeroe-Shetland Basin was carried out on the basis of a published regional seismic section (

Stoker
et al.
1993

). The existing seismic interpretation of the faulted northwest margin of the basin, where the geometry and thickness of the prospective section is obscured by Tertiary basalts, required some modification to be reconciled with the gravity data and is considered a suitable target for detailed gravity modelling. The presence of the interpreted Lewisian basement high in the central area of the basin (Flett Ridge) cannot be reconciled with the gravity data. The preferred interpretation is that the ridge has a cap of pre-Cretaceous sediment about 2 km thick resting on basement. The total thickness of Tertiary sills intruded into the Cretaceous and Paleocene strata in the Flett Basin is estimated to be about 1000 m. In the example described gravity modelling has significantly improved the seismic interpretation and may especially help with understanding the structural interpretation under areas obscured by volcanics.
The importance of fluid pressures and migration to the hydrocarbon prospectivity of the Faeroe–Shetland White Zone
Authors: ILIFFE J. E. , ROBERTSON A. G. , WARD G. H. F. , WYNN C. , PEAD S. D. M. , CAMERON N.
Published: April 2010

The hydrocarbon prospectivity of the Faeroe–Shetland White Zone, located in the area between the Shetland and Faeroe Islands, was assessed in a regional study that integrated seismic and well interpretations with detailed source-rock geochemistry and predictive basin modelling.

The Faeroe Basin formed during a Barremian rifting event followed by subsidence during the Late Cretaceous. The Paleocene began with a period of thermal uplift of basement highs and rapid sedimentation which infilled the submarine topography formed during the Cretaceous, and produced marked overpressuring in the basin. Gradual subsidence continued through the Tertiary except for a significant mid Tertiary inversion event that formed several interesting structures in the basin.

New thermal models of basins and a new pressure mechanism for inducing hydrofractures that allow vertical hydrocarbon migration from Jurassic source rocks through Cretaceous mudrocks to Tertiary reservoirs, which we call the ‘whoopee cushion effect’, provide the key controls on the hydrocarbon charge mechanism, timing and petroleum composition.

The other crucial elements, source, reservoir, and traps which are present at several stratigraphic levels in the White Zone, are summarized in this paper.

The interplay of overpressure, hydrocarbon generation and migration during a complex basin evolution makes the White Zone a highly prospective frontier petroleum province.
Hydrocarbon migration history, West of Shetland: integrated fluid inclusion and fission track studies
Authors: PARNELL J. , CAREY P. F. , GREEN P. , DUNCAN W.
Published: April 2010

In sandstone reservoir rocks in the vicinity of the Rona Ridge, West of Shetland, there is evidence for former episodes of hydrocarbon migration in addition to the episodes responsible for current accumulations. An integrated study of fluid inclusion microthermometry and apatite fission track analysis (AFTA) has helped to clarify the history of fluid migration in the region. The Upper Jurassic Rona Sandstone contains both hydrocarbon and aqueous fluid inclusions. Hydrocarbons occur in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths and authigenic feldspars. This indicates at least 2 phases of migration before the presently trapped oil entered the rock: fluorescence colours indicate that they are of distinct compositions. Associated aqueous inclusions yield minimum trapping temperatures which indicate that the earliest hydrocarbons were associated with a phase of hot fluid migration, and that cooler conditions pertained during later migration. AFTA data provide independent evidence for a passage of hot fluid early in the history of the rock, which may be related to Late Jurassic–Early Cretaceous rifting. AFTA data also suggest a second episode of heating and cooling in the Tertiary following uplift, which may have been responsible for a later stage of hydrocarbon generation and migration. The results reflect repeated expulsion of fluids from deep basinal areas onto adjacent topographic highs, throughout the late Mesozoic to Recent.
Reservoir quality and petrophysical evaluation of Paleocene sandstones in the West of Shetland area
Authors: SULLIVAN M. , COOMBES T. , IMBERT P. , AHAMDACH-DEMARS C.
Published: April 2010

A series of regionally correlatable unconformity bounded sequences has been identified within the Paleocene interval of the West of Shetland area. These sequences contain a succession of sandstones deposited in deep-water environments within the Early to ‘mid’ Paleocene (Danian to early Thanetian) with a gradual shallowing upwards to shelf and deltaic deposits in the Late Paleocene.

Reservoir properties of these sandstones vary widely, with porosities ranging from >30% to below 10%, and permeabilities from >2D to <0.1 mD. Although there is a severe reduction of reservoir quality with depth of burial, certain sandstone units have retained high porosities (>20%) and permeabilities (10–100 mD) at below 3 km burial depth. This preservation of reservoir quality is due in part to depositional facies, but mineralogical/diagenetic variability has also allowed the preservation of porosity and permeability. In particular, the presence of grain rimming chlorite in these sandstones is fundamental in preserving porosity.

Log and core analyses of these sandstones has shown that interpretation of reservoir potential and fluid saturations is not straightforward. The effect of diagenetic minerals on log responses and in particular the salinity of formation waters (both in water- and hydrocarbon-bearing zones) were poorly known at the start of the study, and had to be accurately quantified.

The integration of all available data, including intensive study of core material, allowed the definition of these effects and thus allowed the adequate modelling of hydrocarbon saturations.
The Foinaven Field: managing reservoir development uncertainty prior to start-up
Authors: COOPER M. M. , EVANS A. C. , LYNCH D. J. , NEVILLE G. , NEWLEY T.
Published: April 2010

Foinaven was the first oil field discovery in the deep waters west of the Shetland Islands. The field was (re)discovered in deep water sands of Late Paleocene age in 1992 by Well 204/24a-2, and from late 1993 was the subject of an accelerated appraisal programme leading to field development approval in November 1994. Production started in November 1997 through a subsea system connected to a floating production, storage and offloading (FPSO) vessel with shuttle tanker export.

Continual improvement in the quality of the seismic data across the field coupled with the large amount of well data acquired during the appraisal and development programmes has reduced much of the static uncertainty associated with estimates of the STOIIP (oil initially in place) and reservoir geometry. However, significant issues relating to channel inter-connectivity, the field’s connected reserves, well production decline and pressure maintenance have only been able to be addressed through the acquisition of dynamic data. The key to these data has been an extended well test (EWT) performed on a horizontal appraisal well in Panel 3.

With 42 × 10
6
BBL of reserves, the Panel 3 segment of Foinaven is the second largest of the five segments identified in the field to date. The principal reservoir zone is made up of two separate seismically-identifiable channels, which may or may not be in good communication. For either outcome we planned to achieve good recovery efficiency by appropriate placement of the two pre-drilled horizontal producers. The seismic geometry of the channels, combined with the analysis of a pressure build up (PBU) following a short clean-up flow on one of the wells, indicate the presence of a potential barrier between the two targeted channels. Recent longer-term data from down-hole gauges shows similar 15 psi depletion in the both wells as a result of the extraction of 437016 BBL during the EWT. These data have indicated there is some communication between the channels and have been invaluable in maximizing understanding prior to field start-up.
The Schiehallion development
Authors: LEACH H. M. , HERBERT N. , LOS A. , SMITH R. L.
Published: April 2010

The Schiehallion development is one of the largest on the United Kingdom Continental Shelf in the last ten years, with mean recoverable reserves of 425 × 10
6
BBL. It is the second development West of Shetland, following the Foinaven Field. The Schiehallion development comprises two fields; Schiehallion and Loyal, which are located approximately 200 km west of the Shetland Islands and a similar distance northwest of the Orkneys in water depths of 300–500 m.

The Schiehallion Field was discovered in late 1993 by Well 204/20-1 and side-track -1Z. Oil is trapped in submarine slope reservoir sandstone of Paleocene age. The discovery well penetrated an oil–water contact at 2064 m TVDSS. The field extent was mapped on 3D seismic data through the first half of 1994. This was followed by the drilling of five further vertical appraisal wells in 1994–95, three operated by BP/Britoil and two by Amerada Hess. In 1995, a horizontal well, 204/20-5, was drilled and subsequently tested over an extended period.

The Loyal Field, to the north of Schiehallion, was discovered in late 1994 by Well 204/20-3 which encountered an oil column in sandstone of Paleocene age. The well was successfully side-tracked into a second, adjacent oil-hearing sandstone. An oil–water contact was penetrated at 2397 m TVDSS. A second well, 204/20-6A, drilled in 1995, confirmed the presence of oil at the crest of Loyal in a second separate segment with an oil–water contact inferred at 2160-m TVDSS.

The mapping of net pay thickness directly from 3D seismic underpins the reservoir description of the Schiehallion and Loyal fields. Vertical appraisal wells were targeted to calibrate the net pay model and reduce reservoir uncertainties. This enabled a rapid appraisal of the key elements of the static reservoir description.

Seismic attribute mapping and detailed seismic facies mapping, combined with sedimentological analysis of the cores and borehole imaging electric logs, defined the channelized architecture of the reservoirs. Horizontal Well 204/20–5 and subsequent extended well test (EWT) were designed to answer questions on the dynamic performance of the reservoir, particularly the connectivity and productivity of the sandstone reservoirs. This well was completed in the summer of 1995 after penetrating a gross reservoir length of
c
. 1700 m. The subsequent EWT achieved a stabilized, constrained flow rate of
c
. 18000 BQPD and demonstrated the productivity and good reservoir connectivity of the field. The well was suspended as a future producer.

The development of the two fields was sanctioned in 1996. Development drilling began later in 1996 and oil production commenced on schedule in July 1998. The fields will be exploited using a floating production, storage and off-loading (FPSO) vessel which is jointly owned by the field partners (BP Exploration (Operator), Shell UK Exploration and Production, Amerada Hess Ltd, Murphy Petroleum Ltd, OMV (UK) Ltd and Statoil (UK) Ltd).