bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hydrocarbon prospectivity of the Central Irish Sea Basin with reference to Block 42/12, offshore Ireland
Authors: Maddox Steven J. , Blow Richard , Hardman Martin 
Published: April 2008

Abstract
The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized.
Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion.
Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data.
The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.
Tectono-stratigraphy of the North Celtic Sea Basin
Authors: Rowell Philip 
Published: April 2008

Abstract
The Celtic Sea, offshore southern Ireland, is underlain by several discrete rift basins, which are part of a larger set of such basins that developed marginal to the North Atlantic within the tectonic framework of Pangaean break-up and episodic opening of the North Atlantic Ocean. This paper documents the development of depositional systems within a rift basin setting by analysis of the sequence stratigraphic evolution of the North Celtic Sea Basin, the largest basin in the area, with particular emphasis on the influence of pre-existing structural fabric on that evolution. The Caledonian and Variscan orogenies each imprinted distinct structural grains on this area, and the intersection pattern of their respective tectonic trends provided the structural framework which influenced subsequent Triassic, Late Jurassic and Early Cretaceous phases of rifting. Detailed analysis of an extensive well and seismic database indicates that the mode of reactivation of pre-existing lines of structural weakness depended on the orientation of the principal extensional stress direction relative to these lineaments. Triassic and Late Jurassic northwest-southeast orientated extensional stress reactivated Caledonian lineaments as half-graben boundary faults and Variscan lineaments as accommodation zones and lateral transfer faults which offset the basin margin faults. In contrast, during the Early Cretaceous, extensional stress was orientated approximately north-south, oblique to the Caledonian trend. This resulted in transtensional pull-apart basin geometries dominated by reactivation of Variscan structures. The intrinsic relationship between plate tectonic setting, pre-existing structural fabric and the resulting structural style provides insight into the influence of tectonism on depositional system development in an evolving rift basin. During Late Jurassic and Early Cretaceous rifting, depositional system development was predominantly controlled by tectonism and partly conforms to empirically derived tectono-stratigraphic models of sequence development in non-marine rifts. Depositional environments changed from fluvial-dominant to lacustrine-dominant as each rifting episode evolved through its early to active rift stages. During post-rift subsidence, marine transgressions dominated depositional system development, overriding the influence of local tectonism. Spatially, pre-existing structural fabric and the style of structural deformation are the dominant influences on sediment input and drainage systems within the evolving rift basin. Recognition of the intrinsic structural fabric of a rift basin is one of the key factors in understanding and predicting stratigraphic distributions within this type of basin.
The definition of the Albian ‘A’ Sand reservoir fairway and aspects of associated gas accumulations in the North Celtic Sea Basin
Authors: Taber D. R. , Vickers M. K. , Winn R. D. 
Published: April 2008

Abstract
Log and core data collected from exploration, appraisal and development wells, together with a large quantity of seismic data collected over a 21 year period, have allowed the delineation of a fairway of reservoir quality Cretaceous Albian sandstone (‘A’ Sand) in the north central part of the North Celtic Sea Basin. The ‘A’ Sand is the primary reservoir at the Kinsale Head and Ballycotton gas fields. Burrow types and the presence of glauconite indicate that it was deposited in a marine setting, although the precise depositional environment remains controversial. Regional isopach and lithofacies mapping indicates that only limited development of reservoir quality sandstone exists outside the main ‘A’ Sand fairway, such as at the well 48/18-1 discovery approximately 13 miles west of Ballycotton gas field. The sands are believed to have been derived from the Irish Massif to the north and were transported along a shallow-water shelf. Some northward thinning of the sandstone is recognized in the area that was closer to the interpreted palaeoshoreline. Reservoir quality is controlled, in part, by distance from the source area, while thickness, and to some extent coarseness, appears to have been controlled by palaeobathymetry. The thickest ‘A’ Sand is in the Kinsale Head field area, a local depocentre within the passively subsiding North Celtic Sea Basin. Reservoir quality diminishes to the south and southwest of the Kinsale Head gas field. Topseal for the reservoir is provided by the Albian to Cenomanian Gault Claystone and the associated overlying Cenomanian claystones. Untested structural closures within this ‘A’ Sand fairway may require a similar stratigraphy and structural configuration to the existing fields to prove successful. The Kinsale Head and Ballycotton fields are the result of fortunate geological coincidence. Albian reservoir sand accumulation was followed by deposition of deeper-water clay facies for topseal and subsequent burial beneath the Chalk. The Cretaceous stratigraphic evolution of the basin was succeeded by trap-forming basin inversion in the Early Tertiary, with concomitant and subsequent vertical migration of gas from the deeply buried and very mature thick shales within the Liassic. Additional ‘A’ Sand gas accumulations may be restricted to the structural closures in the general area of the existing fields, where there is well developed claystone facies above the reservoir and where fault offsets are less than the associated topseal thickness.
Sedimentology of the Cretaceous Greensand, Quadrants 48 and 49, North Celtic Sea Basin: a progradational shoreface deposit
Authors: Hartley Adrian 
Published: April 2008

Abstract
The Aptian-Albian Greensand interval is the main reservoir unit in the North Celtic Sea Basin. The Greensand comprises three units: (1) the Lower Claystone, (2) the ‘A’ Sand, and (3) the Gault Claystone. Sedimentological and wireline log studies of the Lower Claystone and ‘A’ Sand units within Irish Quadrants 48 and 49 indicate an overall 35 to 50 m thick, coarsening-upward profile from inner shelf mudstones to middle and possibly upper shoreface fine-grained sandstones. Both the mudstones and sandstones contain glauconite, marine fauna and marine ichnofauna. The ‘A’ Sand can be sub-divided into three coarsening-upward packages which show a progressive upward increase in higher-energy facies and thickness.
Previous interpretations have suggested that the Greensand interval represents a series of tidal sand bars formed during transgression. However, the wide lateral extent and the pulsed, progradational nature of the coarsening-upward units indicates deposition in a (wave-dominated?) shoreface environment during a punctuated regression. The overlying Gault Claystone represents an abrupt return to open marine shelf conditions.
Thickness variations across the study area suggest that faults were active to the south of the basin during Lower Claystone deposition but that activity had largely ceased by ‘A’ Sand times. Facies and isopach maps indicate that shoreface sandstones prograded west, north and east from a source lying somewhere to the south of the present-day position of the Kinsale Head field. Sediment may have been supplied from the uplifted South Celtic Sea Basin/Pembrokeshire Ridge.
A study of the hydrocarbon distribution and Lower Cretaceous Greensand prospectivity in Blocks 48/15, 48/17, 48/18 and 48/19, North Celtic Sea Basin
Authors: Howell Tim J. , Griffiths Paul 
Published: April 2008

Abstract
Well 48/19-2 was drilled in the North Celtic Sea in 1992 to test the Lower Cretaceous Greensand fairway along trend from the Kinsale Head and Ballycotton gas fields. The mapping of the 48/19-2 prospect is discussed and the results of the well are detailed. The discovery of a heavy oil accumulation in the well along this gas trend led to a comprehensive multi-disciplinary reappraisal of the 48/15, 48/17, 48/18 and 48/19 area. The results of this study highlight the adequacy of available source rocks, and suggest a possible Portlandian-Purbeck origin for the oils found in boreholes 48/24-1, 48/24-2 and 48/28-1. Maturation modelling in the study area pointed to a principal pulse of hydrocarbon generation and expulsion during the Late Cretaceous to early Tertiary. It was concluded that the Kinsale Head, Ballycotton and 48/18-1 gas accumulations could have been trapped in closures created after Miocene inversion. Earlier traps, which were pre-Miocene inversion in age, could have received oil which later became subjected to incursions of meteoric water and suffered biodegradation.
The Cretaceous succession in the Porcupine Basin, offshore Ireland: facies distribution and hydrocarbon potential
Authors: Moore J. G. , Shannon P. M. 
Published: April 2008

Abstract
The Porcupine Basin lies on the continental shelf to the west of Ireland. It contains up to 4 km of Cretaceous sediments. A tectono/sequence stratigraphic and seismic facies study has divided the Cretaceous succession into three workable, genetically related, chronostratigraphic sequences (PK1, PK2 and PK3) separated by basinwide sequence boundaries. The PK1 sequence, of Valanginian-Late Aptian age, is dominated by by low-energy, mudstone-prone, marine shelf and basinal facies (transgressive and highstand systems tracts). Higher energy, fault-bounded clastic fans, sandstone-prone deltaic packages and turbidite/mass flow deposits (lowstand systems tract) are also present. The PK2 sequence, of Late Aptian-Cenomanian age, is dominated by low energy, mudstone-prone shelf and basinal deposits (transgressive and highstand systems tracts). Erosional channels associated with the Late Aptian relative sea-level fall are also present, as are turbidite mounds and deltaic packages (lowstand systems tract). The PK3 sequence, of Cenomanian-Danian age, is predominantly a chalk facies and represents low energy deposition in a southward-deepening sea. Some higher energy calciturbidites/slump masses occur along the basin margins, and reefal build-ups are also present. The higher depositional energy deposits offer the best potential for hydrocarbon accumulations.
Fault styles in the Porcupine Basin, offshore Ireland: tectonic and sedimentary controls
Authors: McCann T. , Shannon P. M. , Moore J. G. 
Published: April 2008

Abstract
Analysis of fault style, size and distribution in the Porcupine Basin has demonstrated the existence of a number of fault types. Syn-tectonic growth faults occur on scales ranging from basin-bounding faults to small intrabasinal faults and they range in age from Jurassic through to the end of the Eocene. Tectonic non-growth faults are most pronounced in the post-rift succession. They are generally small and did not significantly influence sedimentation patterns. They frequently occur in clusters and are commonly recorded at mid-Eocene to mid-Miocene horizons. Slump-associated faults occur in association with large-scale sediment failure structures which are most pronounced in the Upper Tertiary succession. Unlike the other fault types, which are extensional in nature and typically planar in shape, these faults can be either extensional or compressional, and frequently have a listric geometry. The interplay of fault types with sedimentation leads to the identification of a number of recognizable seismic facies.
The Erris and eastern Rockall Troughs: structural and sedimentological development
Authors: O’Reilly Brian M. , Hauser Franz , Jacob A. W. Brian , Shannon Pat M. , Makris Jannis , Vogt Ulrike 
Published: April 2008

Abstract

Wide-angle seismic reflection profiles across the western Irish shelf region have resolved the deep crustal structure of the shelf transition into the Rockall Trough. A three-layered sedimentary succession up to 5 km thick is determined within the Rockall Trough. The lowermost layer probably comprises a thin early Mesozoic syn-rift sediment package, while the upper two layers represent Cretaceous to Recent, mostly post-rift, sediments. The seismic response of the crust changes beneath the Erris Trough into the eastern Rockall Trough, as the crust thins from
c.
30 km onshore in Ireland to between 5 and 6 km in the deep water region of the Rockall Trough. However, the thinning of seismic layers in response to stretching is not uniform. Differential stretching of the crust can explain seismic observations of both crustal and upper mantle structure. Low upper mantle velocities may be due to partial serpentinization of the upper mantle lithosphere, facilitated by rheological hardening during lithospheric stretching. The model predicts the occurrence of starved deep marine sediments during the late Mesozoic and Cenozoic.
Tectonic controls on Upper Jurassic to Lower Cretaceous reservoir architecture in the Jeanne d’Arc Basin, with some comparisons from the Porcupine and Moray Firth Basins
Authors: Shannon P. M. , Williams B. P. J. , Sinclair I. K. 
Published: April 2008

Abstract
Seismic, wireline log and core data from three geographically separate basins in the North Atlantic provide an understanding of the regional dynamics of the basin system, and quantify the sedimentary geometry response to a composite rift episode. The response to the Late Cimmerian rift episode in the Jeanne d’Arc, Porcupine and Moray Firth Basins is broadly comparable, with an initial episode of clastic reservoir facies deposition associated with active growth faulting. This was followed by a mid-rift period where subsidence outstripped sedimentation, resulting in organic-rich source rocks and culminating in a mappable maximum flooding surface. During the final phase of the rift episode, the decrease in subsidence rate resulted in the return of reservoir-quality clastics. The reservoir-scale response to the initial rifting is illustrated in cored sequences from the Jeanne d’Arc Basin. These indicate the development of axial and lateral sandy and conglomeratic braided fluvial and alluvial fan systems, with periods of muddy lacustrine deposition. Pedogenic surfaces offer much potential for detailed correlation and the analysis of sandbody geometry. The onset of anoxic conditions conducive to source rock deposition during the mid-rift phase of Late Cimmerian tectonism, and the preferential development of reservoir facies at the base and top of syn-rift sediment fill, are characteristics of considerable importance to exploration of the North Atlantic borderland rift basins.
Anisotropic grain fabric: volcanic and laboratory analogues for turbidites
Authors: Hughes Simon R. , Alexander Jan , Druitt Tim H. 
Published: April 2008

Abstract
A variety of grain fabrics are commonly observed in coarse-grained turbidites that form potential reservoir rocks. Such fabrics may impart an anisotropy to permeability; however, their origin is poorly understood. Observations of analogous volcanic and laboratory deposits are used here to infer the origin and character of clast alignments. Experimental gravity currents were produced by releasing suspensions of silt-grade silicon carbide (10, 15 or 200y volume) with minor volumes of larger, lower-density perspex clasts into a tank of water. Fabric was measured in the experimental deposits and in the Quaternary Upper Laacher See Tephra, Germany. The experimental and pyroclastic deposits both had a clast fabric with grain long-axes predominantly transverse, but not necessarily perpendicular, to the mean flow direction. The perspex clasts in the laboratory currents were initially transported in suspension, and marks on the sediment surface indicated that these clasts rolled along the bed immediately prior to final deposition. The perspex clasts were buried at differing levels and at a range of distances from source in the laboratory deposits, having been deposited at different stages in the flow history. In the volcanic deposits the same fabric is persistent vertically throughout the thickness of the bed at each site, suggesting that progressive aggradation provided a surface on which the clasts rolled prior to deposition. It is proposed here that similar fabrics in high-density turbidites are analogous to those observed in these experimental and pyroclastic deposits, and therefore they may be predicted from palaeocurrent interpretations, or conversely used to assess palaeocurrent directions.
Sandstone megabeds from the Tertiary of the North Sea
Authors: Pauley J. C. 
Published: April 2008

Abstract
Three cored sandstone megabeds of Tertiary (late Palaeocene) age from two quadrants in the Central Graben of the North Sea are discussed. The megabeds are upwards-fining units 3–9 m thick, underlain by gravelly mudstones (1–1.4 m thick) which are interpreted as debrites. The megabeds are divided into five units (M1–M5) based on grain size and sedimentary structures, which are organized into a type vertical sequence. The basal unit M1 comprises a clast-supported mudclast conglomerate. Unit M2 comprises very coarse- to medium-grained, occasionally gravelly, sandstone which is horizontally laminated to cross-stratified. Unit M3 is the thickest unit (2–4 m) and is a laminated, upwards-fining, medium- to fine-grained sandstone with occasional cross-lamination. Unit M4 is a fine- to very fine-grained sandstone characterized by disturbed lamination and water-escape structures. Unit M5 is an upwards-fining silty sandstone to siltstone characterized by discontinuous, irregular, and locally overfolded laminae. The megabeds are interpreted as having been deposited by turbidity currents. There are some similarities to Bouma sequences and high-density turbidite sequences, but massive sandstone is notably absent. The megabeds are associated with mudstones and debrites and only occasional thin turbidites, and as such they are not part of an organized sequence of turbidites. The beds are notably much thicker than those above and below, and lithologies are distinct from those of the surrounding sediments. These features are comparable to those of ‘megaturbidites’ as defined by Bouma. The debrite which immediately underlies the megabeds is significant in that this relationship is also a common feature of megaturbidites. The debrite is thought to represent slope failure which immediately preceded megabed deposition. A hypothesis is presented of earthquake-induced catastrophic slope failure, debrite deposition and associated megaturbidite generation. Such sandstone megabeds are thought to be widespread sheet-like deposits. They are exploited as a hydrocarbon reservoir in two of the wells discussed.
The formation of carbonate cements in the Forth and Balmoral Fields, northern North Sea: a case for biodegradation, carbonate cementation and oil leakage during early burial
Authors: Watson Roseleen S. , Trewin Nigel H. , Fallick Anthony E. 
Published: April 2008

Abstract

The Tertiary reservoirs of the Forth and Balmoral Fields are sand-rich turbidites, which were deposited in a deep marine setting in the vicinity of the Fladen Ground Spur and East Shetland Platform. The Forth Field reservoir contains biodegraded oil and consists of Palaeocene and Eocene unconsolidated to semi-consolidated, well sorted, medium- to fine-grained sub-arkoses at a maximum present burial depth of 1.8 km. Pervasive ferroan and non-ferroan calcite cements dominate the Forth paragenetic sequence. Bitumen-filled inclusions within these cements indicate that oil emplacement and carbonate cementation occurred simultaneously, probably within the first 1000 m of burial. Carbonate δ
18
O data suggest that meteoric water from the East Shetland Platform flushed the reservoir, biodegraded the migrated oil and displaced the original sea water. Biodegradation of oil took place at the palaeo-oil-water contact, producing a complex sequence of laterally extensive non-ferroan and ferroan calcite concretions. Frequent oil leakage during shallow burial may have produced a large number of concretionary cement layers. The cemented layers have the potential to act as major barriers to vertical fluid movement and to significantly reduce the net to gross ratio of the reservoir. Pore-filling ankerite post-dates non-ferroan and ferroan calcite. Ankerite precipitation marks the final stages of bacterial influence within the sediment column.

The Balmoral Field is located
c.
100 km to the south of Forth. The reservoir consists of a series of poorly sorted subfeldspathic arenites with infrequent spherical carbonate concretions. These non-ferroan and ferroan calcitic concretions preferentially precipitated around localized accumulations of organic matter at < 500 m burial depth. Pore fluids were meteoric, and carbonate was provided by both bacterial oxidation and sulphate reduction of organic matter. Meteoric water is thought to have been derived from the East Shetland Platform to the north of Balmoral. Oil migrated into Balmoral during the Oligocene, post-dating meteoric flooding.

Laterally extensive carbonate-cemented sand horizons, formed in association with oil biodegradation, have the potential to compartmentalize a reservoir. The distribution of these cements within Tertiary deep water sandstone reservoirs adjacent to the East Shetland Platform is likely to be controlled by the relative timing of meteoric flushing and oil migration.