Abstract Hydrocarbon exploration in the East Irish Sea Basin began with the identification of surface oil seeps in peat beds in Lancashire, UK. This precipitated the drilling of the first onshore exploration wells. The discovery of the Formby Field in west Lancashire at the end of the 1930s triggered a wave of further drilling. Wells drilled in west Lancashire had limited success, with only minor hydrocarbon shows, whilst the production from the Formby Field was modest. Nonetheless, the invaluable geological information taken from onshore wells and the ratification of the Continental Shelf Act led to a shift in focus to the offshore and a period of significant interest in the East Irish Sea. Two key periods of oil and gas exploration activity stand out in the history of the offshore basin, the first headed by the Gas Council during the 1970s resulted in the discovery of the gas giants of Morecambe Bay, whilst the second fronted by Hamilton Oil during the 1990s heralded the discovery of oil with the Douglas and Lennox fields in Liverpool Bay. Exploration in the basin has waned during the last decade; however, to date, this mature hydrocarbon province has yielded estimated hydrocarbon reserves of over 1.8 BBOE (billion barrels of oil equivalent).

Abstract Seismic mapping of key Paleozoic surfaces in the East Irish Sea–North Channel region has been incorporated into a review of hydrocarbon prospectivity. The major Carboniferous basinal and inversion elements are identified, allowing an assessment of the principal kitchens for hydrocarbon generation and possible migration paths. A Carboniferous tilt-block is identified beneath the central part of the (Permian–Mesozoic) East Irish Sea Basin (EISB), bounded by carbonate platforms to the south and north. The importance of the Bowland Shale Formation as the key source rock is reaffirmed, the Pennine Coal Measures having been extensively excised following Variscan inversion and pre-Permian erosion. Peak generation from the Bowland source coincided with maximum burial of the system in late Jurassic–early Cretaceous time. Multiphase Variscan inversion generated numerous structural traps whose potential remains underexplored. Leakage of hydrocarbons from these into the overlying Triassic Ormskirk Sandstone reservoirs is likely to have occurred on a number of occasions, but currently unknown is how much resource remains in place below the Base Permian Unconformity. Poor permeability in the Pennsylvanian strata beneath the Triassic fields is a significant risk; the same may not be true in the less deeply buried marginal areas of the EISB, where additional potential plays are present in Mississippian carbonate platforms and latest Pennsylvanian clastic sedimentary rocks. Outside the EISB, the North Channel, Solway and Peel basins also contain Devonian and/or Carboniferous rocks. There have, however, been no discoveries, largely a consequence of the absence of a high-quality source rock and a regional seal comparable to the Mercia Mudstone Group and Permian evaporites of the Cumbrian Coast Group in the EISB.

Abstract Interaction between uplift related to the Cretaceous-Paleocene opening of the North Atlantic, Neogene shortening (basin inversion) and Pleistocene glacio-isostasy is illustrated by the complex denudation pattern of Britain; such denudation is greatest over the submergent East Irish Sea basin, some 500 km from the Atlantic margin. This paper reports on analysis of sedimentary porosities using sonic velocity logs from 42 wells in the East Irish Sea basin. We present a new map showing the variation in exhumation magnitude at the uppermost Mesozoic unconformity (i.e. thickness of denuded Mesozoic and Cenozoic sedimentary rocks), today buried beneath a thin veneer of Pleistocene sediment. It indicates that exhumation is mostly < 1500m (632-2132m; mean standard deviation 407m), less than denudation results obtained from vitrinite reflectance and apatite fission-track data. The map also reveals substantial variation in exhumation over short distances, often between adjacent wells sited on opposing walls of individual faults. This is interpreted in terms of the influence of Neogene basin inversion on the exhumation of the EISB. The role of late Tertiary tectonics in western UK exhumation is therefore discussed.

Abstract Compared with the prolific success of the Triassic play in the East Irish Sea Basin (EISB) the lack of hydrocarbon discovery in neighbouring Permo-Triassic basins of the Irish Sea has been an enigma. However, recent exploration of the Peel, Solway and Central Irish Sea basins has provided new insights into the geology of these basins and the controls upon hydrocarbon prospectivity in the Irish Sea area. Regional seismic interpretation suggests that 12 of the 15 exploration wells drilled in the basins adjacent to the EISB tested valid structural closures at top Triassic reservoir level. Re-evaluation of the Irish Sea petroleum system reveals that, although effective reservoirs occur in the Lower–Middle Triassic Ormskirk Sandstone Formation, and evaporites in the Middle–Upper Triassic Mercia Mudstone Group provide a regional top seal, the major factor controlling hydrocarbon prospectivity is the limited presence of effective source rocks in the underlying Carboniferous section. A further control upon prospectivity is the timing of hydrocarbon migration, from those areas where Carboniferous source rocks were deposited and preserved. The Namurian basinal marine oil- and gas-prone shales, which form the principal source of hydrocarbons for the Triassic play in the EISB, are restricted to an east–west fairway extending from the EISB into the Kish Bank Basin. Rocks of this age are absent from the Peel and Solway basins as a result of Variscan uplift and erosion. However, palaeogeographical reconstructions based on well and outcrop data suggest that, even if preserved, the depositional environment was not conducive to the formation of marine oil-and gas-prone source rocks. Well and seismic data suggest that rocks of Namurian age were not deposited in the Central Irish Sea area, which remained high during much of Dinantian and Namurian time. Potential source rock development in the Central Irish Sea area is therefore limited to the Westphalian section, which is organically lean and dominated by inertinitic kerogens. Potential hydrocarbon traps in the Central Irish Sea, Peel and Solway basins formed largely as a result of Early Cretaceous tectonism and were subsequently modified by fault reactivation during Tertiary uplift phases. Trap formation appears to postdate the most likely timing of hydrocarbon charge, which this study suggests would have occurred in Late Triassic and Jurassic time. Hydrocarbon entrapment in the EISB may have been favoured by limited Early Cretaceous uplift, coupled with renewed hydrocarbon generation and re-migration during Early Tertiary time. It is concluded that the remaining prospectivity of the Triassic play in the Irish Sea area is likely to be restricted to the proven play fairway within the EISB.

Abstract The Kish Bank Basin lies in the western Irish Sea c. 20 km east of Dublin. It is one of a number of remnants of a larger Permo-Triassic basin system that may have extended across the whole of the Irish Sea. It has a geological history similar to that of the East Irish Sea Basin, initially developing by the reactivation of Caledonian faults that controlled subsequent deposition during Dinantian and Namurian time, with Westphalian deposition in a sag-basin that overstepped the adjacent basement highs. Variscan dextral transpression resulted in the formation of the Codling and Bray faults, and Permian to Jurassic extension formed a set of north-south-trending faults. Liassic outliers are preserved in the hanging walls of the basin margin faults. Early Cretaceous uplift was followed by chalk deposition. Tertiary movements reactivated older faults, isolating the Kish Bank Basin, and producing 9 km of dextral strike-slip along the Codling Fault Zone. The main reservoir in the hydrocarbon play is provided by the Sherwood Sandstone Group, as successfully exploited in the East Irish Sea. Three wells have been drilled to test this reservoir. These encountered high-quality Sherwood Sandstone reservoirs beneath the good potential seal of the Mercia Mudstone Group (which included thick halites). Source rock potential is from either the Westphalian Coal Measures, as penetrated in well 33/22-1, or from inferred Dinantian to Namurian basinal shales. There is good evidence of an active source system, with oil shows in wells 33/17-1 and 33/22-1, data from geochemical analysis of sea-bed cores, a ‘Seepfinder’ survey, sea-bed mounds and seismic evidence of shallow gas. The main risks of the play are the migration pathway and the timing of trap formation with respect to migration. Migration favours the eastern side of the basin, and many of the tilted fault blocks that formed during Permian to Jurassic time have been modified by Early Cretaceous inversion and by Tertiary strike-slip compression. ALL of the structures that have been drilled to date have been either formed or modified after the time of peak hydrocarbon generation and migration.

Abstract South Morecambe Gas Field is situated in the East Irish Sea and produces gas from the Triassic Sherwood Sandstone Group. Exploration of the basin commenced in 1966 and the discovery well, 110/2-1, was drilled in 1974. Appraisal was complete by 1983 and development was carried out in two phases with the object of providing deliverability to help to satisfy the winter peak in demand. First gas was produced in January 1985 and production duringthe winter can be sustained at 50MMCMD (1750mmscfd). The stratigraphic succession of the East Irish Sea Basin (EISB) consists of Carboniferous (Dinantian to Westphalian) strata unconformably overlain by 15000 to 20000 feet of continental Permo-Triassic strata. The Triassic Sherwood Sandstone Group contains reservoir rocks and the overlying Mercia Mudstone Group evaporites provide a seal. Seismic cover of the area includes 2D and 3D data, the latter providing good images that form the basis of the current structural interpretation. The structural development of the basin commenced with extension in the Permo-Triassic followed by inversions in the late Jurassic and early Tertiary. The reservoir has been zoned using a scheme that recognizes primary depositional facies as the main criterion for correlation. The petrophysical evaluation has introduced new methods of calculating porosity,S w and net pay. The latest reservoir pressure data has been used in a material balance study and a twotank simulation model, both give GIIP estimates which are in line with earlier estimates. The new petrophysically derived reservoir parameters were also used to make a volumetric estimate of GIIP. Remaining recoverable reserves are at least 3 Tcf.

Abstract A thick and relatively complete succession of Carboniferous rocks of Tournaisian to Westphalian age, together with a probable Stephanian section (Floodpage et al. 2001) was deposited throughout the two main areas in the Irish Sea region. The larger Central Province Trough in the south includes the East Irish Sea Basin (EISB)-Quadrant 109 Syncline-Kish Bank Basin, linking the onshore successions of the Craven Basin (Chapter 11) and Dublin Basin (Chapter 21). The smaller Peel Basin-Solway Basin (Fig. 48) represents the extension of the onshore Solway Basin (Chapter 13) and continues westwards between the Drogheda Shelf and the Balbriggan Block (Chadwick et al. 2001; Sevastopulo & Wyse Jackson 2009). Seismic data suggest that Carboniferous strata some 4000–5000 m thick occur in the Solway Basin and EISB (Chadwick et al. 2001). The present day distribution of Carboniferous strata is controlled by the effects of uplift and erosion associated with Variscan basin inversion (Chadwick et al. 2001). Separating the Solway Basin and EISB is the Manx-Lakeland Ridge, which is believed to extend westward to form the Balbriggan Block (Chapter 21). The lithostratigraphical nomenclature of Jackson & Johnson (1996) is retained. In this scheme the Tournaisian and Visean successions are referred to as the Garwood Group, the Namurian succession as the Bisat Group and the Westphalian to Stephanian succession as the Kidston Group. There is insufficient information to subdivide these groups into component formations (Jackson & Johnson 1996).

Abstract Dalton is a sub-sea gas field located in Block 110/2b of the UKCS in the East Irish Sea Basin. The field has been the subject of downward reserve revisions through its life based on increased geological understanding from well, production and simulation data. An innovative infill technique is proposed to reverse those trends and increase recovery by harnessing stranded reservoir gas. Dalton was discovered in 1990 by well 110/2b-9 which encountered a dry gas column of 345 ft within the Ormskirk Sandstone Formation of the Triassic Sherwood Sandstone. The field was developed in 1999 by wells 110/2b-R1 and 110/2b-R2. Wells were located on the eastern field crest in order to attain maximum gas column and maximize recovery from the good quality reservoir (average permeability > 50 mD). Production tests showed the majority of flow from the wells to come from thin, high permeability (> 200 mD) aeolian sands. Two smaller highs located in the west and south, separated from the eastern crest by a saddle and normal faults, were not drilled and were assumed to be in communication through the good quality reservoir and open intra-reservoir faults with the field crest in the east. The risk of fault sealing in the high permeability, high net reservoir was considered to be low. Initial gas production in 1999 of 100 × 10 6 SCFD was followed by a rapid decline to 15 × 10 6 SCFD early in 2001. This rapid decline suggests more complex reservoir geology than was anticipated, with barriers to field-wide reservoir communication occurring due to faulting or variation in reservoir quality. Geological, production and simulation data suggest that current wells located on the eastern crest are not accessing gas reserves elsewhere in the field due to sealing of major intra-reservoir faults. Field production has led to a significant pressure difference between the eastern fault compartment and the rest of the field. A horizontal well is planned to connect the western and eastern fault blocks and induce cross-flow of gas across the sealing fault into existing wells via the high permeability aeolian beds. The horizontal well will not be completed or tied back at surface and, hence, if successful, will be a low cost solution to increasing Dalton Field reserves.

Abstract The Ribblesdale fold belt, representing the Variscan inversion of the Bowland Basin, is a well-known geological feature of northern England. It represents a crustal strain discontinuity between the granite-underpinned basement highs of the northern Pennines and Lake District in the north, and the Central Lancashire High/southern Pennines, in the south. Recent seismic interpretation and mapping have demonstrated that the Ribblesdale fold belt continues offshore towards Anglesey via the Deemster Platform, beneath the Permo-Triassic sedimentary cover of the southern part of the East Irish Sea Basin. The Môn–Deemster fold–thrust belt (FTB) affects strata of Mississippian to late Pennsylvanian age. Variscan thrusts extend down into the pre-Carboniferous basement but apparently terminate at a low-angle detachment deeper in the crust, here correlated with the strongly sheared Penmynydd Zone exposed in the adjacent onshore. Up to 15% shortening is observed on seismic sections across the FTB offshore, but is greater in the strongly inverted onshore segment. Pre-Carboniferous thrusting post-dates formation of the Penmynydd Zone, and is probably of Acadian age, when basement structures such as the southward-vergent Carmel Head Thrust formed. Extensional reactivation of the Acadian structures in early Mississippian time defined the northern edge of the offshore Bowland Basin. The relatively late brittle structures of the Menai Strait fault system locally exhume the Penmynydd Zone and define the southern edge of the basin. The longer seismic records from the offshore provide insights to the tectonic evolution of the more poorly imaged FTB onshore.