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 The Lennox Field, located in blocks 110/15 and 110/14, was the second oil field to be developed in the East Irish Sea Basin. It contains 184 MMBBL of oil in-place within a 143 ft thick oil rim overlain by a large gas cap up to 750 ft thick. The GIIP is estimated to be 497 BCF. The field came on stream in February 1996, and it is now being developed with seven horizontal oil producers, including two multi-lateral wells and two crestal gas injectors. Production from the field can be divided into two distinct phases; the oil rim development phase, and the gas cap blow-down phase. The latter phase is currently anticipated to commence in 2004. The field structure consists of a roll-over anticline formed in the hanging wall of the Formby Point Fault during extensional faulting in Triassic-early Jurassic times, and later readjusted by contractional movements during Tertiary inversion. The oil and gas are trapped in the highly productive Triassic Ormskirk Sandstone Formation. The reservoir comprise high porosity aeolian and fluvial sandstones occurring at a shallow depth (c. 2500 ft) with a maximum gas column of 750 ft above an oil rim of 143 ft. The reservoir quality is principally controlled by primary depositional processes as no significant adverse diagenetic effects are observed. The hydrocarbon filling history was complex, with at least three phases of oil and gas generation. The field contains a light, saturated oil (45° API) with a GOR of 650 SCF/BBL. The crude contains high levels of H 2 S (0.1 mol%) and mercaptans (450 ppm), which are removed during processing at the Douglas complex. Water cut from the field is currently around 2-5%, and no free gas production has been observed to date. Gas production from Lennox is anticipated to start in 2004.

Abstract The Lennox Field is a saturated oilfield with a significant primary gas cap at initial conditions. Located in the East Irish Sea withincBlocks 110/14c and 110/15a, the field was discovered in 1992. First oil was achieved in February 1996. Lennox is a rollover anticline structure. The Triassic Ormskirk Sandstone Formation reservoir comprises good-quality aeolian and fluvial sandstones with typicalcporosities of 11–21%. The gas column reaches a height of c. 850 ft and overlays a 143 ft oil column. Oil initially-in-place is estimated to be 202 MMbbl, whilst total gas initially-in-place is 521 bcf. The field has been developed by a wellhead platform tied-back to the neighbouring Douglas Complex. The field development has been split into two phases: the first phase focused on oil production and involved the drilling of 12 horizontal and multilateral production wells and two gas injection wells. Oil production ceased in 2012 with total produced volumes of 103 MMbbl. The second phase comprised the gas cap blowdown, and the optimization of the existing well stock for gas production. Eni UK acquired the operatorship of the field in April 2014 and has focused on maximizing and accelerating gas production from the field.

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.