Journal of Petroleum Geology, vol. 22(3), July 1999, pp. 243-260

PETROLEUM GEOLOGY AND FUTURE HYDROCARBON POTENTIAL OF THE IRISH SEA

D. G. Quirk^1,2, S. Roy^1,3, I. Knott ^4, J. Redfern¹ and L. Hill¹

The East Irish Sea Basin is hydrocarbon prolific with ten gasfields, two oilfields and another eight gas or oil discoveries. Production is from a widespread Triassic fluvio-aeolian reservoir (the Ormskirk Sandstone) which is sealed by salt-prone mudstones. Three episodes of hydrocarbon generation occurred from a rich, Namurian-age source rock during deep burial in the Late Carboniferous-Early Permian, in the Early Jurassic and in the Late Cretaceous. All of the discoveries are in structural traps which are controlled to some degree by N-S trending normal faults probably active in the Late Jurassic. Consequently, the third (Cretaceous) phase of hydrocarbon generation is the most important. Another phase of uplift and erosion occurred in the Early Tertiary leading to the almost complete removal of Cretaceous and Jurassic strata. This event led to significant primary and tertiary migration as a result of overpressuring in the source rock and gas expansion within the reservoir. Although similar good quality Triassic reservoir occurs in other basins in the Irish Sea, rift-related uplift and erosion in the Middle Permian caused the widespread removal of potential Carboniferous source rocks in these areas, severely limiting the chance of hydrocarbon charge.