SOURCE-ROCK BURIAL HISTORY AND SEAL EFFECTIVENESS - KEY FACETS TO UNDERSTANDING HYDROCARBON EXPLORATION POTENTIAL IN THE EAST AND CENTRAL IRISH SEA BASINS

The timing of hydrocarbon generation from Carboniferous source rocks a nd the lack of evaporites within the Triassic Mercia Mudstone Group se al sequence have contributed significantly to the lack of exploration success in the Triassic Sherwood Sandstone play of the Central Irish S ea Basin. Apatite fission-track analysis (AFTA(R)), combined with vitr inite reflectance data from the Central Irish Sea Basin and adjacent a reas, indicates that maximum burial and heating of the Carboniferous s ection was achieved by the Early Cretaceous. The long residence period for trapped hydrocarbons generated in the Early Cretaceous, combined with the ineffectiveness of the Mercia Mudstone Group seal caused by t he absence of annealing halites and the presence of thief sandstones, led to hydrocarbon loss through trap breaching during subsequent tecto nic events. In contrast, maximum burial of Carboniferous source rocks in the central East Irish Sea hydrocarbon province was achieved in the Late Cretaceous-early Tertiary, giving a shorter residence period for generated hydrocarbons. The combination of a short residence period a nd the presence of a halite-bearing Mercia Mudstone seal sequence sign ificantly improved hydrocarbon retention within the Sherwood Sandstone reservoir. If traps were episodically breached during the late Tertia ry, they were recharged by the inversion process, which caused gas exp ansion, spill from charged traps, and lateral remigration of hydrocarb ons updip. The inversion-related pressure drop also caused retrograde condensate dropout from gas accumulations to provide a supply of late condensate for preferential spm and remigration.