AAPG Bulletin, V. 82 (1998), No. 7 (July 1998), P. 1401-1415.

Source Rock Burial History and Seal Effectiveness: Key Facets to Understanding Hydrocarbon Exploration Potential in the East and Central Irish Sea Basins¹

William I. Duncan,² Paul F. Green,³ and Ian R. Duddy³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received February 13, 1997; revised manuscript received July 9, 1997; final acceptance February 3, 1998.
²Deminex UK Oil and Gas Ltd, Bowater House, 114 Knightsbridge, London, SW1X 7LD, United Kingdom.
³Geotrack International Pty Ltd, 37 Melville Road, West Brunswick, Victoria 3055, Australia.

We wish to thank the Petroleum Affairs Department, Dublin, Ireland, for permission to sample wells 42/12-1 and 42/17-1 and to refer to three confidential reports: a 1987 report by R. Burnett, G. Clayton, J. R. Graham, N. Haughey, D. Robson, and G. D. Sevastopulo; a 1993 Geotrack report (no. 474) prepared for Mobil North Sea Ltd.; and a 1995 Geotrack report (no. 544) prepared for Deminex UK Oil and Gas Ltd. We thank Bulletin referees P. Webb, G. T. Cayley, T. Asprey, and former elected editor, K. T. Biddle, for their constructive reviews of the original manuscript. 

ABSTRACT

The timing of hydrocarbon generation from Carboniferous source rocks and the lack of evapo rites within the Triassic Mercia Mudstone Group seal sequence have contributed significantly to the lack of exploration success in the Triassic Sherwood Sandstone play of the Central Irish Sea Basin. Apatite fission-track analysis (AFTA^®), combined with vitrinite reflectance data from the Central Irish Sea Basin and adjacent areas, indicates that maximum burial and heating of the Carboniferous section 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 the absence of annealing halites and the presence of thief sandstones, led to hydrocarbon loss through trap breaching during subsequent tectonic 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 and the presence of a halite-bearing Mercia Mudstone seal sequence significantly improved hydrocarbon retention within the Sherwood Sandstone reservoir. If traps were episodically breached during the late Tertiary, they were recharged by the inversion process, which caused gas expansion, spill from charged traps, and lateral remigration of hydrocarbons updip. The inversion-related pressure drop also caused retrograde condensate dropout from gas accumulations to provide a supply of late condensate for preferential spill and remigration.