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British Gas Exploration and Production Limited, 100 Thames Valley Park Drive, Reading, RG6 1PT, UK
The hydrocarbon prospectivity of the offshore Central Irish Sea Basin has been assessed with particular reference to Block 42/12, which was formerly operated by Hydrocarbons Ireland Ltd (British Gas Exploration & Production). Data from the two wells drilled in 42/12, and other wells and boreholes in the area, have been integrated with seismic, gravity and magnetic data. Onshore outcrop and sub-surface information were also utilized.
Interpretation of these data shows that the proven basin-fill consists of several megasequences bounded by major unconformities. At least three separate phases of extension are recorded, in the Carboniferous, Triassic and Jurassic. These were followed by post-rift, regional thermal subsidence, but the stratigraphy is further punctuated with evidence of compression and transpression associated with the Variscan, Cimmerian and Alpine inversion events. An assessment of hydrocarbon prospectivity indicates that the primary reservoir interval occurs in the syn-rift Lower Triassic Sherwood Sandstone Group. This is sealed by the overlying Middle-Upper Triassic Mercia Mudstone Group consisting of shales with major halite intervals. Westphalian coal-bearing facies, proven in 42/12-1 and 42/17-1, are potential gas-prone source rocks, which are late to overmature for oil generation to early mature for gas generation. Unproven Namurian marine source rocks may occur in the main basin depocentre. Burial history modelling suggests that source rocks probably generated hydrocarbons by the Early Cretaceous, with possible further generation prior to the main Tertiary inversion. Tilted fault-block and rollover anticline trapping structures were probably formed during Permian to Jurassic rifting, pre-dating the main phases of hydrocarbon expulsion.
Exploration to date has centred on the southern margins of the basin. Future exploration of the deeper and potentially prospective parts of the basin in the north should resolve uncertainties concerning the possible presence of Namurian source rocks and Permian reservoir and seal facies, and the nature of a post-Triassic section identified on seismic data.
The Central Irish Sea Basin is an under-explored Late Palaeozoic to Mesozoic basin which is stratigraphically analogous to the prolific East Irish Sea hydrocarbon province to the northeast. This paper presents the results and interpretations of past exploration activity and reviews these in terms of their impact on the tectono-stratigraphic evolution and future hydrocarbon prospectivity of the basin.
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