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School of Geosciences, Queen’s University, Belfast BT7 1NN, UK
Exploration prospects in the Rathlin Basin depend in part upon the reservoir potential of sandstones in the Permo-Triassic. The sandstones are the deposits of alluvial fan, channel and floodplain environments; aeolian deposits may occur in the basal (Permian) part of the succession. Scattered outcrops around the basin margin in Ireland and Scotland, and boreholes in the deep part of the basin onshore Northern Ireland, allow determination of diagenetic history and reservoir quality.
Cementation is dominated by very early pore-filling sulphate/carbonate, illite-smectite grain-coatings, and overgrowths of potassium feldspar. Other cements are haematite graincoatings, quartz overgrowths and kaolinite pore-fillings. The sulphates/carbonates occlude porosity, and the clays/feldspars reduce permeability by bridging pore throats. Even in the deepest parts of boreholes, porosities can be high owing to dissolution of cements and unstable grains. This secondary porosity has in some cases been occluded by kaolinite, particularly in shallow-buried marginal parts of the basin.
Thick, coal-bearing Carboniferous and gypsiferous Mercia Mudstone Group sections onshore suggest that adequate source and seal rocks exist below and above the sandstones, respectively, thus enhancing the exploration potential of the basin.
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