HYDROCARBON POTENTIAL OF NORTHERN-IRELAND .2. RESERVOIR POTENTIAL OF THE CARBONIFEROUS

Carboniferous sandstones occur in several basins in the north of Ireland, deposited predominantly in fluvio-deltaic environments. The Carboniferous is thickest in the west of the region. Primary compositional variations are evident; in particular, the sandstones are more arkosic in the west. Variations in authigenic mineralogy also occur; sandstones in the west contain greater volumes of authigenic quartz, and exhibit hairy/fibrous illite which is lacking in the east. The distribution of carbonate and sulphate cements is more irregular, and is related to the distribution of marine limestone and evaporite beds. Porosity in the sandstones is secondary after disolution of carbonate and sulphate cements and, to a lesser degree, dissolution of unstable grains. Porosities are generally low (less than 5%) in the west, but sandstones there may be comparable with those in tight gas reservoirs elsewhere. Sandstones in the east exhibit very high porosities in boreholes. Kaolinite is a late phase occupying pore space in the upper parts of most successions, deposited by downward-migrating meteoric fluids, possibly during Permian times. Late iron oxides in some sandstones were similarly precipitated during deep weathering below the sub-Permian unconformity. Sandstones exhibit very patchy bitumen residues in the west, demonstrating that oil has migrated through these rocks.