HYDROCARBON POTENTIAL OF NORTHERN-IRELAND - .3. RESERVOIR POTENTIAL OF THE PERMO-TRIASSIC

Thick Permo-Triassic sandstones occur in two basins in Northern Ireland. The sandstones are fluvio-deltaic, including an oolitic facies, and are interbedded with mudrocks. They exhibit petrographic features which are typical of red beds and are widely found in the Permo-Triassic elsewhere in the British Isles. The predominant cements are haematite, potassium fledspar, illite and carbonates. Gypsum, anhydrite and dolomite cements occur in the regions of deepest burial, which represent the basin centres. There was widespread leaching of carbonate/sulphate cements, but calcite is preserved in the oolitic facies and below the sub-Cretaceous (chalk) unconformity. The dissolution of cements and framework grains was due to leaching by meteoric fluids. Precipitation of kaolinite in secondary pores, and subsequent illitization, represent fluctuations in the meteoric hydraulic head. Deep boreholes exhibit sandstones porosities which indicate that there may be significant reservoir potential where adequate seals and traps are present.