The Kish Bank Basin, offshore Ireland

Abstract

The Kish Bank Basin, a half-graben lying offshore only a few kilometres east and southeast of Dublin in the Irish Sea, is elongated northeast-southwest and measures approximately 48 × 32 km. The basin is bounded to the northwest and north by the Dalkey-Lambay fault system and to the southwest by the Bray Fault. A major dextral strike-slip fault, the northwest-trending Codling Fault, bisects the basin. Three wells have been drilled in the area: Amoco 33/22-1(1977), Shell 33/21-1(1979) and Fina 33/17-1(1986). The Amoco well penetrated a Carboniferous section on the southeastern rim of the basin ( Jenner 1981 ), while the other two holes drilled the Permo-Triassic basin-fill sequence, but without reaching the base of the Permian. The Permo-Triassic succession is similar to that in the East Irish Sea Basin and includes thick Triassic salt sequences. SEM examination of sandstone samples from potential reservoir sections in the 33/21-1 well indicates good porosity in the cleaner sandstones. This is interpreted as largely secondary, following dissolution of early cements. Seismic studies suggest that up to 2.7 km of Jurassic (probably Liassic) strata are present against the western bounding faults. Maturation studies have been carried out on material from two of the wells. The Silesian section penetrated by 33/22-1 ranges in age from Westphalian B to Westphalian D. It is of ‘coal measures’ facies and includes numerous seams of medium to high volatile bituminous coal. Much of the Westphalian section is mature with regard to gas generation, with vitrinite reflectance (Rm) values ranging from c. 0.8% to 1.3%. Maturation levels in the Permo-Triassic section in 33/21-1 range from c. 1.3%–1.6% Rm, suggesting that any underlying Westphalian rocks in deeper parts of the basin would be mature to marginally post-mature with respect to dry gas generation. The burial history of the basin, and the distribution of source rocks within it, are discussed.