INTERPRETATION OF VITRINITE REFLECTANCE PROFILES IN THE CENTRAL IRISH SEA AREA: IMPLICATIONS FOR THE TIMING OF ORGANIC MATURATION

The Central Irish Sea area, from Kish Bank to St. Georgeʹs Channel and Cardigan Bay, consists of a series of Late Palaeozoic to Cenozoic extensional and transtensional basins which have experienced a multiphase inversion history. Potential hydrocarbon source rocks of Carboniferous and Jurassic ages have been recognised in this area. In the Kish Bank and Central Irish Sea Basins, maturation modelling of these source rocks is hampered by the severely truncated rock record and by the relative paucity ofvitrinite throughout much of the preserved post-Palaeozoic (Triassic) section. Vitrinite reflectance data from six exploration wells have been used to quantify the peak palaeotemperatures attained by the rocks in this area and to estimate the magnitude of net exhumation at these locations. An apparent palaeogeothermal gradient of~26°C/ km is recorded by the Jurassic sediments in well 42/21-1, whereas significantly higher palaeogeothermal gradients of 74- 78(degree)C/km are interpreted for the Westphalian/Stephanian sediments in the area. At least two periods of rock exhumation have occurred; during the Late Carboniferous-Late Permian, and again sometime between the latest Jurassic and early Tertiary. Estimates of net exhumation vary from -350 m at well 42/12-1 to -1,900 m at well 42/17-1. Our interpretation suggests that the higher palaeogeothermal gradients recorded by the Westphalian/Stephanian sections reflect elevated heat flows during Stephanian to Early Permian times.