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. George's Channel and Card igan Bay, consists of a series of Late Palaeozoic to Cenozoic extensional a nd transtensional basins which have experienced a multiphase inversion hist ory. Potential hydrocarbon source rocks of Carboniferous and Jurassic ages have been recognised in this area. In the Kish Bank and Central Irish Sea B asins, maturation modelling of these source rocks is hampered by the severe ly truncated rock record and by the relative paucity of vitrinite throughou t much of the preserved post-Palaeozoic (Triassic) section. Vitrinite reflectance data from six exploration wells have been used to qua ntify the peak palaeotemperatures attained by the rocks in this area and to estimate the magnitude of net exhumation at these locations. An apparent p alaeogeothermal gradient of similar to 26 degrees C/km is recorded by the J urassic sediments in well 42/21-1, whereas significantly higher palaeogeoth ermal gradients of 74-78 degrees C/km are interpreted for the Westphalian/S tephanian sediments in the area. At least two periods of rock exhumation ha ve occurred; during the Late Carboniferous-Late Permian, and again sometime between the latest Jurassic and early Tertiary. Estimates of net exhumatio n vary from similar to 350 m at well 42/12-1 to similar to 1,900 m at well 42/17-1. Our interpretation suggests that the higher palaeogeothermal gradi ents recorded by the Westphalian/Stephanian sections reflect elevated heat flows during Stephanian to Early Permian times.