Variation in thermal history styles around the Irish Sea and adjacent areas: implications for hydrocarbon occurrence and tectonic evolution

Abstract

Abstract Thermal history is a major factor in determining hydrocarbon prospectivity in the Irish Sea region. Early application of Apatite Fission Track Analysis (AFTA®) to exploration wells revealed early Tertiary palaeotemperatures around 110°C or more. More recent studies in the Irish Sea and adjacent areas, using a combination of AFTA and vitrinite reflectance (VR), have revealed at least three additional palaeo-thermal episodes: pre-Permian (>290 Ma), late Permian to mid-Triassic (260 to 220 Ma) and Early Cretaceous (140 to 110 Ma). Other evidence of early paleo-thermal effects, for which timing is only constrained to the interval 300 to 150 Ma, may reflect these or additional episodes. In some areas, early effects involved extremely high palaeogeothermal gradients. Early Cretaceous gradients are unconstrained due to a lack of wells in which palaeotemperatures reflect this episode. Early Tertiary palaeotemperatures derived from AFTA and VR in wells from the southern part of the East Irish Sea Basin define a consistent linear depth profile with a gradient of only 10°C km −1 . Wells from the north of the region also define a broadly consistent profile but with a much higher gradient. The low gradients are thought to reflect flow of hot fluids while the high gradients to the north may be associated with igneous activity in that region. Our preferred interpretation of the early Tertiary palaeo-thermal effects involves a combination of burial by around 1.5 to 2 km of post-Triassic sediment and the thermal effects of hot fluid flow, probably from north to south and associated with igneous activity. This interpretation is consistent with geological evidence and is supported by structural data, sonic velocity data and geochemical observations. Different combinations of these episodes lead to a wide variety of thermal history styles across the region. The southern part of the East Irish Sea Basin, containing major accumulations of oil and gas, is characterized by the most suitable thermal history conditions for generation and preservation of hydrocarbons to the present day. In adjacent regions, thermal histories are more complex, and those identified to date are generally less suitable particularly for preservation of hydrocarbons to the present day. Future exploration in these areas will require a thorough understanding of the thermal history of Carboniferous source rocks in order to define the most prospective regions.