Multiple heating episodes in the Wessex Basin: implications for geological evolution and hydrocarbon generation

The timing of maturity development and hydrocarbon generation with respect to structuring is a primary factor controlling oil and gas accumulation. Hydrocarbon generation is largely a temperature-dependent process and in order to predict accumulation it is necessary to reconstruct thermal history. In this study of the Wessex Basin and adjacent areas, direct thermal history data are obtained from Apatite Fission Track Analysis (AFTA) on samples from wells, shallow boreholes and outcrops, enabling the key facets of a complex thermal and tectonic history to be reconstructed. Several distinct styles of thermal history are recognized, with clear differences across the region in the time of cooling from maximum palaeotemperatures. Areas of east Devon show evidence of an early thermal episode, sometime after the mid-Triassic and before similar to 170 Ma. The cause remains uncertain at present. In the area of the 'Wytch Farm block' maximum palaeotemperatures were reached prior to cooling in the early Cretaceous, in the range similar to 140-100 Ma, due mainly to uplift and erosion. This area remained relatively high through the rest of its history compared to the areas to the north and south. The Lias (early Jurassic) was not heated sufficiently to cause significant generation and any that occurred effectively ceased in the early Cretaceous. South of the Portland-Isle of Wight faults, maximum palaeotemperatures were reached prior to cooling due to uplift and erosion in the mid- and late Tertiary, at similar to 40 Ma and similar to 20 Ma, respectively. Tertiary uplift and erosion affected a wide region. Maximum erosion occurred immediately to the south of the Purbeck structure where maturation of Lias source rocks continued through peak oil and into the gas generation phase until terminated by the Tertiary cooling. In most wells, AFTA and vitrinite reflectance data define low or non-linear palaeogeothermal gradients indicative of an element of heating due to fluid movement in addition to heating due to burial, making quantitative estimation of erosion difficult, but any reasonable model requires kilometre-scale uplift and erosion over a wide region. West of Portland in Lyme Bay, a key question is whether maturation history is dominated by early Cretaceous or by mid- to late Tertiary events. At present there are not sufficient data to extrapolate confidently from the area of well control east of Portland. No parts of the basin are currently generating hydrocarbons, implying that surface seepages are from previously reservoired accumulations.