Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and Southern North Sea

Abstract

Apatite fission track analysis (AFTA) is a method of obtaining ther- mal history information in sediments. In sections that have been hotter in the past, AFTA provides estimates of maximum palaeotemperatures, and the time at which a sedimentary section began cooling from maximum palaeotempera- tures. It is useful for studying the thermal history of sequences containing source rocks, and can provide information critical to the understanding of the timing of oil generation. Integration of AFTA and vitrinite reflectance (VR) data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining AFTA and VR data. This approach allows definition of the major facets of thermal history based on directly measurable parameters, rather than on assumed values of palaeo-heatflow, and provides rigorous estimates of the amount of uplift and erosion. The case study demonstrates the application of this approach to the East Midlands Shelf region. Here AFTA data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 Ma. Combined AFTA and VR data show that this cooling was due to kilometre-scale early Ter- tiary uplift and erosion under near normal geothermal gradients. Our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion, but was regional in extent. This has implications for the evolution of source rock maturity in this region and in other basins in around the UK where AFTA has indicated early Tertiary heating and where the extent and amount of Tertiary uplift and erosion may have also been underestimated by previous workers. Apatite fission track analysis (AFTA TM) is a method of obtaining thermal history information in sediments and is particularly useful for studying the thermal history of sequences containing hydrocarbon source rocks. As well as providing estimates of maximum palaeotemperatures, AFTA provides a direct estimate of the time at which a sedimentary section began cooling from its maximum palaeotemperature, a key advantage over traditional geochemical methods. In many situations, this knowledge is critical to the understanding of the timimg of oil generation and migration in relation to trap formation. Information on the timing of maximum palaeotempera- ture can also be of significant value in the interpretation of vitrinite reflectance (VR) data, and combination of AFTA and VR data allows a consistent, integrated thermal history interpretation. The AFTA technique is applied most usefully in sedimentary sections which have been hotter in the past than they are at the present day. It is therefore applicable to large regions of Britain and the UK continental shelf that have been subject to deeper burial in the past, or affected by other sources of heating. The basis of the technique has