Thermal history and depositional environments of source rocks in the Weald and Wessex basins, onshore southern England

Abstract

The Weald and Wessex basins of onshore southern England are host to numerous producing hydrocarbon fields, however vital questions regarding the timing and number of hydrocarbon generation events and the producing source rocks remain unanswered. These basins have experienced multiple phases of uplift and inversion and thus the structural development of the basins is complex, resulting in a similarly complex thermal history of each basin. Therefore, this thesis aims to: i) Identify the timing and number of hydrocarbon generation events in the Wessex Basin. ii) Determine the extent of Cenozoic inversion of the Weald Basin. iii) Quantitatively assess the source rock potential of the Cretaceous Weald Clay and the associated depositional environments of any identified source potential. Aims i and ii were achieved through measuring vitrinite reflectance of fifteen wells and using this data to construct burial models using Genesis® software. The third aim was achieved by measuring the TOC of the Weald Clay preserved in seven wells to identify the distribution of organic matter. Palynofacies analysis was conducted on three of these wells to further understand the depositional environments controlling the development of potential source rock horizons. Burial models of the Wessex Basin revealed two hydrocarbon generation events, however these generation events do not occur uniformly across the basin. Initial generation occurred in the earliest Cretaceous in the Dorset area, such generation was halted in the mid-­‐Aptian due to significant uplift (up to 1 km). Generation was re-­‐established in the Late Cretaceous in the north and eastern areas of the basin, again generation was stopped due to uplift exceeding 1250 m between 61.6-­‐55.8 Ma. The maximum extent of Cenozoic inversion of the Weald Basin is modelled to have exceeded 2200 m with the magnitude of inversion increasing progressively eastwards. The inversion has manifested with an asymmetric component where the northern flank of the basin has experienced reduced uplift in comparison with the southern flank. Hydrocarbon generation commenced in the Late Cretaceous in response to the deposition of thick Chalk sequences, however Cenozoic uplift halted any such generation. TOC data revealed that organic richness of the Weald Clay is typically low (<1 % TOC), although enhanced values of up to 6.7 % TOC are present, however such horizons are restricted in both lateral and vertical extent. Palynofacies analysis indicated that sediments with low TOC values (<1 %) were typically deposited in fluvial and deltaic environments, with deposition of sediment rich in organic matter occurring in freshwater to brackish lacustrine environments.

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ORCID for Ian Harding: orcid.org/0000-0003-4281-0581

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