Interpretation of a large, well-calibrated 3D seismic data volume in the UK Southern North Sea (SNS) suggests that the occurrence of two little known and hitherto poorly documented carbon dioxide (CO2)-rich gas discoveries is strongly controlled by the style and timing of deformation and the presence of a regional Upper Permian (Zechstein Supergroup) evaporite super-seal. It can now be shown that the CO2-rich accumulations are limited to Rotliegend Group, Leman Sandstone Formation (LSF) reservoirs located on the western edge of a major, extensional fault block, the Fizzy Horst, that lies on the eastern flank of a through-going NNW-striking, partially-inverted depocentre, termed the Brown Graben. Significantly, unlike other structures, which experienced Cenozoic compressional reactivation, the traps containing the CO2 are located adjacent to deep-seated faults upon which contractional reactivation occurred only during the Late Cretaceous suggesting a spatial and temporal control on its occurrence. The structural results provide a robust, unifying and testable structural model through which to assess the inherent exploration risk of drilling unwanted, CO2-prone traps in this part of the prospective basin. Conversely, the fact that CO2 was evidently sealed over geological time-scales shows the significance and long-lived (c. 50 Ma) effectiveness of the Zechstein Supergroup evaporite canopy in retaining CO2, as well as larger and less mobile methane (CH4) molecules. The results thus highlight the potential that traps containing LSF reservoirs have as future sites for CO2 storage (carbon sequestration) in the SNS.

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