Turning exploration risk into a carbon storage opportunity in the UK Southern North Sea

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.