Evaluating the role of caprock in controlling natural accumulation and storage of CO₂ in the East Irish Sea Basin, UK

Naturally occurring CO₂ in some gas fields in the East Irish Sea Basin is proof of concept of effective containment there since Paleogene emplacement. A large well and geophysical database affords the opportunity to examine the geological controls responsible for the CO₂ accumulations. Halite-dominated members, such as the Fylde Halite, within the Mercia Mudstone Group caprock are thickest in the Keys Basin, a sub-basin where natural CO₂ resides, but are also present elsewhere before becoming absent southwards, where mudstone-dominated members proportionally increase and contain laterally continuous sandstone interbeds. The lack of petroleum shows above the evaporite-rich units and their consistent presence within underlying argillaceous sections highlights the significance of the halite-dominated members as effective caprocks. Multiple regional pressure trends indicate that the aquifer of the Keys Basin is relatively underpressured and isolated from the wider aquifer by bounding faults and graben. The combination of aquifer isolation, igneous intrusions, and the overlying Fylde Halite Member is considered responsible for the local preservation of natural CO₂. While the thick, halite-dominated caprock will reduce the risk to containment for storage prospects in the Keys Basin, the isolation of the aquifer may cause rapid pressurization from CO₂ injection that limits technically achievable capacities.