Evaluation of the Röt Halite Member as a top seal for carbon dioxide storage in the Southern North Sea

The Röt Halite Member (RHM) is a top seal for planned carbon dioxide (CO2) storage operations in the Bunter Sandstone Formation of the UK Southern North Sea (UKSNS). While previous studies have considered that the RHM, where present, is likely to provide an effective top seal for CO2 storage, few have investigated the RHM in detail to determine its top seal characteristics on a regional scale. Downhole petrophysical logs reveal a distinct, regionally extensive evaporite cyclicity with up to five halite–mudstone–dolomite/anhydrite sequences. These have been mapped across the UKSNS using an extensive catalogue of 600 wells, providing insight into the extent, distribution and thickness of the individual evaporite cycles. The mapping reveals the depositional history of the RHM in the UKSNS, with initial flooding, subsequent expansion and eventual contraction of the evaporite basin. The depositional model is supported by petrographic interpretation from an RHM thin section sample. Additional datasets, including seismic reflection data are also reviewed to complement a discussion on the potential impacts on long-term containment and to identify remaining uncertainties. This regional understanding allows more detailed site-specific containment risk studies, in support of the CO2 licensing and permitting process, to be placed within an appropriate regional context.