The Capability of Faults and Top Seals in the Bunter Sandstone of the UK Southern North Sea to Seal Carbon Dioxide

In the UK sector of the Southern North Sea, the Triassic Bunter Sandstone Formation (BSF) is perceived as having a significant potential for the storage of anthropogenic CO2 within major structural closures. Although most of these structures are saline water-bearing, eight gas fields with BSF reservoirs have been discovered to date. The presence of natural gas fields demonstrates the capability of the various top seals to effectively seal significant gas accumulations over geological time-scales. However, during the formation of the domes, the BSF and its overlying top seals were subjected to extensional stresses and are therefore commonly transected by faults. The seismically resolvable structure of the BSF-reservoired gas fields are considered here, in order to assess the relationship between faulting and pre-production gas/water contacts. The assessment concludes that at least four of the fields have been affected by crestal faults which do not appear to have affected the sealing ability of the overlying top seals. Therefore, similar minor-offset faults occurring within the non-gas bearing structures may be capable of supporting CO2 columns that would exert pressures comparable to those exerted by the pre-production gas columns in the fields.