Could CO₂ storage site performance be compromised by palaeo-gas migration conduits in the overburden?

Formations below the North Sea are proposed as a major repository for anthropogenic C02 to mitigate climate change. Two such sites are already operational offshore Norway. Large areas of the North Sea are known to have leaked natural gas over geological time, so are these areas suitable for C02 storage, given that geological heterogeneities and natural gas palaeo-migration pathways could serve to compromise C02 storage sites in the longer term? We have identified similar features at the Utsira C02 storage site offshore Norway, thus we use this site as a case study to investigate whether C02 leakage is possible and if so under what conditions. We evaluate the main uncertainties affecting this storage site and present a risking methodology and screening tool that could be used to assess future storage sites and minimise the risk of compromise due to inappropriate site location.