The data consists of an extended abstract submitted to 'The Fourth International Conference on Fault and Top Seals', Almeria, Spain, 20-24th September 2015. The abstract describes work carried-out on behalf of the 'Fault seal controls on CO2 storage capacity in aquifers' project funded by the UKCCS Research Centre, grant number UKCCSRC-C1-14. The CO2-rich St. Johns Dome reservoir in Arizona provides a useful analogue for leaking CO2 storage sites, and the abstract describes an analysis of the fault-seal behaviour at the site. http://earthdoc.eage.org/publication/publicationdetails/?publication=82673.