CO2 in the liquid or super-critical state is delivered by at least one carrier vessel from at least one CO2 storage site, which may be an onshore site, to an integrated offshore facility. The integrated offshore facility is provided with at least one on-site storage tank or vessel adapted to store CO2 in the liquid or super-critical state and with equipment for marine transfer of CO2 in the liquid or super-critical state. CO2 is utilised as required from said at least one on-site storage tank or vessel for EOR at said offshore site or for EGR at said offshore site by injection into a sub-sea oil or natural gas bearing reservoir and recovery of oil and/or natural gas from a resulting production stream.