A system for developing offshore hydrocarbon fields includes a floating hydrocarbon processing unit moored to the seabed and connected to a hydrocarbon reservoir via a riser and a wellhead on the seabed, the unit receiving and processing the hydrocarbon mixture received from the reservoir via the riser, and having equipment to separate the hydrocarbon mixture into oil, water and gas and store them in storage tanks. A gas storage buffer reservoir is placed at a distance from and isolated from the hydrocarbon reservoir. A gas export riser is connected to the floating hydrocarbon processing unit and the gas buffer reservoir to export separated gas and inject it into the gas buffer reservoir. A gas riser has one end connected to the gas buffer reservoir and another end connected to the floating gas processing unit for processing gas received from the gas buffer reservoir for enabling transportation to shore.