Cushion Gas is gas left in a gas store to provide the pressure needed to produce stored gas, but which itself remains un-produced. It is typically of the order of 50% of the total stored volume for an Aquifer and depleted fields, but less for other types of storage. When the store is initially established Cushion Gas, unless it is un-produced reserves left in a partly depleted field, may be a large part of the capital cost. Cushion Gas may finally be used when the store is decommissioned. Also known as Base Gas.