Currently in Nigeria because of under utilization of associated gas (AG) , large volumes of gas is flared. This is because of low demand in the domestic market and non completion of the gas gathering system infrastructure in some areas of operation. The Nigerian government gave an ultimatum to stop routine gas flaring and hence most oil companies are working to comply with the directive. The southern swamp fields located in the central and coastal swamp depobelts of the Niger Delta do not have gas gathering systems and hence associated gas produced is stranded gas. Gas injection has been identified as a possible interim solution to gas flaring in this location. This book present the result of a study on the feasibility of gas injection into depleted oil reservoirs and highlights the basic and easily indentifiable reservoir & fluid factors that determine suitability of a reservoir as an underground storage unit & those that affect the response of oil production to gas injection for quick screening of suitable reservoir candidates for gas injection.