Abstract: Determining a production gain index (PGI) for a petroleum reservoir provides a novel leading indicator and metric that is designed to quickly assess the potential for increases in production of petroleum from an operating petroleum reservoir when implementing a recovery plan. The PGI can be determined according to the following equation: P ? ? G ? ? I = ?? ? ? q A ? ? ? q Old where, ??qA=net actual production gain of the reservoir; and ?qOld=sum of current oil rates for existing producers. The PGI can also be determined according to the following equation: PGI=PR×(GPI?1) where, GPI=the global productivity index of the petroleum reservoir; and PR=the interference factor, which accounts for any losses in aggregate production gain due to well interference.