From Wikipedia, the free encyclopedia. Associated petroleum gas (APG), or associated gas, is a form of natural gas which is found with deposits of petroleum, either dissolved in the oil or as a free gas cap above the oil in the reservoir.Historically, this type of gas was released as a waste product from the petroleum extraction industry.It may be a stranded gas reserve due to the remote location of the oil field, either at sea or on land, this gas is simply burnt off in gas flares.When this occurs the gas is referred to as flare gas.istorically, this type of gas was released as a waste product from the petroleum extraction industry. It may be a stranded gas reserve due to the remote location of the oil field, either at sea or on land, this gas is simply burnt off in gas flares. When this occurs the gas is referred to as flare gas.