Two distinct processes produce hydrocarbon gas: biogenic and thermogenic degradation of organic matter. Biogenic gas is formed at shallow depths. In contrast, thermogenic gas is formed at deeper depths by: (1) thermal cracking of sedimentary organic matter into hydrocarbon liquids and gas and (2) thermal cracking of oil at high temperatures into gas. If a gas show is found to be thermogenic in origin, then the possibility exists that the gas derives from a gas cap overlying a down-dip oil leg. That possibility can be further evaluated from specific aspects of the composition of the thermogenic gas