A measured wetness of and a δ13C associated with a gas sample from a hydrocarbon formation is received wherein the wetness is a percentage of C2+ by mass. Calculated wetnesses of and δ13C values associated with a plurality of gas samples taken from one or more analogous hydrocarbon reservoirs is received. Each wetness is calculated as a percentage of mass within the gas sample. The measured wetness received for the gas sample from among the calculated wetnesses is identified. A δ13C is determined from among the δ13C values that corresponds to the measured wetness of the gas sample. A gas maturity for the gas sample is determined using the determined δ13C.