Abstract: According to the σ13C values of methane,the gases in Jizhong Depression can be divided into three groups as follows.-55‰ for shallow biogenetic gas,-53.5‰— -40.4‰ for the gas derived from oil,and -39.6‰— -33.9‰ for the gas derived from coal measures.The ⁴⁰Ar/³⁶Ar values of coal measure gas are similar to those of oil-derived gas and they may be formed in the late stage of gas generation.The coal measure gas in the depression mainly formed by secondary gas generation as the depression resubsided in Tertiary time.It is evident that 99,3 percent of the presently proved gas reserves is discovered at Permo-Carboniferous buried hills,and Ordovician buried hills with Permo-Carboniferous cover while only 0.7 percent of the proved gas reserves is discovered in Tertiary systems.