Abstract: Based on the conprehensive utilization of the data of carbon and hydrogen isotopes,the compositions of light hydrocarbons (C₁-C₇),and the other organic geochemical analyses,the author studied the genesis and maturity of natural gas in the Jiyang Depression.In order to determine different types of gensis and the levels of maturation in the depression,some identifying criteria of the natural gases have been proposed in this paper.Only the gas from biodegration is a specific type of the natural gas in the Jiyang Depression,and the secondary gases derived from biodegradation can be distinguished from various primary gases with these criteria.Meanwhile,the gas source rocks of shallow,middle and deep strata in the depression can be basically determined on the basis of direct correlation on degasing analysis and indirect correlation on hydrocarbon evolutionary equations.