Abstract: This paper gives a discussion on geochemical characteristics of natural gas in the shallow reservoir in Gudao area of Jiyang Depression. The gas suffered serious biodegradation. Original geochemical features are changed because of biodegradation. Correlation of gas-sourece for this gas is difficult. This paper uses technology of heavy hydrocarbon concentration of natural gas to detect biological markers of sterane and terpane, and defines gas-source of biodegradation gas with the aid of data of rare gas isotopes Basis is provided for Clarifying exploratory target.