Associated gases from crude biodegradation possibly contain remnants from microbial degradation as well as products from the anaerobic biodegradation of crude.Studying this kind of associated gas and restoring its original properties are helpful to gas source correlation and exploration of natural gas.The components and isotopic compositions of associated gases from crude biodegradation in Gudao oilfield of Jiyang depression indicate that the natural gas in this field is generated by the sapropelic organic materials mainly in the third member of the Shahejie Formation.The interfusion of secondary biogenic methane with relatively lighter carbon and heavier hydrogen isotopic compositions that are generated from the biodegradation of oils results in the drier components and abnormal negative correlation between the carbon and hydrogen isotopic compositions of methane for the associated gases of crude biodegradation.The attack of micro organisms on propane and n-butane of the na-tural gas causes evident heavier δ

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C values of propane and n-butane in some samples.