Abstract: It is essential to take a detailed study on the geochemical characteristics of heavy oil,which will be useful to conduct the oil-source correlation,the genesis and the development of heavy oil.The oils,which are from the west of Biyang Sag,have been experienced above middle degree of biodegradation.Furthermore,the regular steranes and hopanes have been suffered biodegradation to some extent,which made the common parameters reflected the genesis and maturity of oil ineffective.However,some parameters,which can resist strong biodegradation,such as gammacerane,Ts/Tm and some tricyclic terpane,still can identify the source of these biodegraded oils based on the GC-MS characteristics of 12 oil samples.It indicates that the crude oils in the lower of the Eh₃ formation are derived from the source rock of the lower of the Eh₃ formation,while the source of crude oils in the top of the Eh₃ formation is the mixture of the top and lower of the Eh₃ formation.In addition,some aromatic parameters,which are the component of aromatic hydrocarbon and the relation of triaromatic sterane(C₂₀+C₂₁) /∑(C₂₀-C₂₈) and triaromatic sterane C₂₀/(C₂₀+C₂₈R),can divide biodegraded oils into two types.And this conclusion is also proved by the relation of triaromatic sterane(C₂₀+C₂₁)/∑(C₂₀-C₂₈) and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane.Moreover,it suggests that there might have different biodegraded mechanisms for the biodegraded oils in the west area,for the same degraded degree of oils have a different distribution of saturated compounds.