The western slope of Songliao Basin spans two oil areas, Daqing and Jilin, and 7 oil and gas fields have been found since 1960s. The western slope oil is characteristic of relatively low density, middle viscosity, richness in non-hydrocarbons and low content of asphalt. Analyses of GC and GC-MS on the sampled western slope oils show three features: the first is high contents of isoprenoids and cycloalkanes, and the special baseline indicates the biodegradation of the oils; the second is a certain amount of n-alkanes, which reflects the feature of multiple fillings by oils; the third is the occurrence of various biomarkers for the western slope oil. Compared with the central depression oil, the content of normal steranes decreases while those of pregnanes and diasteranes increase, normal hopane series obviously decrease and anti-biodegradation biomarkers increase, such as long-chain tricyclic or tetracyclic terpanes and gammacerane. Thus, all the GC and MS data show the biodegradation and the feature of multiple fillings by oils. The carbon isotopic values of the western slope oil are —29 to —32%o, those of kerogen are —22 to —24%o for the Jurassic coal rock and —27 to —31%o for the Qingshankou and Nenjiang mudstone. The maturity of source rocks in the western slope is very low, the calculated Ro values of the western slope oil are 0.75-0.95% based on the indices of methyl-phenanthrene and methyl-dibenzothiophene. So, the source rock kitchen of the western slope oils is mainly the Qingshankou lacustrine mudstone of the central depression. The K-Ar chronology data of reservoir authigenic illite in four wells from east to west are 56 Ma, 45 Ma, 35 Ma and 35 Ma, respectively, and they also reflect the migration of the western slope oil from east to west.
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