Abstract: The heavy oil mainly distributes in Fengcheng oilfield of Wuerhe-Xiazijie area in Junggar basin. Affected by the biodegradation, the density and viscosity of heavy oil are very high, normal paraffin hydrocarbons and pentacyclic isoprenoid paraffin hydrocarbons are subjected to serious degradation, so are partial hopanes, steranes and homosteranes, and free carotene, while tricyclic terpanes, pregnanes and homopregnanes are in high content, with high Ts/Tm value of 3.55. This paper presents the six ranks of biodegradations of heavy oils in Wu-Xia area, and accordingly gives six types of heavy oils in this area, based on the degradations of normal-and iso-alkanes, biomarkers and 25-norhopanes as well as the contents of tricyclic terpanes and pregnanes.

Key words: Wuerhe-Xiazijie area, heavy oil, biodegradation, biomarker