Abstract: By analysing the geochemical characteristics of viscous oils from Permian Wuerhe Formation (P₂w) in the southern oil/gas region of V zone within faulted footwall slope area at the northwestern edge of Junggar Basin, it is discovered that the viscous oils have the special properties of low acidity, higher solidifying point and waxy content, alkane-bearing hydrocarbons, oxygen-bearing functional group lacking, unselective losses etc. and have obviously different geneses from secondary oxidized viscous oils in faulted upper plates-Based on the distribution and each geochemical character of hydrocarbons, it is considered that evapofractionation is the major reason to bring about the consistency changes of crude oils in this area.