Water flushing and biodegradation are the most important and most common cause of the formation of heavy oils.The degrees of water flushing and biodegradation mainly depend on the distribution of fractures and unconformities within a basin

the degree of the basin's post stage uplift and bottom water activifies.The first kind of heavy oil in Kenxi oil region

resulted from basin uplift and primary oil pool weathering

is characterized by high density

high viscosity

high content of sulphur and high solidifying point;the second kind of heavy oil

resulted from active bottom water and biodegradation

is characterized by high density

high viscosity

high content of sulphur and low solidifying point.