Abstract: According to different physical properties of heavy oil in Guchen and Xinzhuang oilfield in the northern slope of Biyang sag, and chlorofoum bitumen "A", heavy oil com-plosition and saturated hydrocarbon chromatographic analytical data of oil - bearing sand in nine wells, the geo-chemical features and heavy oil consistency variance degree of the two oilfields have been compared and analyzed, the correlation between heavy oil complosition and unconformity which is close to Neogene has been studied, and the relationship between the heavy oil complosition and burying depth and variance degree has been discussed. The results show that the reservoir generation period is in accordance with the heavy oil consistency variance degree in this two oilfields, that is earlier hydrocarbon generation in Gucheng results in higher heavy oil consitency variancedegree than that of in Xinzhuang.