Geological and geochemical studies of heavy oil reservoirs in China

Thickened heavy oils in China are genetically characteristic of continental fades. As to their physico-chemical properties, these oils are very high in viscosity and low in sulphur and trace element contents. In the group constituents, the concentrations of non-hydrocarbons and asphaltene are very high but those of saturated hydrocarbons and aromatics are very low. The gas chromatograms of alkanes show that these heavy oils have high abundances of iso - alkanes and cyclic hydrocarbons. In all the steroids and terpenoids, bicyclic sesquiterpenoids, tricyclic diterpenoids, re - arranged steranes and gammacerane are strongly biodegradation - resistent. The formation of heavy oil reservoirs is controlled mainly by late basin ascendance, biodegradation, flushing by meteoric water and oxidation in the oil - bearing formations. According to their formation mechanisms, heavy oil reservoirs can be classified as four categories: weathering and denudation, marginal oxidation, secondary migration and thickening of bottom water. Spatially, heavy thick oil reservoirs are distributed regularly: they usually show some paragenetic relationships with normal oil reservoirs. Heavy oil reservoirs often occur in structural highs or in overlying younger strata. Their burial depth is about 2000m. Horizontally, most of them are distributed on the margins of basins or depressions. Weng Weijin, Guo Jiaofeng and Li Huaqi also took part in this work.