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Thickened heavy oils in China are genetically characteristic of continental facies.As to theirphysico-chemical properties,these oils are very high in viscosity and low in sulphur and trace element con-tents.In the group constituents,the concentrations of non-hydrocarbons and asphaltene are very highbut those of saturated hydrocarbons and aromatics are very low.The gas chromatograms of alkanes showthat these heavy oils have high abundances of iso-alkanes and cyclic hydrocarbons.In all the steroids andterpenoids,bicyclic sesquiterpenoids,tricyclic diterpenoids,re-arranged steranes and gammacerane arestrongly biodegradation-resistent.The formation of heavy oil reservoirs is controlled mainly by late basinascendance,biodegradation,flushing by meteoric water and oxidation in the oil-bearing formations.Ac-cording to their formation mechanisms,heavy oil reservoirs can be classified as four categories:weatheringand denudation,marginal oxidation,secondary migration and thickening of bottom water.Spacially,heavythick oil reservoirs are distributed regularly:they usually show some paragenetic relationships withnormal 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 ordepressions.
Thickened heavy oils in China are genetically characterized of continental facies. As to their physico-chemical properties, these oils are very high in viscosity and low in sulfur and trace element con- tent. In the group constituents, the concentrations of non-hydrocarbons and asphaltene are very highbut those of saturated hydrocarbons and aromatics are very low. The gas chromatograms of alkanes showthat these heavy oils have high abundances of iso-alkanes and cyclic hydrocarbons. In all the steroids andterpenoids, bicyclic sesquiterpenoids, tricyclic diterpenoids, re-arranged steranes and gammacerane arestrongly biodegradation-resistent. The formation of heavy oil reservoirs is controlled mainly by late basin conditions, biodegradation, flushing by meteoric water and oxidation in the oil-bearing formations. Ac-cording to their formation mechanisms, heavy oil reservoirs can be classified as four categories: weatheringand denudation, marginal oxidation, secondary migration and thickening of bottom wate r.Spacially, heavythick oil reservoirs are distributed regularly: they usually show some paragenetic relationships with normal oil reservoirs. Heavy oil reservoirs often occur in structural highs or over over younger strata. Their burial depth is about 2000m.Horizontally, most of them are distributed on the margins of basins ordepressions.