GEOCHEMICAL PROCESSES IN PETROLEUM MIGRATION AND ACCUMULATION

The composition of crude oil, although initially controled by the types and maturity of organic matter in source rocks, will be changed thoroughly by the secondary modification during migration and after pool-formation. The dissolution and adsorption subjected by crude oil after expulsion have certain effects on the cpmposition of accumulated crude oil, but the influences of migration fractionation caused by the changing of temperature and pressure conditions in reservoirs, fault activties, denudation, earthquakes or the micropercolation of covers on the composition of crude oil are particularly important. Migration fractionation can form derived condensate and residual oil. Light components are very enriched in derived condensate and usually accumulate in shallower reservoirs, while high molecular weight compounds are enriched in residual oil which is characterized by the more obvious increase of wax content.It is thus clear that the formation of condensate and a high-wax oil is not simply controled by parent sources and maturity. Understanding the effects of migration fractionation will be helpful to avoid the mistaken interpretation of geochemical data.