Abstract: Shuangnan area produces mainly normal crude oil along with small amounts of condensate.It is discovered by observation that the total properties of condensate and normal oil(including density,viscosity,wax content,solidifying point,etc.)and the content of some biomarkers change greatly.Detail geochemical analyses reveal that they are well-matched in maturity.Many kinds of biomarkers are used to make maturity evaluation.The matutity of liquid hydrocarbon R_o is equal to 0.7%-0.8%.These crude oils have the same biological source composition,and are formed in the similar sedimentary environments.The total and molecular composition changes of crude oil are attributed to migration-fractionation process.In this process,gas-condensate is partitioned from the related crude oil,and then migrates to shallower reservoirs and accumulates there.It is very important to recognize the migration-fractionation process and understand its effects on petroleum components,because many geochemical bimarkers applied in oil-oil and oil-source correlation as well as crude oil alteration,oil source types,thermal maturity,etc.may be influenced by the migration-fractionation process.