Abstract: Geochemical analysis of petroleum geology has been made by using comprehensive 2d gas chromatography (GC×GC) and conventional gas chromatography. The parameters of light hydrocarbons and isoprenoids by oil direct injection and by saturated hydrocarbons or aromatics injection are comparable according to a comparative study of geochemistry, whereas part of the aromatics such as naphthalene series and benzfhiore tri-fluorene are characterized with obvious differences. Comparing with aromatics injection, the dimethylnaphthalene in crude oil direct injection is in dominant position and dibenzofurans is significant higher, which shows that the separation of compound-grouped fractions in pretreatment of sample changes the original appearance of aromatics. Therefore the establishment of the benzfhiore tri-fluorene chart which reflects organic deposition environment should not be applied to the analysis results of oil by GC×GC. Oil direct injection can provide more real information because there is no need for pretreatment.