AN INVESTIGATION ON THE GEOCHEMICAL CHARACTERISTICS OF CRUDE OIL FROM WENLIU AREA IN THE DONGPU SAG, BOHAI BAY BASIN

The group composition and carbon isotope of oil,chromatogram and mass spectrum of saturation hydrocarbon in the source rocks and crude oil from Wenliu area in Dongpu Sag,Bohai Bay Basin were analyzed.The studying results show that the crude oil in the Shahejie Formation was mainly immature.The content of saturation hydrocarbon in the samples is 38.3%～79.3%.The carbon isotope of group composition of crude oil is-27.4‰～-28.7‰,and the fractionation effect of the carbon isotope of group composition is not obvious.There is predominance of phytane to pristane,and the ratio of pristane to phytane is lower than 0.5 in most samples.For the most samples,the values of αααC₂₉20R/20(R+S) and C₂₉ββ/(αα+ββ) are lower than 0.40,the content of diasteranes is low,and the sterane of C₂₇ is predominance in the series of steranes.The ratio of diasteranes to regsteranes is related to the maturity of samples.High maturity is correspond to high value of the ratio.For the samples distributing between the Wendong fault and Wenxi fault,the content of Gammacerance is high,the ratio of gamma-cerance to C₃₀hopanes is 0.40～0.99,the ratio of gammacerance to HC₃₁22S is 2.11～4.15 and the geochemical characteristics demonstrated that they formed in the salt lake facies sedimentary environment.For the samples located in the east of Wendong fault,the content of Gammacerance was low,the ratio of gammacerance to C₃₀ hopanes is 0.05～0.15,the ratio of gammacerance to HC₃₁22S is 0.16～0.28,and the geochemistry characteristics demonstrated that they formed in the sedimentary environment of low salty.