GEOCHEMICAL CHARACTERISTICS OF THE HYDROCARBONS IN DONGYING FORMATION RESERVOIR OF WENLIU AREA, THE DONGPU SAG OF THE BOHAI BAY BASIN

The hydrocarbons in Dongying Formation reservoir in Wenliu area,the Dongpu Sag of the Bohai Bay Basin,have undergone various degrees of biodegradation as the results of being relatively shallowly buried.The comparison analyses of the gas chromatography data and the chromatography-mass spectrometry data of the free hydrocarbons in the pores,the adsorbed hydrocarbons on the mineral grain surfaces and the wrapped hydrocarbons in the organic mineral inclusions show that,in the same oil sand sample from the unbiodegrated sections,the above 3 states of hydrocarbons have similar component characteristics with only some indicators being different.The similarities are reflected by the high contents of gammacerane(the gammacerane/C₃₀ hopane ratio is about 0.28 to 0.81) and the relatively low Pr/Ph ratio(about 0.21 to 0.51),indicating that the parent materials are mainly from aquatic orga-nism,which were formed in a kind of saline-lacustrine deposition environment under a strong reduction condition.The differences are reflected by the relatively low carbon number of the highest chromatographic peaks of free saturated hydrocarbons(between C₂₂ to C₂₄),and the unique values of the sterane maturity indicator C_{29 aaa}20S/(20S+20R) of the inclusion hydrocarbons(0.25 to 0.35).Whereas in the sections being biodegrated,the distribution of the normal alkanes in the inclusions are generally intact,while the normal alkanes in free hydrocarbons and adsorbed hydrocarbons are almost disappeared,indicating that the hydrocarbons in the reservoir are the results of different phases of recharge.