Study On Oil Source And Characteristics Of Petroleum Accumulation In Kuche Foreland Basin, Tarim

Based on the theories of petroleum system and reservoir geochemistry, the Kuche foreland basin is selected to study some geochemistry problems on the process of petroleum from source to trap in this work. The results are shown as the followings:On the basis of studies of depositional environment of source rock and its petroleum potential assessment, the biomarker compositions of the five source beds (hq, J2k, Jiy, Tjt, Tsh) in Kuche basin are analyzed in detail and the effects of depositional environment and maturity on the biomarker compositions are also discussed. The results show that there is not clear correlation between the distribution of biomarker composition and the ages of Triassic and Jurassic. It is proposed for the first time in the Kuche basin that the changes of biomarker composition in the coal can be classified into three stages with the maturity, and the regular changes of different biomarker parameters in the coal are revealed.Oil-source correlation studies show that the oil sample in well Yi535 did not mainly derive from coals, but mainly originated from the coal measure mudstones and/or carbonaceous shales. The condensates in Tiergen structure originated from coals with a later stage of maturation. The oils in Yinan 2 well might originate from the coal measure mudstones with relative high maturity, which bear the same source as the oils in Yi535 well but with different mature level.The systematic analyses of light hydrocarbons show that the compositions of light hydrocarbons reveal the mixing of different genetic oils. The light and heavy components of the oils in Yaha and Yangtake have different sources respectively. The light components from marine formations are detected in the No.5 and No. 7 structures in the western of Yaha area. The condensates that derived from coals in Yangtake structure are mixed by the light compositions from Triassic lacustrine mudstones.The distribution of nitrogen compositions proved that differences of compositional fractionation existed in different carrier systems from source to trap in Kuche basin. The petroleum migration fractionation in the south part through unconformity and permeable carriers laterally is more obvious than that in the north part through the fault systems vertically.Based on the distribution of the unknown compounds "X" (between Ts and Tm) in oils and rocks, in combination with the theory of gas-washing evaporative fractionation, the formation of the light oils in Kela 2-3 dry gas reservoir is explained. The light oils originated from high mature rocks. In the process from source to trap with gas washing and evaporative fractionation, the oils got rich in aromatic compounds and poor in other compounds, which indicates that gas washing has great effects on the oil compositions.Based on the light hydrocarbon compositions and the distributions of n-alkanes and the oil physical features, the genetic mechanisms of the pristine oils, residual oils and retrograde oils in Yaha area are discussed, which identified the existence of second gas-condensate reservoirs in the Kuche basin.