Geochemical Characteristics And Mechanism Of Oils From The Wensu Salient And Its Surrounding Areas In The Northern Tarim Basin

The Wensu Salient is located in the northwest of the Tarim Basin and has a large potential for oil and gas exploration.The degree of hydrocarbons exploration and development has been relatively low compared with the other tectonic belts in the basin.The recentlly drilled wells of Xinwendi 1 and Xinwendi 2 proved to be high-yield industrial oil and gas flow wells,suggesting great petroleum exploration potential in the areas.Geochemical and geological methods were used to reveal the the characteristics,genetic mechanisms and accumulation features of oils in the Wensu Salient and its surrounding areas.Based on biomarkers and carbon isotope analysis,the oils in the study area are subdivided into five oil families.(1)Oil families I is marine crude oils of the Awati Sag,with extremely low pristine to phytane ratios and extremely high C₃₅-hopane to C₃₄-hopane ratios;(2)Oil families II is coal-measure marsh facies crude oils from the well of Yinan 2,bearing features of a relatively high pristine to phytane ratios and a relatively high content of Oxyfluorene.(3)Oil families III is the crude oils from the Wensu Salient,which is similar to the crude oils in the Wushi Sag,both of them have similar characteristics,and the ratios of rearranged sterane to regular sterane,tricyclic terpene to pentacyclic terane are very close;(4)Oil families IV is the crude oils from the Quele-Yangtake-Yudong areas,which is typically characterized by descending relative distribution of C₁₉-C₂₂-tricyclic terpenes,and the carbon isotope is relatively low;(5)Oil families V is the crude oils in the Yingmaili,typically characterized by a normal distribution of C₁₉-C₂₂-tricyclic terpenes and the carbon isotope is relatively high.There are mainly two sets of primarily source rocks in the study area,which are the Jurassic Qiakemake Formation(J₂q)and the Triassic Huangshanjie Formation(T₃h),characteristized by typical lacustrine source rocks.The J₂q source rock is featured by a relatively high abundance of C₃₀-diahopane,C₂₉Ts and dibenzofuran series as well as a descending order of the relative abundance of C₁₉-C₂₂-tricyclic terpenes with increasing carbon number.However,the T₃h source rock is characterized by a relatively high abundance of dibenzothiophene series and an increasing order of the relative abundance of C₁₉-C₂₂-tricyclic terpenes with increasing carbon number.The oil-source correlation shows that the mass chromatogram and related biomarker parameters of crude oils in the Wensu Salient are between the two sets of lacustrine source rocks.The hydrocarbon migaration and accumulation model of the Wensu Salient was finally established:"Dual-source,late generation-two stage hydrocarbon charging-composite transport system-structural lithologic reservoir".