GEOCHEMICAL CHARACTERISTICS OF THE CAMBRIAN OIL AND GAS IN WELL TASHEN 1, THE TARIM BASIN

The Well Tashen 1 in Tarim Basin is the deepest subaerial well both in China and in Asia.The brown liquid hydrocarbon had been first discovered in the Upper Cambrian dolomite pore and cave with the formation temperature being 160℃,the formation pressure being 80 MPa and the buried depth being 8 400 m.The systematic geochemical analysis on the gas and liquid hydrocarbon from the Lower Ordovician to the Upper Cambrian in Well Tashen 1 shows that the content of hydrocarbon gas is 97 percent,the dryness factor of the gas is 0.97 and the average carbon isotope of CH₄ is -37.9‰.The R_o of the correspondent source rock is from 1.65% to 1.91%,which is characteristic of typical oily dry gas with high thermal maturity.Various biomarkers and distribution of isotope of petroleum family composition in the saturated fraction from extracts of two samples in Well Tashen 1 indicate that the distribution of n-alkanes is complete(from nC₁₃ to nC₃₈ with the C₂₃ being dominant),the OEP is 1.029,the C₂₁-/C₂₁+ is from 0.49 to 1.46 and the ratio of Pr to Ph(Pr/Ph) is from 0.762 to 0.991 with a predominance of phytane,suggesting a marine sapropelic source rock under the reduction to strong reduction environment.On the other hand,the parameters representing thermal maturity of biomarkers,such as Pr/nC₁₇,Pr/nC₁₈ and MPI,show the great difference between the Ordovician oil in Tahe oilfield and the liquid hydrocarbon in Well Tashen 1.The latter mostly like to the oils in Well Tadong 2 and Well T904 that is believed to come from the Middle and Lower Cambrian source rock.In addition,the liquid hydrocarbon in Well Tashen 1 is characteristic of sterane C₂₇ > C₂₈ << C₂₉ and the isotope of petroleum family composition in the saturated fraction is -29.13‰ to -25.84‰.It can be inferred that the liquid hydrocarbons in the Upper Cambrian dolomite mostly come from the Middle and Lower Cambrian source rock and the high formation pressure is an important reason of the hydrocarbon being preserved in the deep buried formation.