GEOCHEMICAL CHARACTERISTICS AND GENESIS OF THE NATURAL GAS IN THE TAZHONG UPLIFT, THE TARIM BASIN

The genesis of the natural gas in the Tazhong Uplift is very complicated and there have been existing a variety of opinions for a along period.Based on the systematic research results of the composition and carbon isotope characteristics of the natural gas in the Tazhong Uplift,the natural gas in the Tazhong Uplift is mainly composed of hydrocarbon gases,but the composition and the carbon isotope in the west and east parts of the uplift differ apparently form each other.The natural gases in the eastern Tazhong Uplift and the No.1 structural belt have higher values of dry coefficients,higher nitrogen contents and heavier carbon isotopic ratios.It is regarded that these differences are mainly determined by the different thermal evolution degrees of the organic matter,in addition to the organic matter type and the secondary alteration.The maturity of the Lower Paleozoic organic matter in the east part was fina-lized earlier,and the organic matter distributed immediately near the Cambrian source rock layer of the Manjiaer Sag.Therefore,the gas accumulated in the eastern Tazhong Uplift mainly came from the high to over matured Cambrian source rocks.As a contrast,the maturity of the Middle-Upper Ordovician organic matter in the west part was finalized later,and the gas accumulated in the western Tazhong Uplift mainly came from the middle matured Middle-Upper Ordovician source rocks.These results suggest that the natural gas in the Tazhong Uplift is mainly generated from the kerogens of various thermal evolution degrees,and the amount of the oil-pyrolysis gas is not significant.