Geochemical Characteristics and Genesis of Natural Gas in Tarim Basin

According to statistical measurement of 234 gas samples, natural gas in the Tarim basin is dominant hydrocarbon gas, in which more than 80% of hydrocarbon gases accounts for 84% of all the samples. The contents of non-hydrocarbon gas (N2) in the Paleozoic and Mesozoic are higher than those in Cenozoic, with the positive correlation of nitrogen and hydrocarbon gases (C1-4). Based on carbon isotope fractionation of hydrocarbon gases, the natural gases in Paleozoic and Mesozoic are mostly classified as oil-type gas, whereas gas in Cenozoic is coal-type gas derived from humic matters. A positive correlation between δ13C2 and δ13C3 in natural gas from the Tarim basin suggests that natural gases were mainly derived from the thermal decomposition of organic matters. The noble gas isotopes (3He/4He) in the order of n×10-8 indicate natural gas in the Tarim Basin is of crust origin, in which there is no correlation between 3He/4He ratios and types of natural gases. The little higher ratios of 40Ar/36Ar in Paleozoic and Mesozoic than those in Cenozoic resulted from the different K abundance in source rocks. In combination with the geochemical information above, the natural gas is of thermal genesis, mostly derived from organic matters in the Tarim basin.