Geochemistry characteristics and genetic types of natural gas in central part of the Tarim Basin, NW China

According to the chemical, carbon isotopic, and hydrogen isotopic compositions and 3He/4He ratios of 127 natural gas samples, the natural gas in the Tarim Basin is mainly composed of gaseous alkanes dominated by methane. The contents of the gaseous alkanes decrease with the increase of their carbon numbers. The δ13C values of CH4, C2H6 and C3H8 in the platform area of the Tarim Basin vary in the range of −54.4‰∼-24.4‰ (an average value of −39.2‰), −43.1‰∼-26.7‰ (an average value of −36.1‰) and −37.9‰∼-21.6‰ (an average value of −31.8‰), respectively. The overall carbon isotopic composition of the alkanes follows the trend of δ13C1< δ13C2< δ13C3< δ13C4 and in some cases varies with δ13C1>δ13C2 or δ13C3> δ13C4. The lower carbon isotope values of methane (δ13C1< δ13C2) may be attributed to thermal sulfate reduction (TSR), and the higher propane value (δ13C3> δ13C4) was caused by the mixing of the natural gases of the same genetic type formed at different stages. The methane hydrogen isotopes (δ2H-C1) of the natural gases in the Tarim Basin are in the range of −195‰∼-122‰ with an average value of −156‰. The 3He/4He ratios of the natural gases are in the range of 2.3 × 10−8∼66.8 × 10−8, suggesting that there is no deep mantle-derived gas in the platform area of the Tarim Basin. The carbon isotope fractionation of alkane gases (δ13C2<−28‰ and δ13C3<−25‰) indicates that the natural gas in the platform area of the Tarim Basin is oil-type. The modified plots of lnC1/C2vs. lnC2/C3 and C2/C3vs. δ13C2-δ13C3 of the natural gases indicate that the oil-type gas in the Tarim Basin has different predominant formation processes, including kerogen-cracking, oil-cracking, oil- and gas-cracking, secondary gas-cracking and mixing of the kerogen-cracking and oil-cracking gases. The gas evolved from gas-cracking gas, to oil- and gas-cracking gas and oil-cracking gas along the direction from the west side of the Manjar Sag to the central Tarim Basin. The natural gases in some areas of Tazhong are the mixtures of the kerogen-cracking and oil-cracking gases.