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 3 He/ 4 He 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 δ 13 C values of CH 4 , C 2 H 6 and C 3 H 8 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 δ 13 C 1 < δ 13 C 2 < δ 13 C 3 < δ 13 C 4 and in some cases varies with δ 13 C 1 >δ 13 C 2 or δ 13 C 3 > δ 13 C 4 . The lower carbon isotope values of methane (δ 13 C 1 < δ 13 C 2 ) may be attributed to thermal sulfate reduction (TSR), and the higher propane value (δ 13 C 3 > δ 13 C 4 ) was caused by the mixing of the natural gases of the same genetic type formed at different stages. The methane hydrogen isotopes (δ 2 H-C 1 ) of the natural gases in the Tarim Basin are in the range of −195‰∼-122‰ with an average value of −156‰. The 3 He/ 4 He 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 (δ 13 C 2 <−28‰ and δ 13 C 3 <−25‰) indicates that the natural gas in the platform area of the Tarim Basin is oil-type. The modified plots of lnC 1 /C 2 vs. lnC 2 /C 3 and C 2 /C 3 vs. δ 13 C 2 -δ 13 C 3 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. [ABSTRACT FROM AUTHOR]