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The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan Basin is dominated by hydrocarbon gases, of which the methane content is80.16%-98.67%. Typically, the C_2~+ content is larger than 5% in main wet gas. The dry gas is mainly distributed in the western and northern regions of the basin. The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of <2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ~(13)C_1 of-43.8‰ to-29.6‰, δ~(13)C_2 of-35.4‰ to-21.5‰, δ~(13)C_3 of-27.6‰ to-19.8‰,and δ~(13)C_4 of-27.7‰ to-18.8‰. δ~(13)C_3>δ~(13)C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from humic-type source rocks of the Xujiahe Formation. As for the natural gas, δ~2 H_(CH4) values ranged from-195‰ to-161‰,δ~2 H_(C2H6) values ranged from-154‰ to-120‰, and δ~2 H_(C3H8) values ranged from-151‰ to-108‰. The dry coefficient,δ~(13)C and δ~2 H_(CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ~(13)C and δ~2 H_(CH_4) values are, indicative of the characteristic of near-source accumulation. The δ~2 H_(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ~2 H_(C2H6) value is. The Pr/Ph value of the condensate oil ranged from 1.60 to 3.43, illustrating light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon(C_5-C_7) from the Xujiahe Formation presented two characteristics: the first was the relatively high aromatic hydrocarbon content(19%-32.1%), which reveals the characteristic of natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon(0.4%-9.3%),reflecting water-washing during the accumulation of the natural gas. The reported research outcomes indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.
The alternative development of coal-bearing hydrocarbon source rocks and low-porosity and low-permeability tight sandstone reservoirs of the Triassic Xujiahe Formation in the Sichuan Basin is favorable for near-source hydrocarbon accumulation. The natural gas composition of the Xujiahe Formation in the Sichuan The is dominated by hydrocarbon gases, of which the methane content is 80.16% -98.67%. The dry gas is mainly distributed in the western and northern regions of The non-hydrocarbon gases mainly contain nitrogen, carbon dioxide, hydrogen, and helium, with a total content of <2%. The carbon isotope ranges of methane and its homologues in natural gas are: δ ~ (13) C_1 of -43.8 ‰ to-29.6 ‰, δ ~ (13) C_2 of-35.4 ‰ to-21.5 ‰, δ ~ (13) C_3 of-27.6 ‰ to-19.8 ‰, and δ ~ (13) C_4 of-27.7 ‰ to -18.8 ‰. Δ ~ (13) C_3> δ ~ (13) C_4 occurs in some natural gas with a low evolution degree; such gas is mainly coal-related gas from h As for the natural gas, δ ~ 2 H_ (CH4) values ranged from -195 ‰ to -161 ‰, δ ~ 2 H_ (C2H6) values ranged from -154 ‰ to-120 ‰ and δ ~ 2H_ (C3H8) values ranged from -151 ‰ to-108 ‰. The dry coefficient, δ ~ 13 C and δ ~ 2 H_ (CH4) are all positively correlated with the maturity of source rocks. The higher the maturity of source rocks is, the larger the natural gas dry coefficient is and the larger the δ ~ (13) C and δ ~ 2 H_ (CH_4) values are, the indication of the characteristic of near-source accumulation. The δ ~ 2 H_ (C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ ~ 2 H_ (C2H6) value is. The Pr / Ph value of the condensate oil ranged from 1.60 to 3.43, light oxidization-light reduction and partial-oxidization characteristics of the depositional environment of coal-bearing source rocks of the Xujiahe Formation. The natural gas light hydrocarbon (C_5-C_7 ) from the Xujiahe Formation presented twocharacteristics: the first was the relatively high aromatic hydrocarbon content (19% -32.1%), which reveals the characteristic natural gas with humic substances of high-maturity; the second was the low content of aromatic hydrocarbon (0.4% -9.3% , reflecting water-washing during the accumulation of the natural gas. The reported research results indicate a potential mechanism for natural gas accumulation in the Xujiahe Formation, which will further guide natural gas exploration in this region.