Geochemical Characteristics of the Upper Triassic Xujiahe Formation in Sichuan Basin, China and its Significance for Hydrocarbon Accumulation

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 is 80.16%-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: C-13(1) of -43.8 parts per thousand to -29.6 parts per thousand, C-13(2) of -35.4 parts per thousand to -21.5 parts per thousand, C-13(3) of -27.6 parts per thousand to -19.8 parts per thousand, and C-13(4) of -27.7 parts per thousand to -18.8 parts per thousand. C-13(3)>C-13(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, H-2(CH4) values ranged from -195 parts per thousand to -161 parts per thousand, H-2(C2H6) values ranged from -154 parts per thousand to -120 parts per thousand, and H-2(C3H8) values ranged from -151 parts per thousand to -108 parts per thousand. The dry coefficient, C-13 and H-2(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 C-13 and H-2(CH4) values are, indicative of the characteristic of near-source accumulation. The H-2(C2H6) value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the H-2(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.