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

Abstract

AbstractThe 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 C2+ 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: δ13C1 of −43.8‰ to −29.6‰, δ13C2 of −35.4‰ to −21.5‰, δ13C3 of −27.6‰ to −19.8‰, and δ13C4 of −27.7‰ to −18.8‰. δ13C3>δ13C4 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, δ2HCH4 values ranged from −195‰ to −161‰, δ2HC2H6 values ranged from −154‰ to −120‰, and δ2HC3H8 values ranged from −151‰ to −108‰. The dry coefficient, δ13C and δ2HCH4 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 δ13C and δ2HCH4 values are, indicative of the characteristic of near‐source accumulation. The δ2HC2H6 value of natural gas is influenced by paleosalinity to a relatively large extent; the higher the paleosalinity is, the larger the δ2HC2H6 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 (C5–C7) 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.