Bulk and position-specific isotopic compositions of natural gas in the Permian Jiamuhe Formation, Zhongguai Uplift, Junggar Basin, China

This study investigates the genetic types and accumulation characteristics of natural gas from the Permian Jiamuhe Formation in the Zhongguai Uplift, Junggar Basin, through an integrated geochemical analysis of 23 natural gas samples. The analytical techniques included chemical composition analysis, stable carbon (δ13C) and hydrogen (δ2H) isotope analysis of individual alkanes (C1–C4), and position-specific carbon isotope analysis of propane. The results indicate that the natural gases are predominantly dry gas, with an average dryness coefficient (C1/(C1–C5)) of 0.97 and methane content ranging from 75.23% to 95.98%. The stable carbon isotopic compositions generally show an increasing trend from methane to propane, although many samples exhibit partial C2–C3 isotopic rollover, reflecting complex isotopic behavior typical of thermogenic gases. Critically, the ethane carbon isotopic values (δ13C2: − 27.5‰ to − 24.6‰) are predominantly heavier than − 28‰, suggesting a coal-derived origin. Plotting on a Bernard diagram confirms their classification within the thermogenic gas field. The position-specific isotope data of propane reveals a consistent isotopic difference between the methylene and methyl carbon, indicating formation primarily via the n-propyl pathway during kerogen cracking, with no clear isotopic evidence for significant alteration. The study concludes that the natural gas in the Zhongguai Uplift is mainly coal-derived thermogenic gas generated from Type III kerogen, with its accumulation controlled by the interplay of hydrocarbon kitchens, fault-fracture migration pathways, and favorable reservoir conditions in the Jiamuhe Formation.