Geochemical Characteristics of Natural Gas in the Upper Permian Reservoir of the Eastern Sichuan Basin, China: Implication of Multiple Sources Mixing

Abstract

For an improved understanding of gas enrichment mechanism in the eastern Sichuan Basin, South China, twelve natural gas samples were obtained from carbonate reservoirs of the Upper Permian strata to analyze the hydrocarbon and non-hydrocarbon gas compositions, stable carbon and hydrogen isotopes ratios of hydrocarbons, and noble gas isotope ratios. The gas samples in the Upper Permian reservoirs principally consist of alkane gas with a dryness ratio ranging from 127.9 to 1 564.4. The carbon isotope ratio of methane (δ¹³C₁) was almost constant at −34.1 to −31.3‰, but the carbon isotope ratio of ethane (δ¹³C₂) varied from −36.6‰ to −25.8‰. The hydrogen isotope ratio of methane (δ²HC₁) also displayed a wide range from −137‰ to −127‰. The large variations in the dryness ratio, δ¹³C₂, and δ²HC₁ with almost constant δ¹³C₁ suggest the mixing of sapropelic and humic origins for hydrocarbon gases in these reservoirs. A high concentration of hydrogen sulfide (H₂S) originated from the thermochemical sulfate reduction (TSR), which was positively correlated with δ¹³C₁ (or δ¹³C₂) in individual gas fields. TSR altered δ¹³C₁ (or δ¹³C₂) and resulted in the abnormal character of isotopic reversal in the individual samples. The δ¹³C₁ (or δ¹³C₂) in most gas samples, independent of H₂S concentration, further displayed reversed carbon isotopes because of the mixture of thermogenic gases with various thermal maturity levels. The measured argon isotope ratio (⁴⁰Ar/³⁶Ar) varied from 310 to 1 225, which suggests that the oldest 320 Ma source rock age corresponds to Permian shales. The analysis of the gas origin and the identification of primary source rock have made a significant contribution to further understanding the resource potential and distribution of natural gas in the Upper Permian, and have great implications for gas exploration in the eastern Sichuan Basin.

Keywords

gas origin / stable isotopes / thermochemical sulfate reduction / source rocks / gas accumulation / geochemistry

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