Geochemical Characteristics and Differential Accumulation Model of Natural Gas in the Middle Permian Maokou Formation, Puguang area

In recent years, exploration breakthroughs in the Middle Permian Maokou Formation of the Puguang area have demonstrated significant development potential for natural gas reservoirs in this series. However, the genetic types of the gas, the contribution proportions of multiple source rocks, and the mechanisms governing accumulation evolution and differential enrichment remain unclear. In particular, the heterogeneity of gas-source composition across different structural zones and the unclear understanding of accumulation models driven by regional tectonic movements have constrained the deployment of further differentiated exploration. Based on analytical data from recent natural gas samples, combined with source rock characteristics and the fault tectonic background, this study integrates natural gas genetic identification with gas-source correlation. It clarifies the origin and source of natural gas and further discusses the the processes of accumulation and enrichment laws. The results indicate that:Natural gas in the Maokou Formation of the Puguang area is dominated by methane (CH₄) with a mean content of >90 %, while the content of C₂₊ heavy hydrocarbons is low. The dryness coefficient is >0.99, classifying it as a typical highly evolved dry gas. The carbon isotopic composition of methane (δ¹³C₁) ranges from −30.4 ‰ to −28.8 ‰, and that of ethane (δ¹³C₂) ranges from −33.7 ‰ to −30.32 ‰, showing a reversal in the carbon isotope series. Comprehensive identification results suggest that the gas is oil-cracking gas formed at the high-to-over mature stage. Gas-source correlation analysis reveals that the gas is primarily a mixed origin derived jointly from Middle Permian source rocks and the Silurian Longmaxi Formation source rocks. Furthermore, there are significant differences in source contribution proportions across well areas: Well Ming-1 is dominated by the Silurian Longmaxi Formation, whereas the contribution from the Middle Permian source increases significantly in Well Yangliu-1 and Well Chuanyue-84. Combining regional tectonic evolution with the degree of fault development, two differential enrichment models were established for the study area: In zones with developed source-connected faults, it exhibits a “fault conduit-external source dominated” model, primarily charged by Silurian source rocks. In areas with underdeveloped faults, it presents an “intraformational microfracture-proximal source dominated” model, mainly supplied by self-sourced Middle Permian source rocks.