Geological hazard characteristics, sources, and distribution patterns of shallow natural gas in the Dachiganjing structure belt, Eastern Sichuan: an integrated geochemical, isotopic, and petrographic approach

Understanding the structural characteristics and accumulation patterns of shallow natural gas (SNG) is essential for mitigating associated risks in deep engineering projects. The Yuwan high-speed railway project is studied to analyze the sources, migration pathways, and storage layers of SNG in the Dachiganjing area through fieldwork monitoring and geochemistry, including isothermal adsorption, gas chromatography, total organic carbon (TOC), and carbon isotope analyses. The study further explored the spatial accumulation patterns of SNG and delineated hazardous areas. The results indicate that the encountered SNG are derived from the mudstone and shale source rock intervals of the Permian Longtan and Ziliujing formations. SNG migrated through the faults and four groups of steep shear joints, accumulating in the sandstones of the Xujiahe and Shaximiao formations, and in the shales of the Ziliujing Formation. Notably, the sandstones of the Xujiahe Formation exhibit the best reservoir properties and the highest concentration of SNG. The accumulation patterns of gas in the study area can be categorized into three types: “sandwich type”, “self-generated and self-stored type”, and “isolated type”. These different accumulation patterns lead to varied gas outflow characteristics. Ventilation measures can effectively reduce the gas concentrations in the “isolated” and “sandwich” types. The distribution characteristics of SNG in the study area include two main patterns: “higher in the north and lower in the south” and “concentrated in the anticline core”. The research results are expected to provide new perspective and valuable instructions for addressing harmful gas disasters in future deep geological engineering projects.