Gas source of the Middle Jurassic Shaximiao Formation in the Zhongjiang large gas field of Western Sichuan Depression: Constraints from geochemical characteristics of light hydrocarbons

The Zhongjiang gas field is a typical large gas field in terrigenous strata of the Western Sichuan Depression. It remains debatable which member of the Upper Triassic Xujiahe Formation served as the source rocks and how significant the member contributed to the gas accumulations in the Zhongjiang gas field. In this study, we analyzed the essential characteristics of the Lower Jurassic source rocks and the geochemical features of light hydrocarbons in natural gas from the 2nd (T₃x²) and 4th members (T₃x⁴) of the Upper Triassic Xujiahe Formation (T₃x), as well as the Middle Jurassic Shaximiao (J₂s) and Qianfoya (J₂q) formations. Based on this, we explored the sources of the natural gas in the Zhongjiang gas field and determined the natural gas migration patterns and their effects on the properties of light hydrocarbons in the natural gas. The results indicate that the Lower Jurassic lacustrine source rocks of the Zhongjiang gas field contain humic organic matter, with vitrinite reflectance (R_o) values ranging from 0.86% to 0.98%. Samples meeting the criterion for effective source rocks [total organic carbon (TOC) content ≥0.75%] exhibited an average TOC content of merely 1.02%, suggesting significantly lower hydrocarbon generation potential than source rocks in the underlying T₃x, which show higher thermal maturity and TOC contents. For natural gas samples from T₃x², T₃x⁴, J₂s, and J₂q reservoirs, their C_5–7 iso-alkane content was significantly higher than their n-alkane content, and their methylcyclohexane (MCH) index ranged from 59.0% to 77.3%, indicating the predominance of methylcyclohexane in C₇ light hydrocarbons. As indicated by the origin identification and gas-source correlation based on the geochemical features of light hydrocarbons, the natural gas in the Zhongjiang gas field is typical coal-derived gas. The gas from the primary pay zone of the Shaximiao Formation, with significantly high K₁, (P₂ + N₂)/C₇, and P₃/C₇ values, predominantly originated from the 5th member of the T₃x and migrated in the free phase, with a small amount possibly sourced from the Lower Jurassic source rocks. The dissolution and adsorption during gas migration led to a decrease in the aromatic content in C_6–7 light hydrocarbons and an increase in the iso-heptane values. Therefore, their effects must be considered when determining the gas origin and thermal maturity based on the aromatic content in C_6–7 light hydrocarbons and iso-heptane values.