In the Ordos Basin, the Ordovician Majiagou Formation is an important replacement for natural gas exploration. Large-scale natural gas resources, such as those in the Jingbian gas field, have been discovered in the weathered crust at its top. However, the origins and sources of these resources remain highly controversial. In this study, we analyze the components and geochemical characteristics (e. g., carbon and hydrogen isotopic compositions) of natural gas from the Daniudi gas field. Accordingly, the origins and sources of natural gas are clarified within the Ordovician weathered crust reservoirs. The results indicate that the natural gas exhibits dryness coefficient (C₁/C_1-5) values ranging from 0. 911 to 0. 979 and CO₂ content from 1. 68% to 14. 39%. H₂S is observed in some samples, with content varying from 0. 07 × 10^-6 to 3. 37 × 10^-6 and gas sweetness index (GSI) values of merely 10^-7 to 10^-6 order of magnitude. Additionally, the δ¹³C₁, δ¹³C₂, δ¹³C₃, and δD1 values of natural gas range from -40. 0‰ to -33. 2‰,-35. 6‰ to -24. 9‰,-30. 7‰ to -24. 1‰, and -209‰ to -184‰, respectively. The natural gas in the Ordovician weathered crust reservoirs did not undergo significant thermochemical sulfate reduction (TSR). However, CO₂ of inorganic origin, produced by the dissolution of carbonate reservoirs, was mixed into some samples during acid fracturing. Origin identification and gas-source correlation reveal that the natural gas in the weathered crust reservoirs is dominated by coal-derived gas generated from coal-measure source rocks in the Taiyuan Formation. Such coal-derived gas experienced significant fractionation during both vertical and lateral migration into the weathered crust reservoirs. Additionally, the gas reservoirs also contain a minor proportion of oil-associated gas formed by the secondary cracking of crude oil generated from the carbonate source rocks in the Majiagou Formation.

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.