Origins and sources of natural gas in the Ordovician weathered crust reservoirs in the Daniudi gas field, Ordos Basin

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.