Geochemical Characteristics and Origin of Natural Gas from the Ordovician Middle Gas-bearing Assemblage in the Ordos Basin: A Case Study of the Daniudi Gas Field

Abstract

The Ordovician Majiagou Formation (O₁m) in the Ordos Basin is a crucial exploration field for natural gas, and exploration of the Ordovician middle assemblage (O₁m₅^5–7) has recently yielded great breakthrough. The Daniudi gas field provides a good case study to determine the gas source for the strata. The O₁m₅^5–7 gas displays C₁/C_1–5 ratios of 0.932–0.985 and CO₂ contents of 1.56%–11.75%, and the detectable H₂S content ranges from 0.0002% to 1.8617%. The δ¹³C₁, δ¹³C₂, δ¹³C_CO2, and δD₁ values are −39.7‰ to −35.6‰, −30.4‰ to −23.7‰, −12.4‰ to −4.6‰, and −204‰ to −185‰, respectively. Identification of the gas origin and source indicates that the gaseous alkanes are commonly coal-derived gas. The gas was generated from the coal measures in the Taiyuan Formation (C₃t) and subsequently migrated. A small amount of oil-associated gas, mainly from O₁m carbonate source rocks, has been incorporated into the gas reservoir. The natural gas has experienced insignificant alteration by thermochemical sulfate reduction, and the relatively high levels of CO₂ are probably associated with corrosion alteration of carbonate reservoirs by injected fluid during acid fracturing.

Keywords

geochemical characteristics / stable isotopic compositions / genetic types / thermochemical sulfate reduction / gas-source correlation / Ordovician Majiagou Formation / Daniudi gas field

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2026 Geological Society of China