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 (O 1 m ) in the Ordos Basin is a crucial exploration field for natural gas, and exploration of the Ordovician middle assemblage (O 1 m 5 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 O 1 m 5 5–7 gas displays C 1 /C 1–5 ratios of 0.932–0.985 and CO 2 contents of 1.56%–11.75%, and the detectable H 2 S content ranges from 0.0002% to 1.8617%. The δ 13 C 1 , δ 13 C 2 , δ 13 C CO2 , and δ D 1 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 (C 3 t ) and subsequently migrated. A small amount of oil‐associated gas, mainly from O 1 m 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 CO 2 are probably associated with corrosion alteration of carbonate reservoirs by injected fluid during acid fracturing.