The Ordovician Majiagou Formation (O1m) in the Ordos Basin is a crucial exploration field for natural gas, and exploration of the Ordovician middle assemblage (O1m55–7) has recently yielded great breakthrough. The Daniudi gas field provides a good case study to determine the gas source for the strata. The O1m55–7 gas displays C1/C1–5 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, δ13CCO2, 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.