Abstract: Industrial CO₂ gas reservoirs discovered in the northern Songliao Basin locate mostly in the deep strata of the Xujiaweizi Fault Depression.The identification of origin of CO₂ in the reservoirs would be significant to research the model to form deep gas reservoirs.It was suggested that the volcanic rocks in deep strata of this area were calc-alkaline and differential production of mantle-derived magma by petrochemical data of reservoir rocks.In the Changdedong gas reservoir of Xujiaweizi area,the CO₂ contents are 89.73%～90.38% and δ¹³C_CO2 are -6.61‰～-4.06‰,which suggested that CO₂ is inorganic origin;³He/⁴He ratios are 3.9×10^-6 and 4.5×10^-6,which is between mantle-derived and crust derived origin.There is reverse order of carbon isotopic ratio to accompanying methane homologous compounds,which is characteristic of inorganic origin.The CO₂/³He ratio of 1.9×10⁹ indicated that CO₂ in this reservoir should be from degassing of upper mantle.The formation of the Changdedong gas reservoir was related to mantle-derived magma,and the CO₂ gas was inorganic and mantle-derived.