Abstract: With the development of deep gas exploration, some huge, enriched zones of CO₂ have been found in the Changling Fault Depression of the Songliao Basin. The origin of the CO₂ was analyzed for carbon isotope values of and the ³He/⁴He ratio was determined. The CO₂ gas in the Changshen2, 4, 6 well blocks and the Songnan gas field is mainly inorganic gas sourced from the mantle, while that in the Dongling gas field is organic gas. The type was controlled by the distribution of deep fractures in the fault depression. The low-speed and high-conductivity magma chambers resulted from crustal thinning during mantle uplift, the "mesh structure" tectonic detachment belt in the deep Changling Fault Depression and the deep and extensive faults on the basement are the advantageous conditions for the formation of inorganic CO₂ reservoirs.