Abstract: CO₂ gas or CO₂ accompanied with hydrocarbon gas is widely distributed in the Laoyingtai Uplift of the Changling Fault Sag,the Songliao Basin.Contents and compositions of CO₂ are different.Relative contents of CO₂ and isotopic compositions may be obviously different between different wells or different gas formations in the same well,indicating complicate genesis.Studies of genetic mechanism of CO₂ are important not only in theory,but also for natural gas exploration in the region.Based on CO₂ composition and isotopic distribution features,combined with hydrocarbon gas studies,CO₂ deep in the study area is both organic and inorganic origin,or mixed.CO₂ in the Yingcheng Formation which contains much CO₂ is mainly inorganic origin.CO₂ in the Denglouku Formation which contains little CO₂ is organic origin or mixed.Organic origin CO₂ in the Denglouku Formation accumulates at the same time with hydrocarbon gas.Part of CO₂ in the Yingcheng Formation accumulates at the same time with hydrocarbon gas;however,large accumulations of inorganic origin CO₂ take place after Neogene.