Analysis of associated CO2 and Helium isotope composition in oil from 16 oil wells drilled in Carboniferous Donghe Sandstone reservoir in Hadexun Oilfield shows that CO2 content in the associated gas reaches to 0.46,～4.97, and δ13CCO distributes between -12.3‰ and -7.0‰. The values of CO2 content and δ13CCO differ in different areas, in which the values of both shows a regular increase from the north to south and the δ13CCO reaches to climax (-7.0‰) in HD4 trap. Integrated study shows that the associated CO2 mainly originates in anorganogene marine carbonate rock and thermal decomposition product of calcite cement in Donghe Sandstone reservoir. As the local structural high forming during the late stage of Himalayan movement, HD4 trap is the major target zone of hydrocarbon migration and charge. In addition, the strong diffusibility of CO2, possible concentration increasing owing to pressure drop and consequent CO2 exsolution, also makes CO2 accumulates in HD4 trap.