The Natural Gas Origin and Key Control Factors on Gas Reservoir with High Contents CO₂ in Middle Bohai Offshore

The QN-A gas reservoir in Qingnan Depression is the first discovery in the middle part of Bohai Sea, with ten billion cu-bic meters of gas reserves and has high CO₂ contents over 50%. The natural gas origin and key control factors have been analyzed through analysis of gas components, stable carbon isotope, helium isotope and fluid inclusion components and homogenization tem-peratures. The results showed that CO₂ was identified as volcanic mantle-source inorganic gas, and hydrocarbon gas is oil-type gas from Es Formation in adjacent depression. The key control factors of gas reservoir include the quick gas accumulation and strong ac-tivity Neogene boundary fault of uplift, which was beneficial to gas preservation and controlled the CO₂ migration and accumulation separately. In future exploration we should pay attention to the change of CO₂ contents controlled by fault activity.