Origin and distribution of carbon dioxide gas pools in eastern China

Carbon dioxide gas pools occur widely in the basins of eastern China.CO2 gas-bearing bedsare from the Teritary to Ordovician,and reservoirs are sandstone,carbonates and volcanics.The gases fromthese gas pools contain CO2 of 62.86 ‰—99.55 ‰.In the Mesozoic-Cenozoic extensional basins,such asSongliao,Bohai Bay,Subei,Sanshui and Zhujingkou,the δ13CCO2values of CO2 gas pools range from-2.65‰to-8.83‰,mainly from-3.5‰ to-6.0‰,3He/4He ratios are 2.65Ra to 4.96Ra.The regression equa-tion of CO2 content and helium isotope ratio is CO2（%）=61.3852+7.9745R/Ra,correlation coefficient r is0.9430,CO2 is mainly mantle-derived and magmatic origin.δ13CCO2value of CO2 gas from Well X inYinggehai Basin is-3.80‰.3He/4He ratio is 0.07Ra,CO2 is metamorphic origin.Mantle-derived and magmaticCO2 gases are discharged from the cross areas of northeastern trending and northwestern trending faults inthese Mesozoic-Cenozoic extensional basins,in the shallow level,the CO2 gases migrate and accumulate alongnortheastern trending extensional faults.The activity of the Neogene to Quaternary northwestern trendingtectonic-magmatism zones in eastern China is another important discharge event for mantle-derived andmagmatic gases,which have great contribution to the CO2 gas pools in this area.Metamorphic CO2 in theYinggehai Basin is released by the dynamic metamorphism of shear fractures.