Origin and distribution of carbon dioxide gas pools in eastern China

Carbon dioxide gas pools occur widely in the basins of eastern China.CO_2 gas-bearing beds are from the Teritary to Ordovician,and reservoirs are sandstone,carbonates and volcanics.The gases from these gas pools contain CO_2 of 62.86 ‰ —99.55 ‰.In the Mesozoic-Cenozoic extensional basins,such as Songliao,Bohai Bay,Subei,Sanshui and Zhujingkou,the δ~(13)C_(CO_2)values of CO_2 gas pools range from-2.65‰ to-8.83‰,mainly from-3.5‰ to-6.0‰,~3He/~4He ratios are 2.65R_a to 4.96R_a.The regression equa- tion of CO_2 content and helium isotope ratio is CO_2(%)=61.3852+7.9745R/R_a,correlation coefficient r is 0.9430,CO_2 is mainly mantle-derived and magmatic origin.δ~(13)C_(CO_2)value of CO_2 gas from Well X in Yinggehai Basin is-3.80‰.~3He/~4He ratio is 0.07R_a,CO_2 is metamorphic origin.Mantle-derived and magmatic CO_2 gases are discharged from the cross areas of northeastern trending and northwestern trending faults in these Mesozoic-Cenozoic extensional basins,in the shallow level,the CO_2 gases migrate and accumulate along northeastern trending extensional faults.The activity of the Neogene to Quaternary northwestern trending tectonic-magmatism zones in eastern China is another important discharge event for mantle-derived and magmatic gases,which have great contribution to the CO_2 gas pools in this area.Metamorphic CO_2 in the Yinggehai Basin is released by the dynamic metamorphism of shear fractures.