Origins and Fates of CO2 in the Dongying Depression of the Bohai Bay Basin

Abstract

CO2 gas reservoirs have been found in a series of rift basins in East China. The objective of this study is to reveal origins and fates of CO2 in the Dongying Depression in East China by using isotopic (He and C) and molecular compositions. 3He/4He, CO2/3He and δ13CCO2 values indicate that the CO2 gases with CO2 > 79% in the deep fault belts in the depression are predominantly mantle-derived. In contrast, CO2 in most gases in the relative stable areas, where source rocks are developed, is less than 10%. Based on helium isotope and δ13CCO2 ∼ CO2/3He diagram, it was found that 10−30% of these CO2 gases is mantle-derived, 28% to 68% is limestone-decomposed and 18% to 54% sedimentary organic in the relative stable areas. The mantle-derived and limestone-decomposed CO2 in the relative stable areas indicates that the mantle-derived geo-fluid met the source rocks. More hydrocarbons can be generated via its heat influence. As the previously published model is not suitable for study of CO2 fates at least in the case of thermogenic CO2 addition, we proposed two models for CO2 addition and loss: model I for CO2 loss and limestone-decomposed CO2 addition and model II for CO2 loss and addition of sedimentary organic CO2, based on a δ13CCO2 ∼ CO2/3He diagram. By using the new models, it was revealed that the loss rates of mantle-derived CO2 can be as high as 50–99.99% for most gases in the depression, the addition rates of limestone-decomposed CO2 range from 18% to 150%, and the addition rates of sedimentary organic CO2 from 11% to 230%. From this study, it was revealed that more mantle-derived CO2 entered the Dongying Depression than previously recognized.