Geochemistry and Accumulation of Carbon Dioxide Gases in China

We propose the following identification indicators for CO2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples, the CO2 carbon isotope values of 390 samples, and the helium isotope composition of 256 samples collected from wells of 16 oil- and gas-bearing basins and gas seepages in China, and reviewing literature on organic and inorganic CO2 throughout the world. CO2 is of organic origin if CO2 content <15% and δ13CCO2 < −10‰; CO2 is of inorganic origin if δ13CCO2 > −8‰ or CO2 > 60%; and CO2 is of either organic or inorganic origin or a mixture of organic and inorganic origins if −10‰ < δ13CCO2 < −8‰. In China, the δ13CCO2 values of organic CO2 mainly range from −10 to −22‰. In tectonically stable oil- and gas-bearing basins where deep faults and magmatism are undeveloped, organic CO2 occurs almost exclusively. In tectonically active oil- and gas-bearing basins where active faults and magmatism are present or in areas of modern volcanic activity, the δ13CCO2 values of inorganic CO2 range from −2 to −8‰ and are distributed along faults. The inorganic CO2 with a δ13CCO2 value of 0 ±3‰ is of metamorphic origin, and that with δ13CCO2 values of −6 ±2‰ is mantle derived and of magmatic origin. In China, the δ13CCO2 values of inorganic CO2 gas pools range from −3.41 to −8.44‰ and the helium ratio (R:Ra) is >1, which indicates that these gases are typically derived from the mantle.