GEOCHEMISTRY AND ACCUMULATION OF CARBON-DIOXIDE GASES IN CHINA

We propose the following identification indicators for CO2 of differen t origins based on data obtained from analyzing the molecular composit ion of 1198 gas samples, the CO2 carbon isotope values of 390 samples, and the helium isotope composition of 256 samples collected from well s of 16 oil- and gas-bearing basins and gas seepages in China, and rev iewing literature on organic and inorganic CO2 throughout the world. C O2 is of organic origin if CO2 content <15% and delta(13)C(CO2) < -10 parts per thousand; CO2 is of inorganic origin if delta(13)C(CO2) > -8 parts per thousand or CO2 > 60%; and CO2 is of either organic or inor ganic origin or a mixture of organic and inorganic origins if -10 part s per thousand < delta(13)C(CO2) < -8 parts per thousand. In China, th e delta(13)C(CO2) values of organic CO2 mainly range from -10 to -22 p arts per thousand. In tectonically stable oil- and gas-bearing basins where deep faults and magmatism are undeveloped, organic CO2 occurs al most exclusively. In tectonically active oil- and gas-bearing basins w here active faults and magmatism are present or in areas of modern vol canic activity, the delta(13)CO(2) values of inorganic CO2 range from -2 to -8 parts per thousand and are distributed along faults. The inor ganic CO2 with a delta(13)C(CO2) value of 0 +/-3 parts per thousand is of metamorphic origin, and that with delta(13)C(CO2) values of -6 +/- 2 parts per thousand is mantle derived and of magmatic origin. In Chin a, the delta(13)C(CO2) values of inorganic CO2 gas pools range from -3 .41 to -8.44 parts per thousand and the helium ratio (R:Ra) is >1, whi ch indicates that these gases are typically derived from the mantle.