Abstract

We propose the following identification indicators for CO 2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples, the CO 2 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 CO 2 throughout the world. CO 2 is of organic origin if CO 2 content <15% and delta 13 CCO 2 < -10 per mil; CO 2 is of inorganic origin if delta 13 CCO 2 > -8 per mil or CO 2 > 60%; and CO 2 is of either organic or inorganic origin or a mixture of organic and inorganic origins if -10 per mil < delta 13 CCO 2 < -8 per mil. In China, the delta 13 CCO 2 values of organic CO 2 mainly range from -10 to -22 per mil. In tectonically stable oil- and gas-bearing basins where deep faults and magmatism are undeveloped, organic CO 2 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 delta 13 CCO 2 values of inorganic CO 2 range from -2 to -8 per mil and are distributed along faults. The inorganic CO 2 with a delta 13 CCO 2 value of 0 + or - 3 per mil is of metamorphic origin, and that with delta 13 CCO 2 values of -6 + or - 2 per mil is mantle derived and of magmatic origin. In China, the delta 13 CCO 2 values of inorganic CO 2 gas pools range from -3.41 to -8.44 per mil and the helium ratio (R:Ra) is >1, which indicates that these gases are typically derived from the mantle.

You do not currently have access to this article.