Review of natural origin, distribution, and long-term conservation of CO2 in sedimentary basins of China.

Sedimentary basins in China are globally special for retaining large volumes of carbon dioxide. To date, over 40 CO 2 gas fields (>60 vol% CO 2) and many more CO 2 -rich hydrocarbon fields (15–60 vol% CO 2) have been discovered across China. Valuable knowledge of CO 2 migration, accumulation, and long-term sequestration processes in sedimentary basins can be acquired by analysing various CO 2 fields in China. This paper reviews three key aspects of CO 2 reservoirs in Chinese sedimentary basins: (1) CO 2 origin, (2) factors controlling the distribution of CO 2 gas, and (3) the long-term effect of CO 2 emplacement on reservoir rocks. Both the eastern and western parts of China have several giant sedimentary basins. CO 2 -rich gases are predominantly discovered in eastern China among rift basins, with sandstone, carbonate, and volcanic rocks being the principal reservoirs. Analyses of carbon and helium isotopes suggest that the high abundance of CO 2 in eastern China is primarily of mantle origin, with subordinate crustal CO 2. Igneous intrusives and faults control the migration and accumulation of CO 2 -rich gases within the basins. The offshore Yinggehai Basin, however, is an exception in eastern China, where the majority of CO 2 appears to be of crustal origin, resulting from thermolysis of carbonate minerals. For this type of CO 2 , mud diapirs in the deep basin, which serve as upward passages for geothermal fluids, control the distribution of CO 2 -rich gases. In comparison, sedimentary basins in western China generally contain minor amounts of CO 2 (< 5 vol%) in the produced gases. The low-concentration CO 2 is mostly of crustal origin, from the decomposition of organic matter and carbonate minerals or, in a few places, from the thermochemical sulphate reduction process. Pore fluids in CO 2 reservoirs of China are characterised by high salinity, owing to elevated concentrations of Na+ and HCO- 3 induced by CO 2 emplacement. Pervasive dissolutions of feldspars are widely observed in sandstone and volcanic reservoirs in China, and dawsonite is the most common authigenic mineral. Dawsonite growth did not cause significant variations in the reservoir porosity in most cases. Mantle-derived CO 2 has been suggested to promote kerogen maturation and oil migration in sedimentary basins, thereby helping to form some oilfields in China. [ABSTRACT FROM AUTHOR]