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

Abstract

Sedimentary basins in China are globally special for retaining large volumes of carbon dioxide. To date, over 40 CO2 gas fields (>60 vol% CO2) and many more CO2-rich hydrocarbon fields (15–60 vol% CO2) have been discovered across China. Valuable knowledge of CO2 migration, accumulation, and long-term sequestration processes in sedimentary basins can be acquired by analysing various CO2 fields in China. This paper reviews three key aspects of CO2 reservoirs in Chinese sedimentary basins: (1) CO2 origin, (2) factors controlling the distribution of CO2 gas, and (3) the long-term effect of CO2 emplacement on reservoir rocks.Both the eastern and western parts of China have several giant sedimentary basins. CO2-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 CO2 in eastern China is primarily of mantle origin, with subordinate crustal CO2. Igneous intrusives and faults control the migration and accumulation of CO2-rich gases within the basins. The offshore Yinggehai Basin, however, is an exception in eastern China, where the majority of CO2 appears to be of crustal origin, resulting from thermolysis of carbonate minerals. For this type of CO2, mud diapirs in the deep basin, which serve as upward passages for geothermal fluids, control the distribution of CO2-rich gases. In comparison, sedimentary basins in western China generally contain minor amounts of CO2 (< 5 vol%) in the produced gases. The low-concentration CO2 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 CO2 reservoirs of China are characterised by high salinity, owing to elevated concentrations of Na+ and HCO- 3 induced by CO2 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 CO2 has been suggested to promote kerogen maturation and oil migration in sedimentary basins, thereby helping to form some oilfields in China.