The origin and formation of CO2 gas pools in the coal seam of the Yaojie coalfield in China

High concentrations of CO2 have been observed in the No. 2 coal seam in the Yaojie coalfield. A number of instantaneous CO2 outbursts occurred during coal mining. The CO2 concentration range in the eastern coalfield is 18.79 to 96.6%. The δ13CCO2 values are mainly in the range of − 5.0‰ to + 1.0‰ (PDB), suggesting an inorganic origin of CO2. The 3He/4He ratios are (0.6–25.9) × 10− 8, and the R/Ra is 0.0042–0.185, which is a characteristic of crust-derived CO2 gas. The dynamic-thermal metamorphism of the F19 ductile-brittle shear zone is believed to result in the release of CO2 from basement marble formations, which show an inorganic source of CO2 in the Yaojie coalfield. The regional geological evolution and multi-periodical F19 fault movement control the formation, migration, and accumulation of the CO2, in addition to the development of CO2 gas pools in the Yaojie coalfield. The F19 fault played multiple roles in the generation, transport, and sequestration of gas during the CO2 formation. The displacement of CH4, carbonate generation, and pore structure transformation of coal due to a series of physical and chemical effects occurred after CO2 flux into the coal seams.

Research Highlights
► The δ13CCO2 values and 3He/4He ratios of the NO. 2 coal seam gas indicating that the helium is crust derived. Dynamic-thermal metamorphism of the F19 ductile-brittle shear zone made the CO2 release from basement carbonate regions, which is the inorganic source of CO2 in the Yaojie coalfield.
► The regional geological evolution and multi-periodical F19 fault movement controls the CO2 formation, migration, accumulation and forming of CO2 gas pools in the Yaojie coalfield.
► A series of physical and chemical effects took place after CO2 flew into the coal seams between CO2, water and coal seam, such as displacement of CH4, carbonation generation which affects CO2 migration and accumulation correspondingly.