Geochemical characteristics and geological significance of Carboniferous Benxi coal rock gas in central and eastern Ordos Basin

The coal rock gas resources in the 8# coal seam of the Carboniferous Benxi Formation in the central and eastern Ordos Basin have great potential. By analyzing the gas components and carbon and hydrogen isotope compositions of the desorbed gas from 33 coal rock samples of the 8# coal seam in 3 coal rock gas wells of this area, and combining these results with the geological characteristics and gas contents of the coal rocks, the genesis of coal rock gas and the salinity characteristics of the sedimentary water bodies were revealed, and the causes of the abnormal methane carbon isotope in coal rock gas were discussed. The results showed that in the Benxi 8# coal rock gas, the CH₄ content ranged from 88.50% to 97.12%, and the C₂H₆ content ranged from 0.23% to 1.92%. Among the non-hydrocarbon gases, the CO₂ content ranged from 2.51% to 7.18%, and the N₂ content was less than 4.50%. The δ¹³C₁, δ¹³C₂, δ¹³C_CO2, δ²H_C1, and δ²H_C2 values of the Benxi 8# coal rock gas ranged from -37.6‰ to -25.8‰, -19.4‰ to -16.0‰, -12.6‰ to -6.0‰, -185.0‰ to -158.0‰, and -131.0‰ to -103.0‰, respectively. Considering the gas components, carbon isotope compositions, the high level of thermal evolution, the strong gas-generating potential of the coal rocks, and the tectonic stability of the coal rock distribution area, it was concluded that the Benxi 8# coal rock gas in the central and eastern Ordos Basin was high to over mature thermogenic gas, derived from the thermal transformation of humic organic matter within coal seams, representing self-generated and self-stored coal-derived gas. The CO₂ in the coal rock gas was of mixed origin, generated from both the thermal cracking of organic matter and the thermal decomposition of carbonate minerals. The hydrogen isotope characteristics of the coal rock gas revealed that the sedimentary water bodies of the Benxi 8# coal rock in the central and eastern Ordos Basin were brackish to saline, representing a transitional marine-continental sedimentary environment. A good negative correlation existed between δ¹³C₁ of the coal rock gas and the gas content, indicating that better storage and preservation conditions resulted in a higher degree of gas "accumulation". In high to over mature coal rocks, the δ¹³C₁ value of "accumulated" gas was significantly lighter, and the gas-bearing capacity of the coal rocks was better.