Geological Control Mechanism of Coalbed Methane Gas Component Evolution Characteristics in the Daning-Jixian Area, Ordos Basin, China

It is important to study the control mechanism of geological factors (coal bed structure and hydrodynamics) on the evolution of coalbed methane (CBM) gas components. In this paper, the tectonic, hydrochemical, and hydrodynamic field distributions in the Daning-Jixian block, the eastern edge of the Ordos Basin, were obtained by using well logging, seismic, and water sample testing data. The CBM origin was obtained by analyzing methane carbon isotope abundance (delta C-13(1)) in gas sample tests. The results show that (1) the 5# coal seam in the study area is a monoclinic structure with deeper burial in the northwest and shallower burial in the southeast, and the direction of groundwater transportation is consistent with the slope direction of the terrain; (2) CBM in the study area is thermogenic gas and undergoes diffusion, dissolution, and transportation processes. As the hydrodynamics changed from high to low, the percentage of CH4 concentration increased, the percentage of CO2 and N-2 composition decreased, pH became smaller, total dissolved solid increased, and delta C-13(1) became heavier; (3) at the early stage of drainage mining, delta C-13(1) increased significantly, the CH4 component showed a decreasing trend, and nonhydrocarbon gas components increased. As the discharge mining proceeds, the CH4 component tends to increase and the nonhydrocarbon gas components decreases.