Impacts of coal facies on vertical heterogeneity of deep coalbed methane reservoir in the Yulin area of Ordos Basin, North China

Abstract The vertical heterogeneity of coal is a crucial factor influencing the development of coalbed methane (CBM) and it is significantly influenced by coal facies. Utilizing the theories of coal facies and variation coefficient to clearly classify and quantitatively characterize the vertical coal facies types and heterogeneity of deep 8# coal in the Yulin area of the Ordos Basin based on the submaceral content, coal quality, and physical property tests. Through correlation analysis, the influence and control mechanism of coal facies on the vertical heterogeneity of coal reservoir characteristics are analyzed. The results indicate that deep 8# coal reservoir exhibits strong intra-layer heterogeneity, and the middle upper parts of the coal seam are locally favorable intervals. The coal-forming swamp of the deep 8# coal seam was moist forest swamp initially and transitioned into low swamp, undergoing water transgression-regression-transgression. Early coal-forming vegetation was woody plants and gradually replaced by herb/herb and woody plant dominance. The A ad is significantly affected by hydrodynamics, increasing with stronger hydrodynamics (higher GWI and lower F/M) and the Vitrinite has a strong positive correlation with GI but is negatively influenced by F/M. Influenced by coal pore development, microfracture formation, and mineral filling, the porosity and permeability of coal are positively correlated with VI, TPI, GI, and F/M but negatively correlated with GWI. Higher TPI correspond to more mesopores and macropores, with a tri-modal pattern pore distribution characteristic and higher GI indicate more developed ultra-micropores (gas pores), typically resulting in a bi-modal pore size distribution. GWI provides a better indication of gas content, and the vertical gas content of the coal seam deviates from its relationship with coal facies indices due to vertical gas migration.