Microscopic pore characteristics of coal seam and the controlling effect of sedimentary environment on pore structure in No.8 coal seam of the Ordos Basin

To accurately evaluate the distribution and potential of deep coalbed methane resources, it is critical to identify the coal sedimentary and pore distribution characteristics of deep coal reservoirs. Therefore, based on the analysis of coal macerals, nuclear magnetic resonance porosity, and electron microscopy, the characteristics of coal macerals and pore distribution in the Yulin area were identified, as well as the relationship between the coal sedimentary and pore structure characteristics of No.8 coal was evaluated. The results indicated that: ① The organic macerals of No.8 coal seam in the study area were predominantly composed of vitrinite, indicating a predominance of woody plants during the early stages of coal formation. As the degree of water cover increases, it gradually transitions into a mixture of woody and herbaceous plants. ② During the Benxi period, No. 8 coal evolved from an early wetland forest swamp to a later open water swamp with stagnant swamp water, creating a stable environment conducive to organic matter enrichment and coal seam stability. ③ The proportion of macropores in No.8 coal seam was negatively correlated with the structure preservation index (TPI), while it was positively correlated with the gel index (GI). ④ Matrix vitrinite developed dense or sparse banded pores, mass vitrinite developed few pores, and the structure vitrinite developed original tissue pores, mostly filled with minerals, while the homogeneous vitrinite did not develop pores. In conclusion, pore distribution in Yulin's coal seams was controlled by sedimentation and macerals. The pore structure of coal seams in the open water swamp sedimentary presented three peaks, with micropores and macropores developing best, which can simultaneously enrich free gas and adsorbed gas. The pore structure of coal seams in the wetland forest swamp sedimentary presented two peaks, with micropores developing well and macropores developing poorly, mainly enriching adsorbed gas.