The characteristics of coal reservoir pores and coal facies in Liulin district, Hedong coal field of China

The composition, rank, coal facies, and coal pore structure characteristics of 15 coal samples were investigated systemically using methods of lithotype analysis, maceral analysis, proximate analysis, mercury porosimetry analysis, and nitrogen adsorption analysis. These samples were directly collected from the working faces of No.4, 8, 9 seams in Liulin district, eastern margin Ordos Basin, China. Four types of paleoenvironment including a wet forest swamp, an intergradation forest swamp, a drained forest swamp, and a fresh-water peat swamp were distinguished by lithotype and merceral analysis. An R-cluster analysis was performed to demonstrate the correlations between the coal facies and the pore structures. The results showed that coal rank is the primary factor affecting the development of micropores and transition pores, whereas coal facies control the development of seepage pores in similar coalification conditions, especially in a local district and in a seam. This work suggests that the vertical variations observed in coal properties (macerals, vitrinite reflectance, mineral matters, porosity, BET-specific area, pore size distributions and pores connectivity) were controlled by groundwater level fluctuations, indicated by variations in the ground water index (GWI), in the ancient mires due to different accommodation and peat accumulation rates. Comparison of the porosity characteristics of various coal reservoirs developed in various coal facies indicates that wet forest swamp facies is favorable to the formation of a coalbed methane reservoir because of reasonable pore structure and good connectivity. Intergradation forest swamp facies rank the second, and drained forest swamp facies are less advantageous.