Characteristics of the physical parameters and the evolution law of anthracite around the coalification jump: A case of the Jincheng and Guxu mining area, China

To investigate the physical properties of anthracite reservoirs with different metamorphic grades (Ro,max), 38 anthracite samples with an Ro,max value between 2.5 and 4.2% from the Guxu and Jincheng mining areas in China were selected to conduct proximate analysis and determine the macerals, pore size distribution, adsorption capacity, in-situ gas content and permeability. The results showed that a coalification jump at Ro,max=3.7% existed in the anthracite stage and greatly influenced the evolution of physical properties. Before the coalification jump point, a series of micropores and transition pores was formed during metamorphism, which increased the specific surface area of the reservoir. This phenomenon further increased the adsorption capacity and gas content. The formation of methane during metamorphism also provided a material basis for the enrichment of methane. After the coalification jump point, a decreased specific surface area was observed due to compaction of micropores and transition pores, thereby reducing the adsorption capacity and gas content. The end of gas generation also caused the gas content to increase with increasing Ro,max. The burial depth (overlying geostress) and metamorphism degree were key factors in controlling the permeability. The change in the metamorphic degree controlled the formation of microfractures, and the burial depth controlled the degree of fracture closure. The physical properties of reservoirs changed notably before and after the coalification jump point of high-rank coals. Therefore, more attention should be paid to coalification in the process of reservoir evaluation and development of high-rank coal reservoirs