Pore Structure Characteristics of Coal and Their Geological Controlling Factors in Eastern Yunnan and Western Guizhou, China

Coalbed is the carrier for coalbed methane (CBM) enrichment and migration. The pore structure characteristics of coal and their main geological controlling factors are critical to the exploration and development of CBM. In this paper, 20 coal samples were collected from eastern Yunnan and western Guizhou, China. Based on vitrinite reflectance, proximate analysis, maceral analysis, and low-temperature N-2 adsorption/desorption (LT-N(2)GA) experiments, the hysteresis coefficient of low-temperature N-2 desorption was proposed, the types of pore structure were identified, and the effects of coal facies and rank on the pore structure were revealed. The results show that the R-o,R-max values of the 20 coal samples are between 0.74 and 3.38%, which belong to medium- and high-rank coal. In the coal macerals, the vitrinite is mainly collodetrinite. The inertinite is dominated by semifusinite, and some coal samples contain exinite. The coal samples investigated can be divided into two types. Type A samples mainly contain open pores, while type B samples are rich in bottle-shaped pores. Compared with type A coal samples, type B samples have the characteristics of smaller total pore volume (TPV), smaller average pore diameter (APD), larger specific surface area (SSA), and larger hysteresis coefficient. The coal samples are located in three regions of different coal facies, including low-level swamp (reed) facies, wetland herbaceous swamp facies, and wet forest swamp facies. The tissue preservation index (TPI) values of most coal samples are less than unity, which indicates that herbaceous plants have absolute dominance in the coal-forming plants in eastern Yunnan and western Guizhou. The maximum vitrinite reflectance (R-o,R-max), gelification index (GI), TPI, vitrinite content (V), inertinite content (I), Barrett-Joyner-Halenda pore volume (V-BJH), Brunauer-Emmett-Teller SSA (S-BET), and low-temperature N-2 desorption total hysteresis coefficient (H-t) were clustered using the R-type cluster analysis method. It is found that TPI is the main controlling factor of the pore structure of type A coal samples, while the pore structure of type B coal samples are jointly controlled by TPI and coal rank. Type B coal samples are mainly located in Zhuzang and Laochang high-rank coal research areas, while the distribution of type A coal samples is mainly in other medium-high-rank coal research areas. These results will contribute to the exploration and development of CBM and also guide the study of pore structures of other unconventional gas reservoirs.