Dual Control of Macrolithotype and Coal Structure on the Pore Parameters of Middle Jurassic Coals in the Southern Junggar Basin, NW China

Abstract

AbstractCoal pore parameters are closely related to macrolithotypes and coal structures, having a large influence over the gas potential and productivity of coalbed methane (CBM). The Middle Jurassic Xishanyao Formation, located in the southern Junggar Basin of northwestern China, has geological conditions with rich CBM resources. The 46 Xishanyao coal samples gathered from the drilling cores and coal mines cover 4 types of macrolithotypes (bright coal 1, semi‐bright coal 2, semi‐dull coal 3, and dull coal 4) and 2 types of coal structures (primary coal I and cataclastic coal II). Based on a range of pore testing experiments and analytical methods, the dual effects of different macrolithotypes and coal structures on pore structures were intensely studied. The results showed that the specific surface area (SSA) and total pore volume (TPV) of coal samples increased gradually from bright to dull coals. For the same macrolithotypes, the SSA and TPV of the primary coals were lower than those of the cataclastic coals. Generally, the pore structures of bright and semi‐bright coals are simpler when compared to semi‐dull and dull coals with the same coal structure, whereas cataclastic coals have more complicated pore structure systems than primary coals with the same macrolithotypes. The bright and semi‐bright coals have higher vitrinite contents and more endogenous fractures, whereas well‐developed structural fractures were identified in cataclastic coals. Therefore, bright and semi‐bright coals have better pore connectivity than semi‐dull and dull coals with the same coal structure, the pore connectivity of cataclastic coals being slightly better than that of primary coals under the same macrolithotypes. In terms of the CBM adsorption conditions, the eight type samples formed a descending order: II‐4 > I‐4 > II‐3 > I‐3 > II‐2 > I‐2 > II‐1 > I‐1, while they ranked as follows when consideration was given to the CBM seepage capacities: II‐2 > II‐1 > I‐2 > I‐1 > I‐3 > I‐4 > II‐3 > II‐4. As a result, it could be determined that the bright and semi‐bright coals had stronger adsorption capacities, whereas the cataclastic coals had better pore connectivity and seepage capacities. Pore structure characteristics should be analysed under the dual control of different macrolithotypes and coal structures, so that they can provide greater value for guiding CBM exploration and exploitation, as along for preventing underground gas accidents.