The composition, pore structure characterization and deformation mechanism of coal-bearing shales from tectonically altered coalfields in eastern China

Organic rich shales in coal-bearing strata deposited from marine to lacustrine environments are well developed in China. The Paleozoic coal-bearing shales have been significantly altered by a series of tectonic movements. Based on XRD, SEM, MICP, and a nitrogen adsorption experiment and in combination with other parameters in this paper, the mineral composition and pore structure characterization and deformation mechanism of coal-bearing shales were surveyed. The coal-bearing shales in eastern China undergo various types of deformation, including brittle, ductile, and brittle-ductile deformation. In eastern China, the macro pore size of shales grew with increasing quartz content under different types of structural deformation, while the specific surface area decreases as the quartz content increases in different types of structural deformation; With the increasing of clay mineral content, the average pore size and the specific surface area of BET became larger in the various types deformation shale, while the pore volume decreased in the brittle and brittle -ductile deformation shale and increased in the ductile deformation shale. The ductile and brittle-ductile deformation increase the specific surface area, the total pore volume of nano-pores, and the adsorption capacity of liquid nitrogen, and decrease the nano pore diameter. The micropores in the brittle-ductile and ductile shearing of clay minerals may the main factors affecting pore volume and total specific surface area. And it is the mesoporous structure that undergoes evolution in brittle-ductile-deformed shales, leading them to have the maximum pore volume and pore-specific surface area for pore-fracture systems. Brittle shear results in micro-fractures or large pores and thus has an impact on the desorption and percolation capability of shale gas, Ductile deformation increases the specific surface area of shales and enhances their shale adsorption capacity.