Pore structure response of sedimentary cycle in coal-bearing strata and implications for independent superposed coalbed methane systems

Coalbed methane (CBM)-bearing systems are often superposed in coal-bearing strata with multiple coal seams. The formation and development of independent superposed CBM-bearing systems is based on the vertical variation of pore structure in coal-bearing strata. This study examines the potential of the pore structure cycle of the intervening rock formation in coal-bearing strata that compartmentalize the CBM system, which, in turn, assist in the production of the coal gas. The pore structure is characterized using lithology, sequence stratigraphy, and mercury injection porosimetry data generated from 25 samples of the intervening formation. The samples originate from a representative drilling operation located in the Bide-Santang basin, western Guizhou, China. We divided the pore structures into 4 types (I, II, III and IV): type I, well sorted and fine slanting degrees; type II, well sorted and coarse slanting degrees with extremely low mercury saturation; type III, poorly sorted and uniform distribution and type IV, poorly sorted and polarization of micropores and macropores. Pore structure changes cyclically in accordance with the sedimentary cycle: pores developed poorly within rocks that formed in deep water bodies with low energy and reducing sedimentary environments, such as lagoons and limited subtidal and mire facies; pores are better developed within rocks formed in shallow water bodies with high energy and oxidizing environments, such as supratidal flats, intertidal zones, and distributary channel facies. Third-order sequence boundaries (SB) favor the development of secondary porosity, whereas the maximum flooding surface (MFS) possesses a superior condition for forming regional impermeable barriers between different CBM-bearing systems. The sequence stratigraphic framework limits the variation trend of the pore structure and therefore limits the vertical distribution of independent superposed CBM-bearing systems. The findings are of great significance for enriching the geological theory of multi-seam CBM accumulation, optimizing the co-mining methods and improving the co-mining efficiency of superposed CBM systems.