The effect of sedimentary redbeds on coalbed methane occurrence in the Xutuan and Zhaoji Coal Mines, Huaibei Coalfield, China

Abstract

The lithologies and permeabilities of caprocks have significant effects on coalbed methane (CBM) preservation. To study the controlling effects of redbeds deposits on CBM occurrence in the Xutuan and Zhaoji Coal Mines of the Huaibei Coalfield in China, the variations in mineralogical composition, microstructure, pore structure, permeability and diffusion among the redbeds caprock samples, Neogene clay rock and ordinary coal measure strata (sandstone, mudstone and siltstone) have been investigated. The results indicate that redbeds are porous rocks with weaker sealing capabilities than coal strata caprocks, allowing CBM to escape from the coal seam. The formation of the lithological properties of the redbeds is closely related to their mineralogical composition and diagenesis. Redbeds are mainly composed of illite (57–64%), calcite (25–33%), quartz (6–12%), muscovite (3–8%) and pyrite (<1%). The high content of fragile minerals and the later formation age of this rock lead to unapparent compaction and cementation, resulting in a loose structure, low degrees of consolidation and pore structure development. Mercury intrusion experiments show that the porosity of redbeds is 3.8–5.2 times higher than that of sandstone and 8–13 times higher than those of mudstone and siltstone; the pore size distribution of this rock is primarily centralized in transition pores (57.82%–74.72%) and mesopores (9.08%–22.51%), which favors gas diffusion. The permeability and diffusion coefficient tests demonstrate that the sealing capability of redbeds is much weaker than that of coal strata caprocks. Although the origins of the CBM gas pool in the Huaibei Coalfield remain controversial, the findings of this study along with the mining practices indicate that the deposition of thick redbeds can be treated as indicative of low gas contents in the coal mines, which has a significant meaning for the engineering of coal mine gas control.