Heterogeneity of pore structure of late Paleozoic transitional facies coal-bearing shale in the Southern North China and its main controlling factors

Abstract

The pore structure of coal-bearing shale is the space for storage and enrichment of shale gas. As the heterogeneity in characteristics of the pore structures and the controlling factors of shale, we investigated it in the Southern North China. We examined Permian shale in Yuzhou and Hudi coalmines, using low-temperature nitrogen adsorption with the two-dimension non-local density functional theory (2D-NLDFT), field emission scanning electron microscopy combined with argon-ion milling, and high-pressure mercury injection. We found lithofacies have a certain controlling effect on the pore structure characteristics of micropores and mesopores in coal-bearing shale. The organic-lean silicon bearing clay-rich shale lithofacies, and the organic-lean clay/silicon bearing mixed shale lithofacies have similar pore size distribution (PSD) trends and larger pore volume (PV) and specific surface area (SSA) in Yuzhou and Hudi coal mines. The main controlling factors are organic matter maturity, clay mineral and quartz content. The nanopores development in shale of Hudi was promoted by the clay minerals, and organic matter maturity. The PV and SSA in the shale of Yuzhou are smaller than those of other mine wells in the Southern North China, and the analysis shows that quartz inhibited the development of mesopores in the Yuzhou coal mine. The inhibitory effect of quartz on pore development is due to the influence of diagenesis and provenance composition. Due to different stages of organic matter maturation, pore development in shale is positively correlated with organic matter maturity in the Early Permian, and relationship is negatively correlated in the Middle Permian in the Southern North China. The mesopores provide the main PV and SSA in the coal-bearing shale.