Fractal Characteristics of Nanopores and Their Controlling Factors of the Lower Cambrian Shale in the Western Hubei Area, South China

The Lower Cambrian shale is one of the most important marine shale reservoirs in South China, which has huge potential for shale gas resources. However, the Lower Cambrian shale displays strong pore heterogeneity, influencing shale gas exploration. Based on the low-pressure N2 adsorption (LPNA) of the Lower Cambrian shale samples and their isolated kerogens in the western Hubei area, South China, fractal dimensions and their controlling factors of shale nanopores have been investigated. In this study, the pore specific surface area (SSA) and pore volume (PV) of isolated kerogen samples are mainly contributed from micropores and non-micropores, respectively, which are consistent with those of whole-rock samples. The average fractal dimensions of whole-rock samples are higher than those of isolated kerogen samples, suggesting that organic matter (OM) controls the shale pore structure and exerts major influences on fractal characteristics of nanopores and inorganic mineral (IM) enhances the complexity of shale nanopores. The correlations between fractal dimensions and total organic carbon (TOC) and IM contents of whole-rock samples further indicate that the OM simultaneously affects the heterogeneities of the pore surface and structure, while the IM mainly affects the heterogeneity of the pore structure. They both control the fractal characteristics of nanopores in the Lower Cambrian shale. Compared with the Upper Carboniferous-Lower Permian marine-continental transitional (MCT) shale and the Lower Silurian marine shale, the D 1 and D 2 of the studied marine shale display relatively lower and higher values, respectively, revealing that the Lower Cambrian shale has relatively weak adsorption capacity, as well as poor permeability and connectivity.