Microscopic Pore Structures and Their Controlling Factors of the Lower Carboniferous Luzhai Shale in Guizhong Depression, China

Abstract

Microscopic pore structures are of great significance in evaluating shale gas reservoirs and understanding the occurrence state of shale gas. Integrating geochemical analysis, X-ray diffraction (XRD), low-temperature N2 adsorption (LTNA), mercury injection pressure (MIP), small-angle X-ray scattering (SAXS), and field emission scanning electron microscope (FE-SEM), this work systematically characterized microscopic pore structures and then discussed their controlling factors, for the Lower Carboniferous Luzhai (C1lz) shale in Guizhong Depression. The results show that pores in the Luzhai shale are generally shaped by ink bottles, parallel plates, and slits, which are characterized by a larger pore throat ratio, lower efficiency of mercury withdrawal, and poor connectivity. Micropores (<2 nm) and mesopores (2 nm-50 nm) provide the primary specific surface area (SSA) of shale reservoirs, dominantly controlled by the contents of TOC and clay minerals, whereas pore volume is mainly contributed from mesopores, which is positively correlated to quartz content. The fractal dimension (2.94) of macropores is higher than that (2.65) of mesopores, corresponding more complicated and heterogeneous pore structure. With increasing pore size, the correlation between TOC and fractal dimension of macropores weakens. The unconnected pores in mesoscopic range are more complex than connected pores, primarily resulting from morphology and deformation of clay minerals.