Characteristics of the Nanoscale Pore Structure in Northwestern Hunan Shale Gas Reservoirs Using Field Emission Scanning Electron Microscopy, High-Pressure Mercury Intrusion, and Gas Adsorption

Nanostructure morphology and pore size distributions (PSDs) of 10 samples from the Lower Cambrian Niutitang Formation in northwestern Hunan were investigated using field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion (HPMI), low-pressure nitrogen gas adsorption (LP-N2GA), and carbon dioxide gas adsorption (LP-CO2GA). In combination with the geochemical parameters and mineral composition, the factors influencing the nanoscale pore structure were analyzed. The results indicate that the pores in the shale reservoirs are generally nanoscale and can be classified into four types: organic pores, intraparticle pores, interparticle pores, and microfractures, of which the most common are organic nanopores and interparticle pores between clay particles. The nanoscale pores primarily consist of slit-shaped pores with parallel plates and ink-bottle-type pores. The combination of the HPMI, LP-N2GA, and LP-CO2GA curves enabled the creation of the PSD for micro-, meso-, and macroporosities. The PSDs are either bi- or multimodal and include not only predominant mesopores (2–50 nm) but also a certain amount of micropores (50 nm). Micro- and mesopores with a diameter less than 50 nm amount to most of the pore volume, whereas those with a diameter less than 5 nm amount to most of the specific surface area. The total organic carbon (TOC) and clay minerals are the primary factors affecting the nanoscale pore (diameter < 1 μm, especially micro- and mesopores) structure characteristics, whereas micropores are predominantly controlled by the content of the TOC, and meso–macropores are primarily determined by the content of clay minerals, in particular the illite content.