Effect of lithofacies on pore structure of the Cambrian organic-rich shale in northern Guizhou, China

Taking the Cambrian organic-rich shale in northern Guizhou for research object, total organic carbon (TOC), and X-ray diffraction (XRD) were used to study shale lithofacies, and scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen adsorption analyses were combined to reveal pore structure, in order to document the effect of shale lithofacies on pore structure. The results show that the Niutitang shale can be divided into six shale lithofacies, and three (siliceous shale lithofacies, carbonate-rich siliceous shale lithofacies, and carbonaceous/argillaceous containing siliceous shale lithofacies) of them can be identified. Four types of pores (organic matter, interparticle, intraparticle, and dissolved pores) are present in the Niutitang shale. Organic matter and interparticle pores mainly occur in siliceous shales, and intraparticle and dissolved pores typically develop in carbonate-rich siliceous shales and carbonaceous/argillaceous containing siliceous shales. Organic matter and brittle minerals related pores are two significant components of the pore volume in the Niutitang shales. The micropores and mesopores volumes promptly increase with the increase in organic matter content, indicating that organic matter is the primary contributor to micropores and mesopores. Clay content shows weakly negative relations with both the micropore and mesopore volumes. The development of macropores is associated with inorganic minerals (i.e., clay and carbonate). Organic matter and clay contribute the most to specific surface area, while the contribution of brittle minerals is negligible. Siliceous shales are the most favourable reservoir for shale gas due to their tremendous specific surface area and pore volume. In contrast, carbonate-rich siliceous shales have the least storage capacity of shale gas because of their relatively smaller specific surface area and pore volume.