Comparison of the pore structures of Lower Silurian Longmaxi Formation shales with different lithofacies in the southern Sichuan Basin, China

The growing research on the features of shale pore structures has deepened the understanding of shale oil and gas storage capacity. Nevertheless, few studies have focused on the differences of the specific parameters that are characteristic of the nanopore structure in shales with different lithofacies. Such parameters include the full-scale ranges of the pore specific surface area and pore volume distribution, porosity and fractal dimension. In this paper, a dozen Longmaxi Formation shale samples from the southern Sichuan Basin were studied to determine the differences in these parameters. A set of laboratory experiments were performed, such as geochemical experiments, X-ray diffraction, N-2 and CO2 isothermal adsorption, mercury injection capillary pressure tests and scanning electron microscopy experiments. The results of this study indicate that shale samples can be classified into 5 types of lithofacies based on their mineral composition. The surface porosity of organic matter particles is 8-16 times greater than that of mineral particles. The major contributors to the pore volume and surface area of shale samples are mesopores and micropores, respectively. The full-scale pore size distribution is mainly from 0.4 to 0.6 nm, 2-7 nm, and 10-20 nm for the pore volume and from 0.3 to 1.0 nm and 1.5-7 nm for the pore surface area. The average values of the porosity, fractal dimension, pore volume and pore surface area of the M-2 shales are almost equal to those of the S group shales. This result indicates that the M-2 shale has a large gas storage capacity similar to that of shales with siliceous lithofacies. The development of organic matter pores that contribute to more than 50% of the porosity leads to the high porosity and fractal dimension of the M-2 shales and S group shales. In addition to organic matter, illite minerals also play an important role in the constitution of the majority of the storage space in the CM-1 shale.