Pore characteristics of organic-rich shale in the Carboniferous-Permian coal-bearing strata in Qinshui Basin

Abstract

The study of shale pore has always been an important aspect of nonconventional shale reservoir analysis. However, there are relatively few studies on shale that is deposited in transitional environment in the Qinshui Basin. X-ray diffraction, field-emission scanning electron microscopy, N₂, and CO₂ adsorption experiments were used for shale samples from the Taiyuan and Shanxi Formations. Sample pore characteristics based on field-emission scanning electron microscopic photographs were quantitatively characterized by pore fracture characteristics analysis software and fractal characteristics were discussed based on the nitrogen adsorption curve. Results show that sample pores were primarily composed of inter-granular and intra-granular pores. The proportion of micropores (<2 nm) is 25.84–91.65% of the total pore space, with an average of 58.65%. Micropores show three peaks, which occur near 0.38 nm, 0.5 nm, and 0.85 nm. Mesopores (2–50 nm) account for 6.74–56.11% of the total pore space, with an average of 31.20%. The ratio of macropores (>50 nm) less than 100 nm is 1.61–20.64% with an average of 9.17%. The volume of micropores increases with increase in the total organic carbon and clay mineral contents, decreases with the increase of carbonate and quartz mineral contents, and has weak correlation with other factors. The micropore volume per unit organic matter decreases with increase in maximum vitrinite reflectance (R_o,max) in a binomial relationship. The correlations between meso-macropore volume and each factor are rather weak. Form factor, fractal dimension, and probability entropy all correlate well with the depth. Pore structure complexity gradually increases with the increase in the burial depth. The deeper the rock layer increases with the pore size, the more slowly the pore complexity increases. The larger pore diameter, the more smooth the pore wall surface, and the rougher surface is more favorable for gas adsorption. The directionality of pore development increases first and then decreases with increase in the depth.