Micro-pore characteristics of shale from Wufeng-Longmaxi formations in Pingqiao area, Sichuan Basin

The microscopic pore characteristics of shale from the lower Wufeng-Longmaxi formations in the Pingqiao area of the Sichuan Basin were studied using argon ion polished sample scanning electron microscopy and nitrogen adsorption-desorption experiments. Organic-rich siliceous shale and carbonaceous shale micro-pores are dominated by organic pores and micro-cracks. The pore structure is relatively complex, with rich pore morphology and a wide range of pore sizes. Argon ion polishing scanning electron microscopy showed that the organic pores are mainly thin and partly belong to micro-pores. Nitrogen adsorption-desorption experiments showed that the shale has narrow parallel plate-like holes, a small number of conical plate holes and wedge-shaped holes with a poor connectivity, and regular open round holes and ink bottle-shaped holes with open ends. The specific surface area measured by nitrogen adsorption-desorption experiments was 9-32.6 m²/g, with an average of 18.0 m²/g, which was low. The pore diameters measured by the BET method were 3.23 to 4.35 nm, and the average pore volume by the BJH method was 0.016 5 cm³/g. The fine meso-pores were dominant, and they were biased toward the micro-pore boundary. According to the comprehensive analysis, the microscopic pores of shale reservoirs in the study area are mainly mesoporous, and meso-pores and micro-pores contribute most of the specific surface area.