Multiscale characterization of pore structure and connectivity of Wufeng-Longmaxi shale in Sichuan Basin, China

By combining the micro-CT, SEM, FIB-SEM and FIB-HIM, a multiscale characterization method was explored to study the pores in Wufeng-Longmaxi shale, Sichuan Basin, China. This method extends the previous test scale so that the sub-10 nm pores can be visualized in both 2D and 3D. In order to consider the homogeneity or heterogeneity and verify the representativeness of samples, REV and REA (representative elementary volume and area) was determined. Pores in shale were classified as organic pore, inorganic pore and micro-fracture based on SEM, and their surface porosity and pore size distribution were calculated. By 3D reconstruction of FIB-SEM and FIB-HIM, the pore networks were visualized down to sub-10 nm, and the connect patterns of pores through throats were carefully investigated. Results show that, REV and REA are different at different scale. The cube and square growing pattern, grain size, feature distribution and imaging accuracy can affect REV and REA in different degree. SEM quantification results indicate that, the surface porosities of OM particles (ϕS OM) of two 1st member of Longmaxi samples are larger than those of Wufeng samples, indicating the organic pores of 1st member of Longmaxi samples are more developed than Wufeng samples. The size of inorganic pores of Wufeng samples are larger than those of Longmaxi samples. The permeability shows a positive correlation with surface density of micro-fractures. The 3D pore network analysis of FIB-SEM further shows the interconnectivity of 1st member of Longmaxi samples are better than Wufeng samples. The interconnected pores are mainly between 10 and 30 nm, and the throats are concentrated between 0 and 30 nm for both 1st member of Longmaxi samples and Wufeng samples. This is further refined by FIB-HIM, in which, the interconnected pores and throats are concentrated below 10 nm. FIB-HIM also provides clear 3D images of how organic pores can be connected by throats, which is a great progress in shale pore characterization. •A novel multiscale method was employed to extend the pore imaging scale down to sub-10 nm both in 2D and 3D.•REV/REA was carefully determined under the imaging scale of micro-CT and SEM.•Pores and micro-fractures were quantitatively identified.•The network and connectivity of pores were visualized and quantified by the correlative FIB-SEM and FIB-HIM.