Matrix permeability anisotropy of organic-rich marine shales and its geological implications: Experimental measurements and microscopic analyses

Due to the presence of natural and/or preparation-related fractures, the anisotropy of the matrix permeability of shales is usually difficult to characterize. In this study, we used X-ray micro-computed tomography (XRμCT) imaging to select samples from the Lower Cambrian and Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation of the Upper Yangtze Platform, China, showing no natural or artificial fractures. From these samples, cubic specimens were prepared with a wire saw and then re-inspected for fractures. Using a specially developed sample holder, the matrix permeability of these specimens were measured in the three principal directions using the pulse decay method with N2 as the flowing fluid. To analyze the microscopic lithological and structural controls on shale matrix permeability anisotropy, we employed a combined methodology using thin sections, focused ion beam-scanning electron microscopy (FIB-SEM), and permeability simulation.