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

Abstract

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.

The matrix permeability of the 15 shale samples ranged from 19.6 to 189.4 nD (19.6–189.4 × 10⁻²¹ m²) parallel to bedding and from 2.4 to 24.7 nD (2.4–24.7 × 10⁻²¹ m²) perpendicular to bedding, at a constant confining pressure of 1500 psi (10.3 MPa). Tectonic stress resulting from structural deformation leads to the deformation of OM pores within solid bitumen, which is the main factor controlling the matrix permeability. The ratio of the matrix permeability coefficients parallel and perpendicular to bedding (k_x/k_z) varied from 3.4 to 29.6, primarily due to cleavage zones along the bedding-parallel platy mica and clay altered from mica. The matrix permeability ratio in the two orthogonal horizontal directions (k_x/k_y) typically did not exceed a value of 2, but for samples with oriented graptolites and siltstone layers ratios as high as 2.9 and 2.3, respectively, were observed. The microfractures between graptolite and minerals, together with the elongated pores within the graptolites, enhance the permeability and facilitate gas flow along the bedding planes. The higher proportion of brittle minerals in siltstone renders it more prone to cracking compared to shale, which causes the anisotropy in the parallel-to-bedding directions.

For three shales from the strongly deformed northeast Chongqing with high-angle fractures parallel to the lamination, permeability parallel to bedding varied between 627.2 and 6820.3 nD, while permeability perpendicular to bedding ranged between 11.3 and 27.8 nD. The presence of microfracture parallel to the lamination in three shale samples resulted in a permeability higher by factors of 34 to 348 than the matrix permeability, leading to a higher anisotropy ratio between both horizontal and vertical directions.