Anisotropic Characterization of Mechanical Behavior and Fracture Morphology in Marine and Continental Shales under Uniaxial Compression: A Comparative Analysis

Anisotropy is a fundamental structural characteristic of shale, significantly influencing the exploration and exploitation of shale oil and gas resources. Continental shales (CSs), distinguished by their significant development potential, exhibit compositional and fabric distinctions from marine shales (MSs), resulting in diverse mechanical properties and production efficiencies. However, research comparing the mechanical anisotropy between MS and CSs has been limited. This study delves into the differences in mechanical anisotropic behaviors during the progressive failure process of these two types. Samples from China's Longmaxi Formation (MS) and Yanchang Formation (CS) with varied bedding plane angles underwent uniaxial compression tests, acoustic wave analysis, and Computed Tomography (CT) scanning. The research primarily focused on the anisotropic variances in characteristic stresses (crack initiation stress, crack damage stress, and peak stress) throughout the progressive failure process of MS and CS alongside the anisotropic differences in the final fracture morphology. The findings revealed that CS exhibits significantly greater initial wave velocity anisotropy compared with MS. Both shale types display consistent patterns in crack initiation stress, damage stress, and peak stress, which initially decrease and then increase as the bedding angle expands. The anisotropy in CSs is more marked compared with MSs. A comprehensive analysis was undertaken to understand the physical mechanisms behind these phenomena. Furthermore, as the bedding angle nears 45 degrees + phi/2 (phi representing the internal friction angle), sliding along the structural planes intensifies, a phenomenon more evident in CSs. This research advances the understanding of anisotropic deformation and failure mechanisms in MS and CSs, offering critical insights into the development of these two types of shale oil and gas reservoirs.