Microstructural effects on the unconfined mechanical behavior of a tectonically deformed calcareous shale, a study case in the Santiago Formation, México

Abstract

Microstructural properties play a crucial role in the mechanical response of shale rocks. This work examines the impact of fabric anisotropy on the strength and elastic behavior of a highly deformed Jurassic calcareous shale from the Santiago Formation in eastern México by means of unconfined compression strength tests (UCS). The Santiago Formation is attracting considerable interest for the development of unconventional resources of shale gas and shale oil in México. However, the effect of tectonic deformation on the mechanical properties of these Mexican Shales has not been addressed in detail. The anisotropy of the shale was evaluated by means of rock samples cored in the direction normal to layers and two horizontal directions defined along the foliation planes. Petrography of oriented thin sections, X-ray diffraction, scanning electron microscopy (SEM), and petrophysical properties (porosity and permeability), were used to assess the rock composition and microstructural properties. UCS tests results revealed different mechanical responses between the normal direction (V) and the direction parallel to layers, but also differences between the two mutually orthogonal horizontal directions (H1, H2). Tangent Young's modulus (E) showed higher values in the H2 direction (13–22 GPa) and lower values in the H1 and V directions (8–10 GPa). All the specimens yielded very low Poisson's ratios (<0.11). H2 samples displayed the highest strength (48–84 MPa) and reached higher axial strains at failure (0.4–0.7%). Specimens in the H1 direction presented the lowest strength and axial strains (15–29 MPa and 0.22–0.35%, respectively). Compared to H1 samples, vertical samples yielded slightly higher strengths (26–33 MPa) and axial strains (0.31–0.40%). The results reported here confirm that microstructure strongly influenced the mechanical anisotropy and fracture mechanisms of the Santiago Shale. A direct relation was documented between fabric anisotropy, elastic response, UCS, and brittle failure modes validating the importance of fabric characterization for a complete description of anisotropy in tectonically deformed shale rocks.