Grain Fabric Heterogeneity in Strained Shales: Insights From XCT Measurements

Abstract

AbstractUnderstanding the fabric of rigid grains in strained shales is essential for predicting transport or mechanical properties. Fabric analysis of rigid grains is also key to infer deformation mechanisms in fine‐grained materials. In this study, we investigate the quartz shape fabric of two millimeter‐sized drill cores of tectonically deformed shales by means of X‐ray microtomography. The samples originate from the Jaca basin (Spain) and present a slaty cleavage perpendicular to the bedding. The representativeness of fabric data and heterogeneities are characterized at the microscale and compared with published magnetic fabric data. We extract both the individual grain data and the bulk data in sub‐volumes of increasing dimensions, and focus on identifiers, such as feature size, anisotropy, and shape. In the second step, the spatial heterogeneity of the matrix is assessed. We show that the bulk quartz fabric of a single millimeter‐sized sample is consistent with the magnetic fabric obtained based on a large number of centimeter‐sized samples. Yet, the individual grain analysis demonstrates that this bulk fabric hides a competition between two planar fabrics (bedding and cleavage), where both act differently depending on the grain size and morphology. Inter‐sample comparison reveals the existence of a petrofabric with a characteristic length that exceeds the sample size. These insights are directly applicable to the study of the bulk fabric at a larger scale. In this way, X‐ray microtomography complements petrophysical measurements in shales and helps to avoid misinterpretation of the rock fabric based on bulk measurements.