Articles | Volume 8, issue 2
Research article
09 Mar 2017
Research article |  | 09 Mar 2017

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

Guillaume Desbois, Nadine Höhne, Janos L. Urai, Pierre Bésuelle, and Gioacchino Viggiani

Abstract. The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better understanding of microphysical deformation mechanisms. Here we use a combination of scanning electron microscopy (SEM) and broad ion beam (BIB) polishing to study the evolution of microstructure in samples of triaxially deformed Callovo–Oxfordian Clay. Digital image correlation (DIC) was used to measure strain field in the samples and as a guide to select regions of interest in the sample for BIB–SEM analysis. Microstructures show evidence for dominantly cataclastic and minor crystal plastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to the nanometre scale. At low strain, the dilatant fabric contains individually recognisable open fractures, while at high strain the reworked clay gouge also contains broken non-clay grains and smaller pores than the undeformed material, resealing the initial fracture porosity.

Short summary
This work integrates measurements of the mechanical and transport properties with microstructures to understand deformation mechanisms in cemented mudrock. Cataclastic mechanisms are dominant down to nanometre scale. At low strain the fabric contains recognizable open fractures, while at high strain the reworked clay gouge shows resealing of initial fracture porosity. In the future, it will provide a microphysical basis for constitutive models to improve their extrapolation for long timescales.