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,Janos L Urai,Gioacchino Viggiani,Pierre Bésuelle,Nadine Höhne

doi:10.5194/se-8-291-2017

Abstract

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.

Highlights

Mudrocks constitute up to 80 % of the Earth’s sedimentary rocks (Stow, 1981)
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
Subsamples were first embedded in epoxy, extracted with a low speed diamond saw in dry conditions, pre-polished dry using SiC papers and BIB polished in a JEOL SM-09010 cross section polisher

Summary

Introduction

Mudrocks constitute up to 80 % of the Earth’s sedimentary rocks (Stow, 1981). Due to their low permeability and selfsealing properties (Boisson, 2005; Bernier et al, 2007), claystones are considered for nuclear waste disposal and seals for storage in deep geological formations (Salters and Verhoef, 1980; Shapira, 1989; Neerdael and Booyazis, 1997; Bonin, 1998; Ingram and Urai, 1999; ONDRAF/NIRAS, 2001; NA-GRA, 2002; NEA, 2004; ANDRA 2005; IAEA, 2008). Mudrocks constitute up to 80 % of the Earth’s sedimentary rocks (Stow, 1981). Due to their low permeability and selfsealing properties (Boisson, 2005; Bernier et al, 2007), claystones are considered for nuclear waste disposal and seals for storage in deep geological formations It is well established that for long-term predictions a microphysics-based understanding of mechanical and fluid flow properties in mudrocks provides a better basis for extrapolating constitutive equations beyond

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