Physical characterization of fault rocks within the Opalinus Clay formation

Luis Felipe Orellana,Christophe Nussbaum,Pierre Henry,Luiz Grafulha,Marie Violay

doi:10.1038/s41598-022-08236-7

Abstract

Near-surface disposal of radioactive waste in shales is a promising option to safeguard the population and environment. However, natural faults intersecting these geological formations can potentially affect the long-term isolation of the repositories. This paper characterizes the physical properties and mineralogy of the internal fault core structure intersecting the Opalinus Clay formation, a host rock under investigation for nuclear waste storage at the Mont Terri Laboratory (Switzerland). We have performed porosity, density, microstructural and mineralogical measurements in different sections of the fault, including intact clays, scaly clays and fault gouge. Mercury intrusion porosimetry analysis reveal a gouge that has a pore network dominated by nanopores of less than 10 nm, yet a high-porosity (21%) and low grain density (2.62 g/cm3) when compared to the intact rock (14.2%, and 2.69 g/cm3). Thus, a more permeable internal fault core structure with respect to the surrounding rock is deduced. Further, we describe the OPA fault gouge as a discrete fault structure having the potential to act as a preferential, yet narrow, and localized channel for fluid-flow if compared to the surrounding rock. Since the fault gouge is limited to a millimetres-thick structure, we expect the barrier property of the geological formation is almost not affected.

Highlights

As the capacity for fluid transport is related to the connected pore structure and faults, the research comprises a study of the pore structure of clay-bearing fault-rocks within the Opalinus Clay formation, a potential host rock for nuclear waste storage in Switzerland[17–19]
From the X-ray diffraction (XRD) measurements, we have identified phyllosilicates, quartz, calcite, and pyrite as the main constituents of the samples (Table 1)
Through microstructural and pore network characterization, we present an analysis of the hydrological behavior of the clay-rich fault gouge within a major fault system (MF) intersecting the Opalinus Clay (OPA) formation at the Mont Terri Laboratory (MTL), a host-rock candidate for deep nuclear waste storage in Switzerland

Summary

Introduction

Near-surface disposal of radioactive waste in shales is a promising option to safeguard the population and environment. This paper characterizes the physical properties and mineralogy of the internal fault core structure intersecting the Opalinus Clay formation, a host rock under investigation for nuclear waste storage at the Mont Terri Laboratory (Switzerland). As the capacity for fluid transport is related to the connected pore structure and faults, the research comprises a study of the pore structure of clay-bearing fault-rocks within the Opalinus Clay formation, a potential host rock for nuclear waste storage in Switzerland[17–19] For this purpose, we have conducted mineralogical and microstructural analysis, and we have characterized the porosity, grain density and pore structure of more than 60 samples obtained from the internal fault structures of the Main Fault, a clay-bearing fault crosscutting the Mont Terri Laboratory. The MTL will not be the final location for nuclear waste, yet it provides a unique opportunity to foreseen potential hazards

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