Quantification of Lithological Heterogeneity Within Opalinus Clay: Toward a Uniform Subfacies Classification Scheme Using a Novel Automated Core Image Recognition Tool

Abstract

The Opalinus Clay is notable in Switzerland as being the selected host rock for deep geological disposal of radioactive waste. Since the early 1990’s, this argillaceous mudstone formation of Jurassic age has been intensively studied within the framework of national and international projects to characterize its geological, hydrological, mechanical, thermal, chemical, and biological properties. While there is no formal stratigraphic subdivision, the Opalinus Clay lithology is classically divided into several, dam- to m-scale sub-units (or facies), depending on location. Recent multi-proxy studies (combining petrographic, petrophysical, geochemical, and mineralogical analyses) have however demonstrated that high, intra-facies, lithological heterogeneity occurs at the dm- to cm-scale. To constrain this small-scale heterogeneity into distinct lithological units (subfacies), the present study aims at defining and presenting a convenient subfacies classification scheme covering the overall Opalinus Clay lithology across northern Switzerland. Petrographic (macro- and microfacies), mineralogical (X-ray diffraction) and textural (image analysis, machine learning and 3D X-ray computed tomography) analyses are performed on diverse drill cores from the Mont Terri rock laboratory (northwestern Switzerland), and results are extended further to the east (Riniken, Weiach, and Benken). Most of the investigated Opalinus Clay can be described by the use of five distinctive subfacies types (SF1 to SF5), which are visually and quantitatively distinguishable by texture (grain size, bedding, fabric, and color) and composition (nature and mineralogy of components). The five subfacies types can be further refined by additional attributes and sedimentary characteristics (biogenic, diagenetic, and structural). Eventually, the widespread and consistent use of standardized Opalinus Clay subfacies types provides the means to harmonize petrographic descriptions within multidisciplinary research projects, enhance reproducibility of in situ experiments, and further evidence the tight relations between lithology and various rock properties.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

“…The shaly facies (Fig. 1b, d) is composed of rather homogeneous, dark-grey, laminated, argillaceous claystones mainly consisting of clay minerals (illite, illite-smectite kaolinite and chlorite) with silt to fine-sand sized bioclasts (e.g., fossil shells) and detrital quartz and feldspar grains dispersed throughout the clay-rich matrix (Kneuker and Furche 2021;Lauper et al 2021). The OPA sandy facies (Fig.…”

“…The OPA sandy facies (Fig. 1a, c) is a heterogeneous, siliceous-argillaceous claystone, characterized by an alternation of dark-grey, clay-rich layers and undulating lenses and layers of silt to sand-sized quartz and feldspar grains, cemented by carbonates (Lauper et al 2018(Lauper et al , 2021Schuster et al 2021). Well-pronounced bedding planes of OPA are caused by the preferential alignment of phyllosilicates, leading to a transverse isotropy of petrophysical, mechanical and hydraulic properties (e.g., NAGRA 2002;Wenk et al 2008;Siegesmund et al 2014;Favero et al 2018;Schuster et al 2021).…”

“…Brittle deformation of clay-rich rocks is favored with decreasing clay mineral content and is further influenced by the present stress field and burial history (e.g., Gutierrez et al 1996;Ingram and Urai 1999;Nygård et al 2006;Bourg 2015). At the Mont Terri URL, several sub-units have been distinguished within the formation, which vary in grainsize, color as well as the proportions of the main mineral components: phyllosilicates, quartz and carbonates (Hostettler et al 2017;Lauper et al 2018Lauper et al , 2021Kneuker and Furche 2021). Accordingly, OPA is generally subdivided into a carbonate-rich sandy, a shaly and a sandy lithofacies type at the URL (Thury and Bossart 1999;Pearson et al 2003).…”

Geochemistry and palaeo-hydrogeology of the weathered zone in the Opalinus Clay

“…On average, it consists of 60 ± 12 wt% clay minerals (illite, illite/smectite mixed layers, kaolinite, chlorite), 22 ± 6 wt% quartz and feldspars, 13 ± 10 wt % calcite and minor contributions of siderite, pyrite and organic matter (Mazurek, 2011). In the vertical dimension, lithological sub-units, reflecting slight variations in sedimentary facies, were defined by Lauper et al (2021). Owing to maximum burial in the Cretaceous and subsequent basin inversion (Mazurek et al, 2006;Giger et al, 2015), the Opalinus Clay is an indurated and over-consolidated shale.…”

The influence of sedimentary heterogeneity on the diffusion of radionuclides in the sandy facies of Opalinus Clay at the field scale

“…Such heterogeneities are responsible for the observed heterogeneous diffusion patterns at the core scale (Bollermann et al, 2022). At the core scale, Lauper et al (2021) quantified the lithological heterogeneity within OPA from northern Switzerland. They concluded that most of the investigated OPA can be described by five distinctive subfacies types that are visually and quantitatively distinguishable by texture and composition.…”