Shaly Facies Research Articles

State of the engineered barrier system and host rock of the HE-E experiment after 12 years of heating

This paper presents the status of the HE-E heater experiment, which is currently running at the Mont Terri Underground Research Laboratory (URL) in Switzerland. The experiment is located in a 10 m-long section of a micro-tunnel in the shaly facies of the Opalinus Clay and includes two 4 m-long heater sections isolated by plugs. Heater 1 is located in a sand/bentonite (65/35) buffer section and Heater 2 is located in a granular bentonite buffer section. Heating started on 28 June 2011, and this paper reports results up to 30 June 2023. During the 12 years of heating the system has otherwise been left undisturbed, with ongoing but very slow resaturation from the Opalinus Clay. The observed response to heating is considered in two phases: (1) the initial transient period of about 500 days when heater temperatures rose to the target 140°C and (2) the subsequent evolution. Recently a novel telescoped drilling procedure has been performed to recover and characterize material from the bentonite buffer without interruption of the heating. This paper describes the experimental long-term evolution and status prior to this disturbance, together with expectations regarding the current state of the buffer.

Characterization of heterogeneities in the sandy facies of the Opalinus Clay (Mont Terri underground rock laboratory, Switzerland)

SUMMARY This contribution is presenting a multidisciplinary investigation of heterogeneities in a clay rock formation, based on seismic tomography, logging and core analysis, as a reconnaissance study for a diffusion experiment. Diffusion experiments in clay rock formations provide crucial experimental data on diffusive transport of radionuclides (RN) in extremely low hydraulic conductivity media. Previous diffusion experiments, conducted, for example, in the Mont Terri underground rock laboratory within the relatively homogeneous shaly facies of Opalinus Clay, and modelling studies of these experiments have demonstrated that the clay rock could sufficiently well be described as a homogeneous anisotropic medium. For other lithofacies, characterized by larger heterogeneity, such simplification may be unsuitable, and the description of heterogeneity over a range of scales will be important. The sandy facies of the Opalinus Clay exhibits a significantly more pronounced heterogeneity compared to the shaly facies, and a combined characterization and RN diffusion study has been initiated to investigate various approaches of heterogeneity characterization and subsequent diffusion in a heterogeneous environment. As an initial step, two inclined exploratory boreholes have been drilled to access the margins of the experiment location. These boreholes have been used to acquire a cross-hole tomographic seismic data set. Optical, natural gamma and backscattering logging were applied and rock cores were analysed. The integrated results of these investigations allowed the identification of an anomalous brighter layer within the investigated area of the sandy facies of approximately 1 m thickness and with its upper bound at roughly 10 m depth within the inclined exploratory wells. Mineralogical analyses revealed only slight variations throughout the rock cores and indicated that the anomalous layer exhibited a slightly higher quartz content, and locally significantly higher calcite contents, accompanied by a lower content of clay minerals. The anomalous layer was characterized by reduced natural gamma emissions, due to the lower clay content, and increased neutron backscattering likely indicating an increased porosity. Seismic P-wave velocities, derived from anisotropic tomography, exhibited a maximal gradient near the top of this layer. The transition from the overlaying darker rock matrix into this layer has been identified as an appropriate location for the setup of a tracer diffusion experiment in a heterogeneous environment.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures (pc) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing pc, we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by pc at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing pc. Microstructural analysis of deformed samples revealed a positive correlation between increasing pc and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy (µ ~ 0.48) compared to the shaly facies (µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history.

Performance of high-resolution impact and vibration sources for underground seismic exploration of clay formations at the scale of a rock laboratory

SUMMARY Low permeability, high retention capacity and self-sealing ability are advantageous characteristics that are attributed to argillaceous rocks. In contrast, other properties of clay, such as internal heterogeneities, strong attenuation and anisotropic behaviour, pose major challenges for underground exploration techniques. Although with regard to the underground storage of nuclear waste, the seismic exploration in the underground itself is of great importance to fill the gap between surface and borehole investigations. Furthermore, to prevent destruction of the host rock during exploration this demands low to non-invasive techniques. To approach these issues, a seismic survey was carried out in the Mont Terri Underground Rock Laboratory (Switzerland) using a gallery-based acquisition with an operating range up to several decametres. The seismic campaign included three-component borehole sensors and two different seismic source types (pneumatic impact and magnetostrictive vibroseis source). An executed source comparison analysed the characteristics of the different source types, for example frequency or amplitude behaviour of the generated wavefield, to assess their performance under similar conditions at the meso scale and to reveal their strengths and weaknesses in clay. Based on these findings, we performed traveltime and reflection analyses that demonstrate their potential to characterize clay formations and to map internal structures. The highest seismic velocities are found in the carbonate-rich sandy facies (vPmax = 4000 m s−1, vSmax = 2050 m s−1), slower velocities are found in the sandy facies (vPmax = 3720 m s−1, vSmax = 1840 m s−1) and the slowest velocities are found in the shaly facies (vPmax = 3220 m s−1, vSmax = 1480 m s−1). The seismic velocity anisotropy is larger within the shaly facies (AvP = 23 per cent, AvS = 32 per cent) compared to the sandy facies (AvP = 9 per cent, AvS = 12 per cent) and it is more pronounced for S-waves than P-waves. Thus, non-invasive meso-scale seismic techniques are suited to characterize the Opalinus Clay in great detail.

Uranium migration through the Swiss Opalinus Clay varies on the metre scale in response to differences of the stability constant of the aqueous, ternary uranyl complex Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>

Abstract. The simulation of uranium migration through the Swiss Opalinus Clay is used as an example to quantify the influence of varying values of a stability constant in the underlying thermodynamic database on the migration lengths for the repository scale. Values for the stability constant of the neutral, ternary uranyl complex Ca2UO2(CO3)3 differ in literature by up to one order of magnitude. Within the studied geochemical system, either the neutral or the anionic complex CaUO2(CO3)32- is the predominant one, depending on the chosen value for the neutral complex. This leads to a changed interaction with the diffuse double layers (DDL) enveloping the clay minerals and thus can potentially influence the diffusive transport of uranium. Hence, two identical scenarios only differing in the value for the stability constant of the Ca2UO2(CO3)3 complex were applied in order to quantify and compare the migration lengths of uranium on the host rock scale (50 m) after a simulation time of one million years. We ran multi-component diffusion simulations for the shaly and sandy facies in the Opalinus Clay. A difference in the stability constant of 1.33 log units changes the migration lengths by 5 to 7 m for the sandy and shaly facies, respectively. The deviation is caused by the anion exclusion effect. However, with a maximum diffusion distance of 22 m, the influence of the stability constant of the Ca2UO2(CO3)3 complex on uranium migration in the Opalinus Clay is negligible on the host rock scale.

Capturing the structural and compositional variability of Opalinus Clay: constraints from multidisciplinary investigations of Mont Terri drill cores (Switzerland)

Opalinus Clay is considered a potential host rock for radioactive waste disposal. The Jurassic claystone formation is composed of several facies and subfacies types, which are characterized by varying amounts of quartz, carbonates (cements and fossils) and clay minerals. To provide samples for ongoing experimental and numerical studies, a complete core section was drilled in the Mont Terri rock laboratory. The lithological and structural variability (including tectonic fault structures) from borehole BAD-2 was investigated using a multidisciplinary approach including high-resolution geoelectric in situ borehole measurements, mineralogical/geochemical and (micro)structural analyses.The lithological–compositional variability was captured by successfully applying a modified subfacies approach, which is independently confirmed by the geochemical data and ERT (Electrical Resistivity Tomography) measurements. The variability on the cm to dm scale perpendicular to bedding can be determined based on the mean resistivity and variation of amplitude. In particular, the facies transitions could be precisely located. The new results suggest that both shaly facies types form the homogenous part of the investigated section, whereas the sandy facies and especially the carbonate-rich sandy facies represent the more heterogeneous lithofacies types of the Opalinus Clay. The varying resistivity can be attributed to differences in clay mineral and carbonate content. Regarding the structural variability, brittle faults were observed with varying frequency throughout the investigated section. Most fault planes occur in the shaly facies types, some of them concentrate along heterogeneities on the subfacies scale. The striking reproducibility of the measurements and observations was confirmed by a comparison with boreholes drilled in parallel, indicating a rather low compositional–structural variability parallel to bedding. The applied multidisciplinary approach is well suited to depict the vertical and lateral variability of a claystone formation, allowing an assessment of the degree of homogeneity/heterogeneity based on the subfacies concept.

Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric

The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (dot{varepsilon }) of 1 × 10–3–5 × 10–6 s−1. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed pc–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at pc = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σmax) increases with increasing pc, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and dot{varepsilon } do not significantly affect deformation behavior of dried samples. However, σmax and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.

Surrogate Model for Multi-Component Diffusion of Uranium through Opalinus Clay on the Host Rock Scale

Multi-component (MC) diffusion simulations enable a process based and more precise approach to calculate transport and sorption compared to the commonly used single-component (SC) models following Fick’s law. The MC approach takes into account the interaction of chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32−. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.

Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay

Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide pCO2, varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex CaUO2(CO3)32−. Hence, anion exclusion was taken into account and further reduced the migration distances by 30 %. The concentrations of calcium and carbonates reflected by the set pCO2 determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository.

A new continuum-based constitutive model for the simulation of the inherent anisotropy of Opalinus clay

Opalinus clay has been extensively investigated, as it is considered as potential host rock for underground nuclear waste repositories. The behavior of the Opalinus clay is a transverse isotropic possessing anisotropic characteristics in both strength and stiffness. Also, the lab investigations conducted on the Opalinus clay found that it behaves as bi-linear elasto-plastic strain softened material in general and as brittle plastic in particular. Two case studies of tunnel excavated in shaly facies formation of the Opalinus clay have been numerically analyzed by applying a new continuum-based constitutive model called “Transubi model” providing an Instantaneous hydromechanical coupling short-term simulation by generating the pore pressure considering no fluid flow. Lab test results obtained from literature have been used to validate this model: triaxial experiments and direct shear tests. The Transubi model is able to duplicate the pre-yield hardening and the post-yield softening for loading perpendicular and parallel to bedding planes, S- and P- samples, respectively. Finally, two in-situ tests in the Mt. Terri URL are used to demonstrate the applicability of the Transubi model to predict the short-term hydro-mechanical coupled behavior. The results from numerical simulations for both excavations are compared to those from field observations showing better agreements than other previous numerical analyzes from literature.

Static and dynamic stiffness measurements with Opalinus Clay

ABSTRACTIn this work, an experimental study was carried out with the aim of reconciling static and dynamic stiffness of Opalinus Clay. The static and dynamic stiffness of core plugs from a shaly and a sandy facies of Opalinus Clay were characterized at two different stress states. The measurements included undrained quasi‐static loading–unloading cycles from which the static stiffness was derived, dynamic stiffness measurement at seismic frequencies (0.5–150 Hz) and ultrasonic velocity measurements (500 kHz) probing the dynamic stiffness at ultrasonic frequencies. The experiments were carried out in a special triaxial low‐frequency cell. The obtained results demonstrate that the difference between static and dynamic stiffness is due to both dispersion and non‐elastic effects: Both sandy and shaly facies of Opalinus Clay exhibit large dispersion, that is, a large frequency dependence of dynamic stiffness and acoustic velocities. Especially dynamic Young's moduli exhibit very high dispersion; between seismic and ultrasonic frequencies they may change by more than a factor 2. P‐wave velocities perpendicular to bedding are by more than 200 m/s higher at ultrasonic frequencies than at seismic frequencies. The static undrained stiffness of both sandy and shaly facies is strongly influenced by non‐elastic effects, resulting in significant softening during both loading and unloading with increasing stress amplitude. The zero‐stress extrapolated static undrained stiffness, however, reflects the purely elastic response and agrees well with the dynamic stiffness at seismic frequency.

Organic Geochemical Analysis of Shaly Facies from Two Wells Within Anambra Basin, Southeastern Nigeria

Abstract Organic geochemical analysis of two selected wells penetrating shale facies of the Anambra basin was conducted with the view of evaluating the section in terms of quantity and quality of organic matter, genetic potential, organic matter type, thermal maturity as well as determining the type of hydrocarbon that could be generated. Geochemical parameters such as Total Organic Carbon (TOC), S1 (representing free and adsorbed hydrocarbons present), S2 (representing hydrocarbons generated directly from the kerogen), S3 (carbon dioxide CO2 present) and maximum temperature (Tmax) as well as Hydrogen Index (HI), Oxygen Index (OI), Production Index (PI) and Genetic Potential (GP) were derived and calculated from the pyrolysis data. Result indicated that Well 1 samples have an average TOC of 1.21 wt % which is considered good in organic matter quantity and fair in quality, while Well 2 samples are organically lean, poor in quantity and quality with average TOC value of 0.15 wt %. The Genetic Potential (GP) expressed as (S1+S2) for Well 1 and Well 2 averages 2.03 and 0.68 mg HC/g respectively, indicating, a poor generational potential. The HI, OI and S2/S3 values of Well 1 samples are 146.56 mg HC/g, 226.78 mg HC/g and 0.86 respectively which on plots suggest the kerogen as type IV although few samples fall within the type III area. This contrasts with Well 2 samples having HI, OI and S2/S3 values as 343.67 mg HC/g, 276.78 mg HC/g and 1.26 respectively. Thus making the kerogen type to be interpreted as type III. Judging from Tmax (average of 441.67°C for Well 1 and 470.44°C for Well 2) and PI (average of 0.13 for Well 1 and 0.24 for Well 2) values, Well 1 samples are within the oil generating window whereas Well 2 samples are overmatured generating dry gas. Deductions from the result of geochemical analysis show that the kerogen of Well 1 samples will generate oil while that of Well 2 samples have propensity to generate dry gas.

Application of amplitude spectrum and genetic inversion to determine shaly facies distribution: a case study: Murzuq Basin, Libya

ABSTRACTLate Ordovician glacial deposits of the Mamuniyat Formation are the main oil reservoir in the Murzuq Basin in Libya. Autopicking the strong reflection at the base of the Silurian shales can be used to map the top of the Mamuniyat reservoir in the area where it is in direct contact with the Silurian shales. However, in areas where the Bir Tlacsin Formation, a mud‐prone unit, is between the Silurian shales and the Mamuniyat reservoir, the top of the Mamuniyat is difficult to pick because the units juxtaposed across the boundary are too similar to produce a strong reflection. Defining the Bir Tlacsin facies is important because it impacts hydrocarbon accumulation and migration. To predict the distribution of the shaly facies of Bir Tlacsin and enhance mapping of the top Mamuniyat reservoir, we utilized a continuous wavelet transform to identify the distinctive thickness of the Hot Shale and Bir Tlacsin units. We also used genetic inversion to distinguish the bulk density of the Bir Tlacsin facies. A 64 Hz frequency gave good time resolution to the amplitude spectrum and was used to predict the facies distribution of the Bir Tlacsin. In contrast, the 24 Hz frequency showed good frequency resolution of the amplitude spectrum and was used to estimate the temporal thickness of the non‐reservoir unit of Bir Tlacsin and Hot Shale. That estimate was then used to modify the autopick horizon for the base of the Silurian reflector to approximate the top of the Mamuniyat reservoir. Because of the large density contrast between the shaly facies of the Bir Tlacsin and the underlying and overlying units, inverted density also provides a way to predict the distribution of the Bir Tlacsin through estimated temporal thickness and to enhance mapping of the top Mamuniyat reservoir through mapping the base of the inverted density of the Bir Tlacsin. A comparison between mapping of the top reservoir using spectral decomposition and inverted density with respect to autopick shows that both methods improved the top of the Mamuniyat reservoir mapping. Prediction of the presence of Bir Tlacsin and improved accuracy of the top of the Mamuniyat reservoir mapping reduce the risk of drilling the shaly facies of Bir Tlacsin and provide a better estimate of the reservoir reserve.

Analysis of acoustic emissions recorded during a mine-by experiment in an underground research laboratory in clay shales

During a mine-by experiment performed at the Mont Terri Underground Research Laboratory located at the transition between the sandy and the shaly facies of the Opalinus Clay formation, excavation induced micro-acoustic events were recorded in the so-called Gallery 08 (the EZ-G experiment). A first cluster of events occurred in the vicinity of the eastern sidewall of Gallery 08, and a second cluster was observed ahead of the advancing tunnel face. For each recorded micro-acoustic event (AE), all located in the sandy facies of the Opalinus Clay formation, the total stresses associated with the onset of inelastic deformations were estimated using a three-dimensional numerical model. The numerical analysis is based on the assumption that the rock mass behavior in the vicinity of the excavation is essentially elastic before the stress redistribution causes damage evidenced by the triggered AE activity. For the cluster located at the tunnel sidewall, the source mechanism analysis reveals the predominance of extensional failure (tensile) events. The numerical analysis of each individual micro-acoustic event suggests that the differential stresses at the onset of damage range between 3 and 10 MPa. These values are in reasonable agreement with crack initiation thresholds obtained in the laboratory from samples of the various sub-facies types of the sandy facies in the Opalinus Clay formation, which range between 2 and 18 MPa. For the cluster located ahead of the tunnel face, the source mechanism analysis indicates the predominance of local shear failure events. This is consistent with observed shear dislocations on bedding planes within weak beds in the sandy facies that have similar strength properties as the shaly facies. Thus, the modelled shear and total normal stresses acting across the average bedding plane orientation at each event location were modelled and used to estimate the in situ shear strength along the bedding planes. The model suggests a friction angle of 33.4° and a cohesion of 0.43 MPa. The results are overall consistent with the bedding plane strength obtained through undrained direct shear tests on specimens of the shaly facies, suggesting that the observed weak beds with a strength similar to the shaly facies govern the behavior at the tunnel face.

Multi-proxy facies analysis of the Opalinus Clay and depositional implications (Mont Terri rock laboratory, Switzerland)

Located in NW Switzerland, the Mont Terri rock laboratory is a research facility primarily investigating the Opalinus Clay as potential host rock for deep geological disposal of radioactive waste. In the Mont Terri area, this Jurassic shale formation is characterized by three distinctive lithofacies: a shaly facies, a carbonate-rich sandy facies and a sandy facies. However, the lithological variability at dm- to cm-scale is not yet fully understood and a detailed lithofacies characterization is currently lacking. Within the present study, petrographic descriptions at micro- and macro-scale, geophysical core logging (P-wave velocity and gamma-ray density), geochemical core logging (X-ray fluorescence) and organic matter quantification (Rock-Eval pyrolysis) were combined on a 27.6 m long Opalinus Clay drillcore comprising the three major lithofacies. The high-resolution investigation of the core resulted into a refinement of the three-fold lithofacies classification, and revealed high intra-facies heterogeneity. Five subfacies were defined and linked to distinctive depositional regimes. The studied succession is interpreted as a shallowing-upward trend within a storm-wave-dominated epicontinental sea characterized by relative shallow water depths.

Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

Abstract. The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around 1 km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. We mapped the scaly clay fabric at 1 m–10 nm scale, examining scaly clay for the first time using broad-ion beam polishing combined with scanning electron microscopy (BIB-SEM). Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 10 cm and 10 µm in diameter, with slickensided surfaces. Our results show that microlithons are only weakly deformed and that strain is accumulated by fragmentation of microlithons by newly formed shear zones, by shearing in the micron-thick zones and by rearrangement of the microlithons.The scaly clay aggregates can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the microlithon size by sieving indicate a power-law distribution model with exponents just above 2. From this, we estimate that only 1 vol % of the scaly clay aggregate is in the shear zones.After a literature review of the hypotheses for scaly clay generation, we present a new model to explain the progressive formation of a self-similar network of anastomosing thin shear zones in a fault relay. The relay provides the necessary boundary conditions for macroscopically continuous deformation. Localization of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures, evolve into complex microscale re-partitioning of shear, forming new shear zones while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain-scale deformation mechanisms are microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This study provides a microphysical basis to relate microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long-term deformation.

The effect of microstructural heterogeneity on pore size distribution and permeability in Opalinus Clay (Mont Terri, Switzerland): insights from an integrated study of laboratory fluid flow and pore morphology from BIB-SEM images

Abstract Opalinus Clay (OPA) is considered as a potential host rock for the deep geological disposal of radioactive waste. One key parameter in long-term storage prediction is permeability. In this study we investigated microstructural controls on permeability for the different facies of OPA. Permeability and porosity were determined under controlled pressure conditions. In addition, the pore space was investigated by SEM, using high-quality surfaces prepared by broad ion beam (BIB) milling. Water permeability coefficients range from 1.6×10 −21 to 5.6×10 −20 m 2 ; He-pycnometer porosities range between approximately 21 and 12%. The sample with the highest He porosity (shaly facies) is characterized by the lowest permeability, and vice versa (carbonate-rich sandy facies). This inverse behaviour deviates from the generally reported trend of increasing permeability with increasing porosity, indicating that parameters other than porosity affect permeability. Visible porosities from SEM images revealed that 67–95% of the total porosity resides within pores smaller than the SEM detection limit. Pore sizes follow a power-law distribution, with characteristic power-law exponents ( D ) differing greatly between the facies. The carbonate-rich sandy facies contains a network of much larger pores ( D (shaly) ≈2.4; D (carbonate-rich) c. 2.0), because of the presence of load-supporting sand grains that locally prevent clay compaction, and are responsible for a higher permeability.

Anisotropic volumetric behaviour of Opalinus clay shale upon suction variation

Opalinus clay shale is under consideration to serve as the host geomaterial for the underground storage of nuclear waste in Switzerland. Water retention behaviour and anisotropic behaviour represent two main features of this geomaterial; however, a comprehensive understanding of the interaction between these two features is still lacking. This paper aims to provide a detailed experimental analysis of the coupling between the water retention capacity and the anisotropic behaviour of two facies (shaly and sandy) of the Opalinus clay shale. The response of the tested geomaterials is characterised by an unequal swelling and shrinkage response in directions parallel and perpendicular to the bedding during wetting and drying episodes. Although the shaly facies showed a higher volumetric response to suction variations, the anisotropic behaviour is more pronounced for the sandy facies.

Seismic and mechanical properties of Opalinus Clay: comparison between sandy and shaly facies from Mont Terri (Switzerland)

For the safe disposal of high-level radioactive waste, different host rocks are currently being considered. The favorable properties of clay are low permeability, some retention capacity concerning radionuclides, and the ability to self-seal cracks and fissures, e.g. by swelling or time-dependent compaction creep. In Switzerland, the Jurassic Opalinus Clay is envisaged as a potential host rock which—at Mont Terri—is subdivided into the sandy, shaly, and carbonate-rich facies, the latter being less abundant. For long-term safety assessments, the understanding of the relations of properties (e.g. mineralogical composition and microstructure) and performance (e.g. mechanical behavior) of clays and claystones is essential. In the case of the sandy Opalinus Clay, the mechanical strength increases with increasing carbonate content, because carbonates form the matrix. The mineralogical investigation of a set of sandy facies samples proved a significantly larger carbonate content (20–40 mass %) when compared to the shaly facies (10–20 mass %). The carbonates of the shaly Opalinus Clay, on the other hand, are mostly localized fossils aligned parallel to the bedding, acting as predetermined breaking points. Image analysis of SEM images of polished sections proved the determined microstructural differences. In addition, carbonate particles of the sandy facies are mostly isometric, whereas carbonate particles of the shaly facies cover a greater range of shapes. The mechanical tests were accompanied by investigations of the p- and s-wave velocities, which revealed that the anisotropy of the sandy facies is less pronounced than sedimentological analyses would suggest. The mechanical strength, which, for the first time, presents results of real triaxial tests of the sandy facies. The samples of the sandy facies exhibit a failure strength of σ eff,B, approximately twice as high as was found for the shaly facies considering the deformation axis parallel to the bedding. Similar values were obtained when measuring perpendicularly to the bedding.

Pore morphology and distribution in the Shaly facies of Opalinus Clay (Mont Terri, Switzerland): Insights from representative 2D BIB–SEM investigations on mm to nm scale

We used Broad-Ion-Beam polishing in combination with Scanning Electron Microscopy to study the microstructure and porosity in the Shaly facies of Opalinus Clay (northern Switzerland). With resolutions of a few nm, we studied areas of around 1mm2, sufficiently large to be statistically representative for the microstructure. In three samples from a 1m long drill core from the Mt Terri laboratory, a qualitative description of the microstructure was combined with quantitative description of porosity. Eight different mineral phases and six different pore types are identified, in the clay matrix, siderite, pyrite framboids and fossil shells respectively. Pore sizes in the clay matrix have a power law distribution with an exponent of 2.4. Porosity inferred from the BIB–SEM method is comparable to that measured using mercury porosimetry. Based on micrographs made perpendicular and parallel to the bedding we show that pores and minerals are homogeneously distributed in 3D, and present a microstructural model for the Shaly facies of Opalinus Clay. This model defines predictable homogeneous porous and non-porous regions with similar pore characteristics. Combined, these form the heterogeneous fabric of Opalinus Clay (Shaly facies).