Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian-Oxfordian clayey formation of Bure (France)

International audienceThe Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ34S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ34S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ34S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87Sr/86Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ18O consistent with a marine environment. Its δ13C is however lower than those of marine carbonates, suggesting a contribution of 13C-depleted carbon from degradation of organic matter. δ18O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ18O values (not, vert, similar+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates

Modelling the porewater chemistry of the Callovian–Oxfordian formation at a regional scale

International audienceIn ANDRA's studies to characterize the Callovian-Oxfordian formation, porewater chemistry is a key topic. Indeed, chemistry determines the durability of the repository materials (bentonite, concrete, metals, nuclear glass) and the speciation (and thus the mobility) of radionuclides. The method developed in the frame of the THERMOAR project enables the acquisition of a complete set of data from core samples to model the porewater chemistry. The method requires a detailed mineralogical study, a model of free-water/bound-water distribution, leaching experiments, adsorbed ion measurements, ion-exchange constant acquisition, and CO2 partial-pressure measurements. These experiments and measurements were done on samples from the site of the Meuse/Haute-Marne laboratory and from ANDRA's regional boreholes. The regional stability of a great number of parameters can be observed, except for a decrease of the Na and Cl concentration following a NE-SW axis passing through the laboratory. The water/rock equilibrium model makes it possible to calculate the chemical composition of interstitial waters of the formation

Qualitative and time analysis of the gaseous constituents transport processes in a micro-tunnel on the basis of in-situ measurements

International audienceThis paper describes air pressure, relative humidity (RH), temperature and oxygen molar fraction (xO$_2$) measurements that have been carried out over several years in a hundred meter long and 75 centimeters diameter micro-tunnel (MT) at the Andra Meuse/Haute-Marne underground research laboratory (MHM URL). Sensors were placed all along, every twenty meters inside the MT, and in the access drift. RH and xO$_2$ time evolutions exhibit very complicated profiles, showing many singularities. These features can be well explained by taking into account mainly the corrosion of the steel casings, the water evaporation from the surrounding saturated clay and a leak between the head of the MT excavation and the access drift. Airflow through the leak is driven by the drift pressure seasonal and short time fluctuations. A simple model involving coupled infinite and finite reservoirs made it possible to reproduce xO$_2$ evolution and to calculate the air fluxes associated. Drift pressure variations induce longitudinal compression-dilatation processes in the system that are closely estimated using suitable analytical formulas. By taking into account diffusion driven leaks at the casings interfaces, it was possible to build a global model from which the results compare very well with experimental data

Les alcanes dissous, traceurs de transfert des solutés entre réservoirs souterrains

International audienceLes alcanes dissous, traceurs de transfert des solutésentre réservoirs souterrains. Raymond Michels1,*, Irina Panfilova2, Agnès Vinsot3, Stefan Wechner41 Université de Lorraine, CNRS, GeoRessources, UMR 7359, CREGU - France2 LEMTA, CNRS-Université de Lorraine, UMR7563 - France3Andra, Centre de Meuse/Haute-Marne - France4Hydroisotop GmbH, Schweitenkirchen - AllemagneDans le cadre de ses études géologiques et hydrogéologiques, l’Andra a foré le puits EST433 de 2000m de profondeur à Montiers-sur-Saulx. Pour chaque échantillon de roche prélevé, la concentration relative à la masse d’eau de l’échantillon ainsi que la composition isotopique en carbone et hydrogène ont été déterminées pour les constituants individuels de la classe C1-C4. L’étude de la nature, de l’origine et de la distribution de ces alcanes en concentrations traces a été entreprise afin d’apporter de nouvelles contraintes sur les possibilités de migration des solutés dans la pile sédimentaire.Dans les diagrammes géochimiques les alcanes C1-C4 présentent des caractéristiques compatibles avec une origine thermogénique. Bien que le profil global d’évolution des alcanes avec la profondeur présente une augmentation dans la proportion relative en méthane, un examen détaillé des distributions révèle des variations plus complexes en relation avec l’organisation des lithologies. Ainsi, les données géochimiques ont-elles été interprétées en association avec ces dernières le long du profil géologique.Afin de mieux appréhender les variations des concentrations relatives en méthane, une modélisation de la diffusion 1D utilisant le logiciel Comsol a été réalisée. Parmi les 28 variables considérées, le modèle prend en compte les lithologies, perméabilités, le gradient géothermique actuel, le coefficient de diffusion effectif du méthane en solution. Le potentiel en hydrocarbures des roche-mères, les faciès argileux et les communications avec les aquifères sont parmi les paramètres majeurs gouvernant les origines et la diffusion des alcanes. Les grandes tendances du profil de distribution du méthane sont reproduites par le modèle qui apporte ainsi des contraintes physiques aux interprétations géologiques et géochimiques.Cette étude montre que les alcanes en solution sont des traceurs originaux dans la compréhension des transferts de fluides et de solutés dans les systèmes géologiques même dans des sections de bassins dans lesquelles les roches-mères sont thermiquement immatures.Mots-Clés : Bassin sédimentaire, réservoirs, transferts, alcanes, gaz hydrocarbures, transferts

Kinetic experiments in order to determine the rate of oxygen consumption by the Callovian-Oxfordian argillaceous rock

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Design of a robust and compact gamma sensor: Assessment of uranium detection in underground COx mudstone for nuclear waste management

International audienceIn the context of deep geological radioactive waste disposal in France, various experiments have been designed, including in situ hydrogeological tests, chemical measurements and diffusion experiments. In this paper, we focus on in situ Diffusion of Radionuclides (DRN) tests, typically for actinide diffusion. These tests involved the construction of a powerful and qualified instrument that fulfills requirements for in situ radionuclides measurements. This implied a very specific design of four channels 1x1 inch cylinder YSO gamma-ray sensors, satisfying severe geometric and operational constraints while displaying high sensitivity and robust, real time on-line performance. The calibration of such sensors was assessed regarding their ability to quantify the minimum quantity of natural uranium that could be detected with and without the presence of Callovian–Oxfordian mudstone (COx) screens. Its design, calibration,and detection sensitivity are described in the present paper. The sensor sensitivity at 12 and 6 counts/second/g(U) without and with COx screening effect were determined for energy peak counting windows (50–105 keV) respectively. This was equivalent to a limit of detection (LOD) of uranium at 42.5 mg and 126 mg without and with a 7 cm thickness of COx sample. These sensors provided the first trial for in situ actinide diffusion tests in the context of nuclear waste management

The influence of natural trace element distribution on the mobility of radionuclides. The exemple of nickel in a clay-rock

International audienceThe natural distribution of nickel (Ni) in the Callovian–Oxfordian clay-rich rock of Bure (France) was investigated, together with that of cobalt (Co), zinc (Zn) and lead (Pb). The most Ni-enriched phases are pyrite (∼400 × 10−6 g g−1 Ni), sphalerite (∼300 × 10−6 g g−1), chlorite (∼285 × 10−6 g g−1), organic matter (∼300 × 10−6 g g−1), muscovite (100–200 × 10−6 g g−1) and possibly carbonate minerals (mainly calcite and minor dolomite, ∼10 × 10−6 g g−1). Despite their high abundance (up to ∼80% in the upper part of the formation), carbonate minerals are quantitatively a minor Ni reservoir; most of the Ni is borne by chlorite and pyrite, which are minor mineral phases. Co and Ni have a similar distribution, whereas Pb and Zn have partly different reservoirs (e.g. sphalerite – ZnS).In parallel, the equilibrium between rock and pore water was investigated, and importance of pyrite (or reduced sulfur minerals) as Ni reservoir in the formation was confirmed from an in situ experiment perturbed by oxidation processes. Under unperturbed conditions (laboratory kinetics experiments and in situ pore water sampling), Ni solubility ranges from ∼0.2 to 1 × 10−6 mol L−1. The rock sorption capacities with regards to radioactive Ni (in the range of ∼1.5 × 10−10 mol L−1 to ∼1.5 × 10−9 mol L−1 of spiked 63Ni) were also tested using batch experiments. Sorption kinetics of 63Ni on the Callovian–Oxfordian clay-rich rock was explained by two mechanisms: fast and reversible sorption onto clay minerals and slow incorporation in another phase. Indirect evidences point out the likely major role of calcite in this latter process albeit pyrite and organic matter may also be involved. Reversible sorption of Ni onto clay minerals surfaces is shown to be hindered by competition processes with other naturally occurring bivalent metals (e.g. naturally-occurring Ni and Zn), thus decreasing the 63Ni Rd value as compared to the value predicted from a bottom-up approach considering only 63Ni sorption on clay minerals. At longer time scale, irreversible 63Ni trapping is observed and compensates the competition processes. These two mechanisms were implemented in a reactive transport model to illustrate their influence on the prediction of long-term behavior of radioactive Ni. Migration distances are increased by the effect of cation competition, and reduced in a comparable extent by irreversible trapping in the time frame investigated

Biogeochemical processes in a clay formation in-situ experiment: Part A - Overview, experimental design and water data of an experiment in the Opalinus Clay at the Mont Terri Underground Research Laboratory, Switzerland

International audienceAn in situ test in the Opalinus Clay formation, termed porewater chemistry (PC) experiment, was carried out for a period of 5 years. It was based on the concept of diffusive equilibration whereby a traced water with a composition close to that expected in the formation was continuously circulated and monitored in a packed-off borehole. The main original focus was to obtain reliable data on the pH/pCO(2) conditions of the porewater. but because of unexpected microbiologically-induced redox reactions, the objective was extended to elucidate the biogeochemical processes occurring in the borehole and to understand their impact on pH/pCO(2) and porewater chemistry in the low permeability clay formation. The behaviour of the conservative tracers (2)H and Br(-) could be explained by diffusive dilution in the clay and moreover the results showed that diffusive equilibration between the borehole water and the formation occurred within about 3 year's time. However, the composition and pH/pCO(2) conditions differed considerably from those of the in situ porewater. Thus, pH was lower and pCO(2) was higher than indicated by complementary laboratory investigations. The noted differences are explained by microbiologically-induced redox reactions occurring in the borehole and in the interfacial wall area which were caused by an organic source released from the equipment material. The degradation of this source was accompanied by sulfate reduction and - to a lesser extent - by methane generation, which induced a high rate of acetogenic reactions corresponding to very high acetate concentrations for the first 600 days. Concomitantly with the anaerobic degradation of an organic source, carbonate dissolution occurred and these processes resulted in high pCO(2) and alkalinities as well as drop in pH. Afterwards, the microbial regime changed and, in parallel to ongoing sulfate reduction, acetate was consumed, leading to a strong decrease in TOG which reached background levels after about 1200 days. In spite of the depletion of this organic perturbation in the circuit water, sulfate reduction and methanogenesis continued to occur at a constant rate leading to near-to-constant concentrations of sulfate and bicarbonate as well as pH/pCO(2) conditions until the end of the experiment. The main sink for sulphur was iron sulfide, which precipitated as FeS (am) and FeS(2). The chemical and isotopic composition was affected by the complex interplay of diffusion, carbon degradation rates, mineral equilibria and dissolution rates, iron sulfide precipitation rates, and clay exchange reactions. The (13)C signals measured for different carbon species showed significant variations which could only be partly explained. The main cations, such as Na, Ca and Mg remained remarkably constant during the experiment, thus indicating the strong buffering of the formation via cation and proton exchange as well as carbonate dissolution/precipitation reactions

Natural gas extraction and artificial gas injection experiments in Opalinus Clay, Mont Terri rock laboratory (Switzerland)

International audienceTwo experiments have been installed at Mont Terri in 2004 and 2009 that allowed gas circulation within a borehole at a pressure between 1 and 2 bar. These experiments made it possible to observe the natural gases that were initially dissolved in pore-water degassing into the borehole and to monitor their content evolution in the borehole over several years. They also allowed for inert (He, Ne) and reactive (H2) gases to be injected into the borehole with the aim either to determine their diffusion properties into the rock pore-water or to evaluate their removal reaction kinetics. The natural gases identified were CO2, light alkanes, He, and more importantly N2. The natural concentration of four gases in Opalinus Clay pore-water was evaluated at the experiment location: N2 2.2 mmol/L ± 25%, CH4 0.30 mmol/L ± 25%, C2H6 0.023 mmol/L ± 25%, C3H8 0.012 mmol/L ± 25%. Retention properties of methane, ethane, and propane were estimated. Ne injection tests helped to characterize rock diffusion properties regarding the dissolved inert gases. These experimental results are highly relevant towards evaluating how the fluid composition could possibly evolve in the drifts of a radioactive waste disposal facility

Natural tracer profiles across argillaceous formations

Argillaceous formations generally act as aquitards because of their low hydraulic conductivities. This property, together with the large retention capacity of clays for cationic contaminants, has brought argillaceous formations into focus as potential host rocks for the geological disposal of radioactive and other waste. In several countries, programmes are under way to characterise the detailed transport properties of such formations at depth. In this context, the interpretation of profiles of natural tracers in pore waters across the formations can give valuable information about the large-scale and long-term transport behaviour of these formations. Here, tracer-profile data, obtained by various methods of pore-water extraction for nine sites in central Europe, are compiled. Data at each site comprise some or all of the conservative tracers: anions (Cl(-), Br(-)), water isotopes (delta(18)O, delta(2)H) and noble gases (mainly He). Based on a careful evaluation of the palaeo-hydrogeological evolution at each site, model scenarios are derived for initial and boundary pore-water compositions and an attempt is made to numerically reproduce the observed tracer distributions in a consistent way for all tracers and sites, using transport parameters derived from laboratory or in situ tests. The comprehensive results from this project have been reported in Mazurek et al. (2009). Here the results for three sites are presented in detail, but the conclusions are based on model interpretations of the entire data set. In essentially all cases, the shapes of the profiles can be explained by diffusion acting as the dominant transport process over periods of several thousands to several millions of years and at the length scales of the profiles. Transport by advection has a negligible influence on the observed profiles at most sites, as can be shown by estimating the maximum advection velocities that still give acceptable fits of the model with the data. The advantages and disadvantages of different conservative tracers are also assessed. The anion Cl(-) is well suited as a natural tracer in aquitards, because its concentration varies considerably in environmental waters. It can easily be measured, although the uncertainty regarding the fraction of the pore space that is accessible to anions in clays remains an issue. The stable water isotopes are also well suited, but they are more difficult to measure and their values generally exhibit a smaller relative range of variation. Chlorine isotopes (delta(37)Cl) and He are more difficult to interpret because initial and boundary conditions cannot easily be constrained by independent evidence. It is also shown that the existence of perturbing events such as the activation of aquifers due to uplift and erosion, leading to relatively sharp changes of boundary conditions, can be considered as a pre-requisite to obtain well-interpretable tracer signatures. On the other hand, gradual changes of boundary conditions are more difficult to parameterise and so may preclude a clear interpretation