Martin Mazurek

PREFERRED ORIENTATIONS AND ANISOTROPY IN SHALES: CALLOVO-OXFORDIAN SHALE (FRANCE) AND OPALINUS CLAY (SWITZERLAND)

Anisotropy in clay-rich sedimentary rocks is receiving increasing attention. Seismic anisotropy is essential in the prospecting for petroleum deposits. Anisotropy of diffusion has become relevant for environmental contaminants, including nuclear waste. In both cases, the orientation of component minerals is a critical ingredient and, largely because of small grain size and poor crystallinity, the orientation distribution of clay minerals has been difficult to quantify. A method is demonstrated that relies on hard synchrotron X-rays to obtain diffraction images of shales and applies the crystallographic Rietveld method to deconvolute the images and extract quantitative information about phase fractions and preferred orientation that can then be used to model macroscopic physical properties. The method is applied to shales from European studies which investigate the suitability of shales as potential nuclear waste repositories (Meuse/Haute-Marne Underground Research Laboratory near Bure, France, and Benken borehole and Mont Terri Rock Laboratory, Switzerland). A Callovo-Oxfordian shale from Meuse/Haute-Marne shows a relatively weak alignment of clay minerals and a random distribution for calcite. Opalinus shales from Benken and Mont Terri show strong alignment of illite-smectite, kaolinite, chlorite, and calcite. This intrinsic contribution to anisotropy is consistent with macroscopic physical properties where anisotropy is caused both by the orientation distribution of crystallites and high-aspect-ratio pores. Polycrystal elastic properties are obtained by averaging single crystal properties over the orientation distribution and polyphase properties by averaging over all phases. From elastic properties we obtain anisotropies for p waves ranging from 7 to 22%.

Solute transport in crystalline rocks at Aspo ¨— I: Geological basis and model calibration

Water-conducting faults and fractures were studied in the granite-hosted Aspö Hard Rock Laboratory (SE Sweden). On a scale of decametres and larger, steeply dipping faults dominate and contain a variety of different fault rocks (mylonites, cataclasites, fault gouges). On a smaller scale, somewhat less regular fracture patterns were found. Conceptual models of the fault and fracture geometries and of the properties of rock types adjacent to fractures were derived and used as input for the modelling of in situ dipole tracer tests that were conducted in the framework of the Tracer Retention Understanding Experiment (TRUE-1) on a scale of metres. After the identification of all relevant transport and retardation processes, blind predictions of the breakthroughs of conservative to moderately sorbing tracers were calculated and then compared with the experimental data. This paper provides the geological basis and model calibration, while the predictive and inverse modelling work is the topic of the companion paper [J. Contam. Hydrol. 61 (2003) 175]. The TRUE-1 experimental volume is highly fractured and contains the same types of fault rocks and alterations as on the decametric scale. The experimental flow field was modelled on the basis of a 2D-streamtube formalism with an underlying homogeneous and isotropic transmissivity field. Tracer transport was modelled using the dual porosity medium approach, which is linked to the flow model by the flow porosity. Given the substantial pumping rates in the extraction borehole, the transport domain has a maximum width of a few centimetres only. It is concluded that both the uncertainty with regard to the length of individual fractures and the detailed geometry of the network along the flowpath between injection and extraction boreholes are not critical because flow is largely one-dimensional, whether through a single fracture or a network. Process identification and model calibration were based on a single uranine breakthrough (test PDT3), which clearly showed that matrix diffusion had to be included in the model even over the short experimental time scales, evidenced by a characteristic shape of the trailing edge of the breakthrough curve. Using the geological information and therefore considering limited matrix diffusion into a thin fault gouge horizon resulted in a good fit to the experiment. On the other hand, fresh granite was found not to interact noticeably with the tracers over the time scales of the experiments. While fracture-filling gouge materials are very efficient in retarding tracers over short periods of time (hours-days), their volume is very small and, with time progressing, retardation will be dominated by altered wall rock and, finally, by fresh granite. In such rocks, both porosity (and therefore the effective diffusion coefficient) and sorption K(d)s are more than one order of magnitude smaller compared to fault gouge, thus indicating that long-term retardation is expected to occur but to be less pronounced.

Thermal constraints on crustal rare gas release and migration: Evidence from Alpine fluid inclusions

The Palfris marl of the Helvetic Alpine Nappes contains four distinct vein fill generations. CH4-rich gas is found in abundant fluid inclusions within these carbonate veins, while free CH4 gas has also been produced from exploratory boreholes through this formation. The stable isotope and helium, neon, and argon isotopic composition of these fluids has been determined. A constant radiogenic 40Ar concentration of 1.25 ± 0.13 (1σ) ppm in these differently sited fluids requires an intimate association between the 40Arrad source and the hydrocarbon phase. This can only be reasonably explained if the 40Arrad was input into the hydrocarbon phase during hydrocarbon generation, migration, or storage prior to entrapment in the fluid inclusions. Stable isotope results constrain the maturity of hydrocarbon production, while fluid inclusion formation pressures and temperatures record values of up to 2.5 kbars and 250°C. These values place limits on the range of thermal conditions in which the hydrocarbon/40Arrad relationship was established. All fluids within inclusions also contain radiogenic 4He40Ar values at predicted crustal production ratios. These observations provide the first evidence that both 4Herad and 40Arrad can be quantitatively released on a regional scale bounded by the thermal conditions required to produce the hydrocarbon phase and the conditions under which the fluid inclusions were formed (T = 190–250°C). These results require that negligible quantities of excess 40Arrad, decoupled from 4Herad, have been released into this system. Given the wide array of mechanisms which can potentially cause decoupling of these two species, this result provides an important constraint on the role of these processes within the sedimentary fluid regime. In contrast, the free borehole gas contains excess radiogenic 4He and 21Ne, relative to 40Arrad, in proportions which can be accounted for by local production and subsequent diffusion from the surrounding marl. The latter pattern is consistent with rare gas migration in lower temperature environments. A conceptual model which considers both diffusional and metamorphic release of helium and argon, and the ability of the surrounding fluid regime to transport the rare gases from their respective mineral production sites, is consistent with both these results and data from regional rare gas studies.

Geological and Hydraulic Properties of Water-Conducting Features in Crystalline Rocks

Geological and hydrogeological field evidence from several sites (Grimsel Test Site, Aspo Hard Rock Laboratory, deep boreholes in northern Switzerland, various mines) shows that in spite of contrasting geological settings, evolutions and ages, several common characteristics of water-conducting features exist in crystalline basement rocks. Geometric and hydraulic properties of water-conducting features depend mainly on the mechanism of brittle deformation (e.g. faulting, jointing), on the nature and intensity of water/rock interactions (e.g. hydrothermal fracture sealing) and on rock type. Leucocratic rocks, such as aplite/pegmatite dykes, have higher fracture frequencies and transmissivities when compared to more basic rocks. Brittle deformation in most crystalline-rock environments occurred recurrently, and pre-existing structures (e.g. lithologic contacts, ductile shear-zones, older fault and fracture generations) were preferentially reactivated. Faults of different sizes, ranging from small cataclastic zones to regional lineaments, are the most important structures in which flow occurs. Due to the complex architecture of faults in directions parallel and perpendicular to strike, the spatial distribution of flow in faults is very heterogeneous. Hydrothermal alteration events lead to fracture scaling by mineral precipitation or to increased apertures due to the dissolution of pre-existing fracture infills, thereby enhancing the heterogeneity of the flowpaths on a small scale.

Anisotropy in shale from Mont Terri

Anisotropy of shales is the subject of this report, and we use an example of the Jurassic Opalinus Clay from Mont Terri (Switzerland) that is being investigated in the context of radioactive waste disposal. The study is targeted at the geomechanical characterization of shale by laboratory testing. The overall aim is to improve the constitutive material laws and their application in numerical models.

Solute transport in crystalline rocks at Äspö — II: Blind predictions, inverse modelling and lessons learnt from test STT1

Based on the results from detailed structural and petrological characterisation and on up-scaled laboratory values for sorption and diffusion, blind predictions were made for the STT1 dipole tracer test performed in the Swedish Aspö Hard Rock Laboratory. The tracers used were nonsorbing, such as uranine and tritiated water, weakly sorbing 22Na(+), 85Sr(2+), 47Ca(2+)and more strongly sorbing 86Rb(+), 133Ba(2+), 137Cs(+). Our model consists of two parts: (1) a flow part based on a 2D-streamtube formalism accounting for the natural background flow field and with an underlying homogeneous and isotropic transmissivity field and (2) a transport part in terms of the dual porosity medium approach which is linked to the flow part by the flow porosity. The calibration of the model was done using the data from one single uranine breakthrough (PDT3). The study clearly showed that matrix diffusion into a highly porous material, fault gouge, had to be included in our model evidenced by the characteristic shape of the breakthrough curve and in line with geological observations. After the disclosure of the measurements, it turned out that, in spite of the simplicity of our model, the prediction for the nonsorbing and weakly sorbing tracers was fairly good. The blind prediction for the more strongly sorbing tracers was in general less accurate. The reason for the good predictions is deemed to be the result of the choice of a model structure strongly based on geological observation. The breakthrough curves were inversely modelled to determine in situ values for the transport parameters and to draw consequences on the model structure applied. For good fits, only one additional fracture family in contact with cataclasite had to be taken into account, but no new transport mechanisms had to be invoked. The in situ values for the effective diffusion coefficient for fault gouge are a factor of 2-15 larger than the laboratory data. For cataclasite, both data sets have values comparable to laboratory data. The extracted K(d) values for the weakly sorbing tracers are larger than Swedish laboratory data by a factor of 25-60, but agree within a factor of 3-5 for the more strongly sorbing nuclides. The reason for the inconsistency concerning K(d)s is the use of fresh granite in the laboratory studies, whereas tracers in the field experiments interact only with fracture fault gouge and to a lesser extent with cataclasite both being mineralogically very different (e.g. clay-bearing) from the intact wall rock.

Derivation and application of a geologic dataset for flow modelling by discrete fracture networks in low-permeability argillaceous rocks

Abstract Argillaceous rock formations are targets of exploration for radioactive waste disposal sites in several countries. Groundwater flow in most indurated argillaceous rocks is very limited and occurs (if at all) mainly in brittle discontinuities, such as faults. On the basis of surface observations, core logging and hydraulic measurements in boreholes penetrating an argillaceous marl formation in the Swiss Alps, the relationships between internal fault architecture, larger-scale arrangement of the fault network and fault transmissivity are explored. Only a fraction of all faults observed in the cores correlates with water inflow points into the boreholes, and this is taken as evidence of variable transmissivity within each fault. Such flow channeling is also supported by geologic evidence. Stochastic discrete fracture network models are used for the upscaling of measured fault transmissivities, namely for the calculation of effective hydraulic conductivities ( K eff ) of model cubes with lengths of side of 50–500 m. Input data to these models include size, spacing, orientation, heterogeneity (flow channeling) and transmissivity of the faults. Fault size is not well known, but a sensitivity analysis shows that even the extreme assumption of infinite size yields K eff only less than 1 order of magnitude higher when compared to the base case. The effect of different arrangements of flow channels within each fault is also explored but has no appreciable effect on K eff . It is concluded that calculated values for K eff are robust because those input parameters that are least adequately known have only limited effects on the model results.

Fieldbook and geodatabases: tools for field data acquisition and analysis

Abstract We introduce FieldBook and GeoDatabase, 2 new and effective tools for geologic field data acquisition and analysis. FieldBook is an application for Apples Newton MessagePad. Geological data collected at the outcrop, including notes and drawings, can be entered directly and on-site. The formalization of the multiparameter information leads directly to a consistent database. This procedure results in a complete, up-to-date database where all information collected by different researchers in a project is available anytime, and no data are lost. GeoDatabase is an application based on FileMaker™ Pro, representing the FieldBook interface on PC/Macintosh. GeoDatabase provides extensive search possibilities and strong export features that are needed for field-data analysis, either in the field or in the office. It can be used as a central database within a local network with several users on either PC or a Macintosh. FieldBook and GeoDatabase both are simple to use, yet they satisfy the demands of field campaigns involving numerous scientists. Applications of field projects in the crystalline basement of the Salalah area and the Masirah ophiolite are given.

Evolution of gas and aqueous fluid in low-permeability argillaceous rocks during uplift and exhumation of the central Swiss Alps

Abstract Fluid chemistry and the hydraulic regime in a marl formation of the Swiss Alps were studied by a number of techniques. Fluid inclusions record the conditions of maximum burial and regional low-temperature metamorphism, whereas fluid samples and hydraulic tests derived from deep boreholes reflect present-day, near-surface conditions. The characterization of the different types of fluids places constraints on the geochemical and hydraulic evolution of low-permeability argillaceous rocks during uplift and exhumation. Fluid inclusions were studied by microthermometry and sampled directly by decrepitation techniques. They contain a two-phase system consisting of an aqueous fluid and a coexisting CH4-rich gas (T=190–250°C, Plith≈2500 bar). Bulk and isotopic compositions of aqueous fluid inclusions are consistent with a mixture of connate seawater and water derived from the dehydration of clay minerals. Methane was generated in situ by thermal cracking of kerogen. Textural evidence and stable isotopic signatures of carbonates in veins and in the rock matrix indicate local buffering of fluid compositions and very low water/rock ratios. Free fluids residing in the present-day fracture and matrix porosity consist of CH4-saturated Na–Cl groundwater with minute amounts of free CH4 gas which occurs in druses. Their chemical and isotopic compositions are very similar to those of the fluid inclusions, suggesting a common origin. Post-metamorphic admixtures of externally derived waters cannot be identified, and it is suggested that present-day Na–Cl groundwaters that occur in the central parts of the marl have resided in the formation since the time of metamorphism some 20 Ma b.p. The only major change in the fluid composition has been the outgassing of CH4 from the formation, most probably by diffusion. The hydraulic regime during metamorphism was characterized by localized fluid underpressures in open veins because widely scattered, sub-hydrostatic pressures were often identified in fluid inclusions. The central part of the argillaceous rock body, approximately coinciding with the region where Na–Cl groundwaters occur, has sub-hydrostatic pressures today, as indicated by hydraulic tests in deep boreholes. Both the closed-system behavior derived from the chemical and isotopic characteristics of the fluids and the (recurrent or continuous) existence of hydraulic underpressures suggest very low permeabilities of argillaceous rocks during metamorphism and throughout subsequent uplift and exhumation. All fluids present in the central parts of the formation are either connate or produced in situ. Even though major events of brittle faulting and unloading due to uplift occurred since the peak of metamorphism, fluid flow through the formation has been negligible.

MINERALOGICAL AND ISOTOPIC RECORD OF DIAGENESIS FROM THE OPALINUS CLAY FORMATION AT BENKEN, SWITZERLAND: IMPLICATIONS FOR THE MODELING OF PORE-WATER CHEMISTRY IN A CLAY FORMATION

Argillaceous rocks are considered to be a suitable geological barrier for the long-term containment of wastes. Their efficiency at retarding contaminant migration is assessed using reactive-transport experiments and modeling, the latter requiring a sound understanding of pore-water chemistry. The building of a pore-water model, which is mandatory for laboratory experiments mimicking in situ conditions, requires a detailed knowledge of the rock mineralogy and of minerals at equilibrium with present-day pore waters. Using a combination of petrological, mineralogical, and isotopic studies, the present study focused on the reduced Opalinus Clay formation (Fm) of the Benken borehole (30 km north of Zurich) which is intended for nuclear-waste disposal in Switzerland. A diagenetic sequence is proposed, which serves as a basis for determining the minerals stable in the formation and their textural relationships. Early cementation of dominant calcite, rare dolomite, and pyrite formed by bacterial sulfate reduction, was followed by formation of iron-rich calcite, ankerite, siderite, glauconite, (Ba, Sr) sulfates, and traces of sphalerite and galena. The distribution and abundance of siderite depends heavily on the depositional environment (and consequently on the water column). Benken sediment deposition during Aalenian times corresponds to an offshore environment with the early formation of siderite concretions at the water/sediment interface at the fluctuating boundary between the suboxic iron reduction and the sulfate reduction zones. Diagenetic minerals (carbonates except dolomite, sulfates, silicates) remained stable from their formation to the present. Based on these mineralogical and geochemical data, the mineral assemblage previously used for the geochemical model of the pore waters at Mont Terri may be applied to Benken without significant changes. These further investigations demonstrate the need for detailed mineralogical and geochemical study to refine the model of pore-water chemistry in a clay formation.