Jean-Jacques Royer

Curvature Attribute from Surface-Restoration as Predictor Variable in Kupferschiefer Copper Potentials

This work explains a procedure to predict Cu potentials in the ore-Kupferschiefer using structural surface-restoration and logistic regression (LR) analysis. The predictor in the assessments are established from the restored horizon that contains the ore-series. Applying flexural-slip to unfold/unfault the 3D model of the Fore-Sudetic Monocline, we obtained curvature for each restored time. We found that curvature represents one of the main structural features related to the Cu mineralization. Maximum curvature corresponds to high internal deformation in the restored layers, evidencing faulting and damaged areas in the 3D model. Thus, curvature may highlight fault systems that drove fluid circulation from the basement and host the early mineralization stages. In the Cu potential modeling, curvature, distance to the Fore-Sudetic Block and depth of restored Zechstein at Cretaceous time are used as predictors and proven Cu-potential areas as targets. Then, we applied LR analysis establishing the separating function between mineralized and non-mineralized locations. The LR models show positive correspondence between predicted probabilities of Cu-potentials and curvature estimated on the surface depicting the mineralized layer. Nevertheless, predicted probabilities are particularly higher using curvatures obtained from Late Paleozoic and Late Triassic restorations.

Permeability characterization of the Soultz and Ogachi large-scale reservoir using induced microseismicity

Hydraulic fracturing is a common procedure to increase the permeability of a reservoir. It consists in injecting high-pressure fluid into pilot boreholes. These hydraulic tests induce locally seismic emission (microseismicity) from which large-scale permeability estimates can be derived assuming a diffusion-like process of the pore pressure into the surrounding stimulated rocks. Such a procedure is applied on six data sets collected in the vicinity of two geothermal sites at Soultz (France) and Ogachi (Japan). The results show that the method is adequate to estimate large-scale permeability tensors at different depths in the reservoir. Such an approach provides permeability of the medium before fracturing compatible with in situ measurements. Using a line source formulation of the diffusion equation rather than a classical point source approach, improvements are proposed for accounting in situation where the injection is performed on a well section. This technique applied to successive fluid-injection tests indicates an increase in permeability by an order of magnitude. The underestimates observed in some cases are attributed to the difference of scale at which the permeability is estimated (some 1 km 3 corresponding to the seismic active volume of rock compared to a few meters around the well for the pumping or pressure oscillation tests). One advantage of the proposed method is that it provides permeability tensor estimates at the reservoir scale.

A new methodology to account for uncertainties in 4D seismic interpretation

Summary Multi-attribute interpretation with discriminant analysis is an efficient tool to interpret 4D seismic data in terms of reservoir changes. Yet, because this met hod cannot take into account data uncertainties which are in most cases significant, the interpretation may be questionable. The following paper presents an algorithm based on interval arithmetic, which allows to assess the impact of the data errors on the interpretation. An application is proposed on a heavy oil pool produced by steam injection, where a base and two repetitive surveys are available. It includes two parts: first, the quantification of 4D data uncertainties and secondly, their propagation and final impact on the interpretation results.

Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: II. Thermal modelling and implications for the mechanisms of uranium mineralization

A two-dimensional (2-D) thermal modelling of the Saint Sylvestre—western Marche leucogranite complex (northwestern Limousin, French Massif Central, FMC) was conducted to help constrain the cooling history and the uranium mineralization postdating the time of intrusion by 40–50 m.y. Numerical simulation of the post-emplacement cooling of the complex (<700°C) indicates that the pluton thermally equilibrated with its country rocks (at around 360–400°C and a mean depth of 10.5 km) by conduction in as little as 4 m.y. after its intrusion age (324 ± 4 Ma). Integration of the regional 40Ar/39Ar muscovite data in the 2-D model with an assumed universal Ar closure temperature of 325 ± 25°C reveals several sub-stages in the subsolidus cooling of the complex. The cooling pattern regionally defined by the muscovite data indicates that cooling was driven by the extensional exhumation and erosion of the thickened crust (+ intrusion) at a mean denudation rate of 0.3 mm y−1 Complete cooling below 325°C ended at 301 Ma via a transient faster denudation rate of 1.5 mm y−1 at the Westphalian-Stephanian boundary. The genetic relationships between the fluid circulations, the mineralization, and the cooling history of the pluton are discussed with particular emphasis on the tectonic process driving exhumation (extension). The initiation of the regional uplift at ∼320 Ma triggered at depth the circulation of in situ derived low-density aqueous fluids that reacted with the granite to form large vertical dissolution conduits (episyenites) characterized by the strong leaching of SiO2. The hydrothermal alteration was further enhanced during uplift by the structurally focused throughput of large volumes of aqueous fluids along brittle faults cutting across the laccolith. These conduits acted more than 20–30 m.y. after the trap formation as preferential channelways for the U-ore deposition at 270–280 Ma, due to sustained hydrothermal circulation adjacent to the high-heat producing terminal injections emplaced along the vertical fault zones and the metasomatic columns.

3D modeling of uranium-bearing solution-collapse breccias in Proterozoic sandstones (Athabasca Basin, Canada)-Metallogenic interpretations

Unconformity-related uranium deposits are the highest grade, large tonnage uranium resources in the world. In the Athabasca Basin (northern Saskatchewan, Canada), which is the premier host for unconformity-type deposits, the ore deposits are frequently hosted and surrounded by breccias in sandstone. The significance of these breccias and their relation to mineralization are of major importance for the genesis of these high-grade deposits. Therefore, a modeling study, integrating results from structural geology and petrology, was performed with the gOcad 3D modeling software, in order to decipher geometrical and geological relationships between breccias, faults and mineralization zones. Mineralized bodies and the sudoite-dravite breccia bodies display strong spatial correlations. They appear to be controlled by reverse shear zones cross-cutting the unconformity and containing graphite in the basement. Geochemical computations evidenced that volumetric water-rock ratios up to 10,000 could be obtained in these breccia bodies for volume losses of up to 90%. Assuming reasonable values for quartz saturation, hydraulic conductivity and connected porosity, the minimal fluid volume and the time duration necessary to generate the sudoite-dravite breccia bodies were estimated at ca. 2km^3 and ca. 1Myr, respectively. The comparison of these results with literature data suggests that the formation of sudoite-dravite breccia and mineralization could have been coeval. It may be proposed that within the space created by the quartz dissolution in the breccia body, a mixing between basement and basinal fluids could have induced U deposition and allowed the development of high-grade mineralization. The first-order uranium solubility that this coeval formation would imply is consistent with literature data, which suggests that this conceptual model is reasonable.

Relationships between granitoids and mineral deposits: three-dimensional modelling of the Variscan Limousin Province (NW French Massif Central)

Multidisciplinary three-dimensional modelling, involving geophysical, structural and geochemical data, has been used to study the relationships between magmatism, tectonics, fluid circulation and mineralisation in the northern Limousin, and to provide P-T-Z-t paths constrained by the available dating. The ore deposit occurrence displays little spatial relationship with granites emplaced in the 360-320 Ma period, probably because the low global permeability and tectonic regime did not allowed vertical fluid exchanges to be established. In contrast, the change in the tectonic regime induced by the delamination of the lower lithosphere (320-300 Ma), and characterised by the passage to general extension, has played a major metallogenic role. However, the ore deposit processes appear to be specific to each metal. Most of the W-Sn deposits appear to be synchronous with rare metal granites emplacement, at c. 310 Ma, that allowed the focus of fluids of different origins towards the apex of plutons. In contrast, for Au and U, the whole mineralisation process covers several tens of-millions of years. It is controlled by the regional tectonic evolution of the Limousin area during the same period, and especially by a rapid exhumation of the ductile crust which occurred in the 310-300 Ma period.

L'altération naturelle des scories de la métallurgie ancienne : un analogue de déchets vitrifiés

The study of the natural alteration of ancient vitreous slags (100 to 4 000 years) suggests a single global mechanism. In a first stage, weathering consists of a selective extraction of the modifier cations of glass (including Pb and Ba). This phenomenon is associated with an increase of the solution pH, inducing the glass dissolution. The elements with a weak limit of solubility remain in place and can form hydroxides (Al, Fe). Pb and Ba are extracted from weathered glass. Pb is located in Fe-hydroxides, Ba may form sulfides. The rate of alteration varies from 20 to 180 μm per 1 000 years.

A new nonparametric discriminant analysis algorithm accounting for bounded data errors

In a statistical pattern recognition context, discriminant analysis is designed to classify, when possible, objects into predefined categories. Because this method requires precise input data, uncertainties cannot be propagated in the classifying process. In real case studies, this could lead to drastic misinterpretations of objects. A new nonparametric algorithm based on interval arithmetic has thus been developed to propagate interval-form data. They consist in calculating interval conditional probability density functions and interval posterior probabilities. Objects are eventually assigned to a subset of classes, consistent with the data and their uncertainties. The classifying model is thus less precise, but more realistic than the standard one, which we prove on a real case study.

Estimating heterogeneous reservoir permeability from induced microseismicity

Few methods give direct insights on reservoir permeability. The induced micro-seismicity from hydro-fracturing experiments provides indirect information on reservoir permeability. Several existing S.B.R.C.-based inversion methods have been proposed to estimate the diffusivity of reservoir before fracturing. In this paper, we extend this approach to estimate the heterogeneous diffusivity of an anisotropic 3D medium and its time evolution during the stimulation. This approach is applied on the HDR geothermal reservoir at Soultz-Les-Forets, France.

Stochastic Imaging of Environmental Data

Interpretation of environmental data is currently confronted with the problem of estimating the spatial variation of a parameter from a limited number of sample points irregularly distributed in space. The challenge is to extract the relevant information for a given problem from the individual observations at control points. For example, in risk analysis (water resource monitoring, overflow forecasting, pollution monitoring), the emphasis is on detecting the maximum value or the anomalies of the appropriate parameter.