Caractérisation géologique et géométrique de la fracturation naturelle d'une formation granitique à partir de carottes de forages

Résumé.

Un site granitique non affleurant pressenti pour installer un laboratoire souterrain de recherche sur le stockage des déchets radioactifs a été échantillonné par deux forages aux carottes réorientées, l'un vertical, l'autre oblique. Ils ont permis une étude systématique de la fracturation, aspect considéré comme déterminant pour un projet de stockage. On présente ici, dans le contexte structural du site, celui du Seuil du Poitou, une description hiérarchisée des discontinuités, des failles supposées hectométriques aux fissures décimétriques. On propose un classement morphogénétique, préalablement à une répartition en familles directionnelles donnant une image simplifiée mais réaliste de l'organisation structurale de la formation magmatique traversée. Afin de caractériser les classes de fractures définies, on essaie d'apprécier leur rôle hydraulique et leur extension, propriétés inconnues en forage bien qu'essentielles pour une estimation du comportement hydro-mécanique du massif.

Abstract

As part of a long-term radioactive waste management consideration, the feasibility of creating a deep geological repository in a granitic formation is being studied in France. In 1994, geophysical maps led to the selection of a 225 km² study area in the Seuil du Poitou region. In this area, the granitic rocks of Variscan age overlain by a sedimentary cover were investigated through 16 cored boreholes. In 1996, the geological and hydrogeological results indicated that a site in a 20 km² zone, away from NW-SE trending regional faults, would be sufficiently consistent in character for an underground research laboratory to be constructed. Weaknesses of the rock mass determine its hydraulic and strength properties which have a major influence on the suitability of a site.

The 20 km² site was initially explored with one vertical borehole (Cha112) and subsequently with an oblique hole (Cha212) drilled at 60° to the horizontal on a bearing 78°N, based on the statistical maximum of the fracture orientations measured in Cha112. Systematic structural and petrographical data were obtained from the cores of these boreholes.

The purpose of this paper is to outline a classification for the various discontinuities observed in the cores and to appreciate two properties that are not directly measurable – hydraulicity and size – which have to be assumed from other parameters such as filling thickness. Nearly all the fractures cross-cut by the two boreholes are sealed by hydrothermal mineral paragenesis, including epidote, quartz, chlorite, carbonates and clay minerals. Based on the occurrence of macroscopic voids or porosity and breccia filling, it appeared that only a small number of major fracture zones would be likely to be water-bearing. From a hydrogeological qualitative point of view, it was proposed that fractures with a thickness filling more than 10 mm could be significant. This medium category of fracturation can be considered statistically. After detailed observation, the smaller fracturation category was found to consist of veins, fractures (metric scale), fissures (core scale) and cracks.

A consideration of the genesis and morphology of the fractures suggested three groups: magmatic veins, tension fractures and faults. After observation of trace persistence in pluton outcrops similar to the site rocks, empirical relationships between filling thickness and discontinuity size were assumed. It was then considered realistic to divide the population of discontinuities in each class into directional sets, identified from stereography and cluster analysis. Cinematic data were not sufficiently clear to determine any paleostress state.

The geological and geometrical description and classification of natural discontinuities reported in this paper were required in order to define relevant rock structure models for hydro-mechanical simulations.