Relative chronology of deep circulations within the fractured basement of the Upper Rhine Graben

Chrystel Dezayes (1) , Catherine Lerouge (1) , Claire Ramboz (2) , Guillaume Wille (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327

In the Upper Rhine Graben, geothermal projects are strongly under development, particularly for the exploitation of fluid within the top of the basement. This type of reservoirs constitutes a fractured dominated system. However, the hydraulic behaviour of the fracture network is poorly known, such as connexion, main fluid flow direction, role of the fault zone versus minor fractures, the deformation types (cataclase, fault, crack...). We propose two approaches to understand the fluid flow for a better geothermal exploitation in the Upper Rhine Graben. The first approach is to study the fillings of the fracture which plug partially or totally the current fluid flow. The second approach is to determine the nature of the palæo-circulations by studying fracture fillings, their textures and establish mineral sequence of fillings in relation to the tectonic phases and the geological history of the basement. And finally we compare chemistry of palæo-fluids with those of present-day fluids which flow in the deeper part of the Upper Rhine Graben. In this work, we propose a combined structural and mineralogical analysis of the fractures and their fillings based on samples collected on the EPS1 Soultz cores and outcrops on the both flanks of the Upper Rhine Graben. These mineralogical fillings have been sampled within fractures identified by measuring their orientation and defining their relative age. Petrological study (Optical microscope, Scanning Electron Microscope noted SEM, cathodoluminescence noted CL) completed by chemical analyses (Electron Microprobe Analyses, noted EPMA) have been performed in order to determine the filling minerals and their relationships. Among filling minerals, quartz, carbonates and illite are the major ones. Different types and generations of carbonates (calcite, dolomite, ankerite, Mn-bearing carbonates and siderite) have been highlighted at the scale of the major fracture zones in the upper part of granite, using CL and EPMA. Combined mineralogical, textural and structural data allow establishing a chronology of the palæo-circulations relative to the tectonic phases and the geological history of the basement. We identify the major phases which plug the fractures and, consequently, the set of fractures which remain more permeable to the current fluid flow.

Mots clés: Rhine Graben; granite; fracture; mineralogical filling; paleocirculation

Domaines: Sciences de la Terre

Chrystel Dezayes, Catherine Lerouge, Claire Ramboz, Guillaume Wille. Relative chronology of deep circulations within the fractured basement of the Upper Rhine Graben. European Geothermal Congress, EGC 2013, Jun 2013, Pise, Italy. 10 p. ⟨hal-00788988⟩