Occurrences of Clay Minerals in Permeable Fracture Zones in the Granitic Basement of Geothermal Wells at Rittershoffen, France

Abstract

Two geothermal wells, GRT-1 and GRT-2, were drilled in a granite at Rittershoffen (Alsace, France), in the Upper Rhine Graben, in order to exploit the geothermal resource at the sediment-basement reservoir. Brine circulations occur in a permeable fracture network and lead to hydrothermal alteration. The goal of the study is to characterize the hydrothermal alteration associated with permeable fracture zones in Rittershoffen. As clay minerals are very reactive to hydrothermal alteration and could be good indicator to past and present circulations. A special attention has been paid to the textural, structural and chemical properties of these minerals. The clay fractions (lower than 5 microns) were analyzed in fifty cuttings samples selected around permeable fracture zones in order to analyze the microstructural properties of clays from X-Ray Diffraction and the chemical composition of clays from Scanning Electron Microscope coupled with EDS. Chloritization (ferromagnesian chlorites) and sericitization (well crystallized illite) associated with pervasive alteration are observed along wells. Iron-rich chloritic materials were observed in fracture fillings as well as illitic materials enriched in poorly crystallized illite and I/S mixed layers (lower than 10% of smectite) related to hydrothermal alteration as evidenced in both wells in permeable fracture zones. The heterogeneous assemblages of I/S mixed layers and the small-size of illite crystallites could be a response to an abrupt change of physical and chemical conditions such as the mixing of an oversaturated ascending geothermal fluid with a cooler fluid favoring nucleation of metastable phases. The intense illitization associated with fluid circulations may lead to a total replacement of chloritic materials as observed in the major permeable fracture zone of GRT-1 and a reduction of the natural permeability. The well GRT-2 presents four permeable fracture zones in the granitic basement with a good natural permeability probably because they are less affected by illitization.