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Hydrothermal Alteration in the New Deep Geothermal Well GIL-1 (Strasbourg Area, France)

Abstract : The first geothermal well of Illkirch located South of Strasbourg (France), GIL-1, has been drilled to 3.8 km deep in a granitic basement. Drill cuttings and geophysical logs from basement were investigated in terms of hydrothermal alteration and natural fractures respectively. Petrographic observations of the 250 cuttings samples between 2900 and 3800 m MD were conducted on-site during the drilling with binocular loupe and enabled to identify the hydrothermal alteration grades in the open-hole granitic section of the well. From binocular examination, secondary minerals like drusy quartz, carbonates and anhydrite were spatially correlated to the occurrences of natural fractures. In the granitic section corresponding to propylitic alteration, phyllosilicates include primary biotite, muscovite and secondary chlorite. Then, 48 cuttings samples were analyzed by X-ray diffraction (XRD) to identify the secondary clay mineralogy corresponding to poorly crystallized illite (PCI) and illite-rich illite-smectite mixed layers (I/S ML) which generally takes place within fractured zones (FZs). Mud logging and geophysical logs acquired in the granitic section of GIL-1 well have been used for characterizing fracture location (calcimetry, temperature log) and fracture orientation (electrical image logs). The granitic section of the well is characterized by a dense network of natural fractures. From 2900 to 3300 m MD, about 220 electrical conductive fractures have been observed on the image logs. From temperature logs, three main permeable zones have been identified and correlated with the occurrences of natural conductive fractures. From 2900 to 3200 m MD, the clay signature is mainly governed by the occurrences of PCI and I/S ML which are correlated to permeable FZs. In the deepest part of the granitic section, secondary chlorite and possible secondary well crystallized illite (WCI) have been observed. In parallel, an innovative shortwave infrared (SWIR) spectroscopy method was developed for characterization of clay minerals in those fractured reservoirs. It was first applied to the granitic section of the Soultz and Rittershoffen geothermal wells (Alsace, France). This time-saving method allows the quantification of illitic hydrothermal alteration and indirectly the recognition of permeable FZs. In perspective, SWIR data acquired in GIL-1 will also be analyzed in order to confirm the applicability of this routine and cheap method for deep geothermal well in the Upper Rhine Graben (URG). 1. INTRODUCTION Since more than 30 years, a geothermal rush was observed in the Upper Rhine Graben (URG). The pioneer project of Soultz-sous-Forêts investigated the naturally fractured granitic reservoir within four geothermal deep wells (5 km). The nearby Rittershoffen project took then advantage of the lessons learned from Soultz by drilling two deep geothermal wells (3km) in the granitic basement targeting the multiscale network of fractures. Two more geothermal projects are under drilling operations in the Strasbourg area, in Vendenheim and in Illkirch. This contribution focuses on the preliminary geological results (petrography, secondary mineralogy, natural fractures) collected in the granitic section of the Illkirch GIL-1 well drilled to 3.8 km. The mid-Carboniferous granitic basement of the URG has been affected by several extensional and compressional tectonic phases which developed a multi-scale fracture network (Schumacher, 2002; Villemin and Bergerat, 1987). In deep geothermal projects, the target is to cross these natural fracture zones (FZs) which are today the seat of hydrothermal circulations and act as the main pathways for the natural brine through wide convection cells (Baillieux et al., 2013; Dezayes and Lerouge, 2019; Schellschmidt and Clauser, 1996). At the top of the highly fractured granitic basement, several temperature anomalies localized along the main faults are the expression of the fluid circulation in the FZs (Figure 1). In these complex FZs, specific mineralogical assemblages are evidences of paleo or present hydrothermal circulations. Clay minerals like illite, chlorite and tosudite are very sensitive to the fluid/rock (F/R) ratio, to the pH and to the temperature and are systematically precipitated in FZs (Ledésert et al., 2010). But the occurrence of other secondary minerals like drusy quartz, carbonates, barite, and anhydrite is also associated to fracture infillings (Smith et al., 1998; Traineau et al., 1992). Hence, this study will investigate the alteration mineralogy in the granitic section of the deviated GIL-1 well drilled between 2.9 and 3.8 km. Alteration mineralogy from about 250 cuttings will be derived from petrographic observation with binocular loupe and with X-ray diffraction (XRD). These petrographic results will be linked to the FZs occurrences on the electrical image logs. The goal will be to identify the clay signature of FZs in the granitic basement. These preliminary results aim to calibrate the shortwave infrared (SWIR) data acquired in the GIL-1 well. This innovative SWIR method already demonstrated its applicability to the granitic basement of the URG with the Soultz and Rittershoffen case studies whose examples will also be presented (Glaas et al., 2019; Vidal et al., 2018a).
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Carole Glaas, Patricia Patrier, Jeanne Vidal, Daniel Beaufort, Jean-François Girard, et al.. Hydrothermal Alteration in the New Deep Geothermal Well GIL-1 (Strasbourg Area, France). Proceedings World Geothermal Congress, Apr 2020, Reykjavik, Iceland. ⟨hal-02435192⟩

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