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

• Main Authors: Glaas, Carole, Patrier, Patricia, Vidal, Jeanne, Beaufort, Daniel, Girard, Jean-François, Genter, Albert
• Other Authors: ESGéothermie (GroupeES), Électricité de Strasbourg Géothermie (ES Géothermie), Institut de Chimie des Milieux et Matériaux de Poitiers UMR 7285 (IC2MP Poitiers ), Université de Poitiers = University of Poitiers (UP)-Institut national des sciences de l'Univers (INSU - CNRS)-Institut de Chimie - CNRS Chimie (INC-CNRS)-Centre National de la Recherche Scientifique (CNRS), Institut de physique du globe de Strasbourg (IPGS), Université de Strasbourg (UNISTRA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Bureau de Recherches Géologiques et Minières (BRGM) (BRGM)
• Format: Conference Object
• Language: English
• Published: HAL CCSD 2020
• Subjects: illite; clay minerals; fracture zone; granitic reservoir; X-ray diffraction (XRD); Short Wave Infrared (SWIR); Illkirch; Upper Rhine Graben (URG); [SDU.STU.AG]Sciences of the Universe [physics]/Earth Sciences/Applied geology; [SDU.STU.PE]Sciences of the Universe [physics]/Earth Sciences/Petrography; [SDU.STU.MI]Sciences of the Universe [physics]/Earth Sciences/Mineralogy; Iceland
• Online Access: https://cnrs.hal.science/hal-02435192; https://cnrs.hal.science/hal-02435192/document; https://cnrs.hal.science/hal-02435192/file/Glaas%20et%20al%20WGC%202020.pdf

International audience 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 ...