Hydrothermal Alteration in the New Deep Geothermal Well GIL-1 (Strasbourg Area, France)
Carole GLAAS, Patricia PATRIER, Jeanne VIDAL, Daniel BEAUFORT, Jean-François GIRARD, Albert GENTER
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 are investigated in terms of hydrothermal alteration and natural fractures respectively. Petrographic observations of the 250 cutting 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, primary biotite, muscovite and secondary chlorite have been observed. Then, 48 cutting samples were analyzed by X-ray diffraction (XRD) to identify the secondary clay mineralogy which generally takes place within fractured zones (FZs), corresponding to poorly crystallized illite (PCI) and illite-smectite mixed layers (I/S ML). 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, primary muscovite and/or secondary well crystallized illite (WCI) have been observed. In parallel, an innovative short wave 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 quantifying both hydrothermal alteration and the occurrences 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).
|        Topic: Geology||Paper Number: 12097|