Abstract

The localization of permeable fracture zones (FZ) that channelize the geothermal resource in the Upper Rhine Graben (URG) is a key challenge for geothermal projects. The study focuses on the results of short-wave infrared (SWIR) spectroscopy and more specifically the variations of the 2200 wavelength for cuttings of the two deep geothermal wells of Rittershoffen, drilled in the French part of the URG. The goal is to develop a useful tool for geological identification of permeable FZ in the granitic basement. SWIR results were correlated to XRD and mineralogical observations done in the granitic basement of those two wells. They allow identifying the extension of the main permeable FZ in the production well GRT-2 where structural data are not continuously available. Surprisingly, compared to previous mineralogical studies done with standard methods, variations of the ~2200 nm wavelength indicate a more intense illitization of the well GRT-2. The higher permeability of this well could be explained by a main permeable FZ more than three times thicker than the one in the injection well GRT-1. Also, in the GRT-2 well, the higher fracture density allows a better connection between the well and the reservoir. This application of the SWIR for identifying secondary clay minerals brings a promising, rapid and non-destructive tool to locate permeable FZ within the granitic basement for future geothermal projects in the Strasbourg area.