Contribution of SWIR to the Clay Signature of Permeable Fracture Zones in the Granitic Basement. Overview of Soultz and Rittershoffen wells

Abstract : The high potential of the Upper Rhine Graben (URG) for geothermal relies on more than 30 years of expertise in crystalline geothermal reservoirs developed in Soultz (France), the near presence of direct users in the area of the Eurometropolis of Strasbourg and undeniably, the acceptance of the population and the presence of investors. Hence, Strasbourg area is very attractive and is very active in the development of deep geothermal plants. In this context, this study proposes an innovative method of geothermal exploration to detect the highly hydrothermally altered zones corresponding to permeable fracture zones (FZ). They are acting as natural pathways for the fluid in the crystalline rocks of the URG. Applied on cuttings of granite samples with reduced costs and time, the short wave infrared (SWIR) spectroscopy method was realized on geothermal wells from Northern Alsace (Soultz, Rittershoffen) for calibration. Indeed, the FZ of these wells were first characterized by structural and mineralogical studies done from borehole image logs and cuttings analysis respectively. In the Rittershoffen wells, the occurrence of small crystallites of illite and illite-smectite mixed layers minerals (<10% smectite) were systematically and spatially linked with the occurrence of permeable FZ and thus provide specific clay signatures. In the Soultz wells, mineralogy and FZ structure were deeply studied with core samples descriptions. SWIR spectra were acquired on more than 2400 cuttings in Soultz and Rittershoffen wells. The area of the SWIR peak at 2200 nm wavelength correlates with the amount of illitic minerals and can thus be a direct indicator of argilization and alteration. Observations show that SWIR results correlate with the former X-ray diffraction (XRD) results, binocular magnifier or core observations identifying the several granitic facies affected by hydrothermal alteration. Higher the hydrothermal alteration is and higher the SWIR signal is. SWIR spectroscopy is thus a promising tool to estimate the alteration intensity, and could be a complementary tool for the characterization of FZ architecture. Using routinely field SWIR spectroscopy on crystalline cuttings could be a pioneer method to characterize FZ and their permeability at early stages of geothermal exploration wells.
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Carole Glaas, Jeanne Vidal, Patricia Patrier, Daniel Beaufort, Albert Genter. Contribution of SWIR to the Clay Signature of Permeable Fracture Zones in the Granitic Basement. Overview of Soultz and Rittershoffen wells. European Geothermal Congress, Jun 2019, Den Haag, Netherlands. ⟨hal-02435184⟩

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