How Do Secondary Minerals in Granite Help Distinguish Paleo- from Present-Day Permeable Fracture Zones? Joint Interpretation of SWIR Spectroscopy and Geophysical Logs in the Geothermal Wells of Northern Alsace

Abstract

The investigation of permeable hydrothermally altered and fractured zones and their distribution is a key issue for the understanding of fluid circulation in granitic rocks, on which the success of geothermal projects relies. Based on the use of short-wave infrared (SWIR) spectroscopy applied to rock cuttings coupled with interpretation of geophysical logs, we propose an investigation of the clay signature of fault and fracture zones (FZ) inside the granitic basement. This methodology was applied to two geothermal wells: GRT-2 from the Rittershoffen and GPK-1 from the Soultz-sous-Forêts (Soultz) geothermal sites, both located in the Upper Rhine Graben (URG). A total of 1430 SWIR spectra were acquired and analysed. Variations in the 2200 nm absorption band area are correlated with hydrothermal alteration grades. The 2200 nm absorption band area is found to reflect the illite quantity and its variations in the granitic basement. Low, stable values are observed in the unaltered granite facies, showing good reproducibility of the method, whereas scattered high values are associated with high hydrothermal alteration and FZs. Variations in the 2200 nm absorption band area were correlated with the gamma ray and electrical resistivity logs. This procedure allowed us to confirm that illite mainly controls the resistivity response except inside the permeable FZs, where the resistivity response is controlled by the geothermal brine. Thus, the architecture of these permeable FZs was described precisely by using a combination of the 2200 nm absorption band area data and the electrical resistivity log. Moreover, by correlation with other geophysical logs (temperature (T), porosity, and density), paleo-permeable and currently permeable FZs inside the reservoir were distinguished. The correlation of SWIR spectroscopy with electrical resistivity logs appears to be a robust tool for geothermal exploration in granitic reservoirs in the URG.

The use of newly formed minerals to characterize geothermal reservoirs and their cap rock: examples from intracontinental extensional basins and volcanic islands in subduction context

“…The fracture zone is surrounded by two major open fractures with a highly altered zone in-between visible on flow logs and borehole imagery performed thanks to acoustic logging. Euhedral drusy quartz fragments were also found in the rock chips recovered during drilling at Rittershoffen at the same depths as temperature anomalies [42,112,117] indicating fluid flow and hence permeable zones. The permeability of the FZs seems to be influenced by the crystal growth of secondary quartz in fracture voids [118].…”

Contribution of the Paragenetic Sequence of Clay Minerals to Re-Examination of the Alteration Zoning in the Krafla Geothermal System

“…This equipment, which is largely used by mi-ning companies, is still poorly developed in geothermal system prospecting and is most often used for qualitative approaches (determination of alteration zoning). However, quantitative development is feasible, as illustrated by [93,94], if a careful calibration is performed at the field scale beforehand. Therefore, combining EM surveys, which are mandatory to obtain 3D imaging of the field, and SWIR logging could be a very promising approach for geothermal system prospecting in the future and could achieve a better dynamic picture of these systems.…”

Clay Minerals in the Deep Reservoir of the Cerro Pabellón Geothermal System (Northern Chile)

“…During the life of geothermal systems, the fluid circulations into the reservoir lead to dissolutions of primary minerals and glassy or fine-grained groundmass of volcanogenic host rocks and subsequent deposition of secondary minerals. The nature and amount of mineralogical parageneses associated with alteration of the deep rock samples could help to understand paleo and present-day circulations of the geothermal system [8][9][10][11][12][13][14][15][16]. As clay minerals are very sensitive to physicochemical changes (temperature, F/R ratio, time, nature of hydrothermal fluids, fluid state), special attention was paid to the clay minerals [17][18][19][20][21].…”