Potential interest areas for the development of geothermal energy in La Reunion Island
Résumé
La Réunion Island was always considered as a high interest for the geothermal exploitation and several exploration surveys were performed since the 70's. However, at the present day, no geothermal exploitation has been built. Nevertheless, the French environmental law, Grenelle de l'Environnement, has set ambitious goals for the French overseas territories and La Réunion must be self-powered by 2030. In this framework, BRGM and the Geoscience Laboratory of La Réunion University in collaboration with Région Réunion, LMV of Clermont-Ferrand University and La Réunion National Park, with the financial support of DREAL and ADEME, the French energy agency, have conducted a new exploration project. This project aims to perform a synthesis of the knowledge of the island and to define the most favourable areas for exploiting geothermal moderate and high temperature resources. This study is based on a bibliographic compilation integrating the previous geothermal exploration surveys from the 70's to the 2000's, the recent results of the La Réunion University studies and the heliborn magnetic and transient electromagnetism (TEM) survey of 2014. All these data were integrated in a Geographic Information System (GIS) in order to provide maps of geothermal interest. Therefore, three thematic maps have been performed (geology and geochemical data, geophysical data and environmental issues) and a global synthesis map showing the favourable areas for geothermal energy development based on the classical criteria used in volcanic context exploration, and also the criteria of non-conventional geothermal energy as Engineered Geothermal System (EGS):-High thermal gradient in boreholes-Hot springs, fumaroles, travertines, gas anomalies;-Geological discontinuities such as fractures, faults, and magmatic intrusions;-High electric resistivity layer constituting a reservoir overlaid by a conductive layer that can be a cap rock (model of Johnston et al., 1992). This synthesis leads to the definition of several geothermal targets, which can be grouped in three sets of interest:-The proven interest areas;-The interest areas to be confirmed;-The interest areas located in protected areas (inside the National Park). The analysis of the previous surveys shows that if thermal anomalies exist, permeability of the geological formations seems to be poor and insufficient for geothermal exploitation. There is a moderate chance that a classical volcanic system exists at La Réunion Island. Therefore, the next exploration phases should take into account the non-conventional geothermal exploitation, like EGS, which consist in improving the development of the wells for optimizing their productivity in order to reach a sufficient economic return.
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