Imaging of a Geothermal Reservoir in Volcanic Environment with Magnetic Data Supported by Structural and Petrophysical Analyses

Abstract

The GEOTREF Program aims to develop new concepts for the prospection of high enthalpy geothermal reservoirs in volcanic environments. The main target of the program is in Basse-Terre Island, Guadeloupe, French West Indies. One of the main ways to assert a conceptual model for this kind of reservoirs is to analyze an analogue outcrop developed in similar conditions than those expected for the prospected one. The studied analogue is in Terre-de-Haut Island, in Les Saintes archipelago, 15 km south to Basse-Terre. The central part of the island displays highly hydrothermalized materials surrounded by fresh or weakly altered materials. The structural analysis conducted onshore highlights three structural directions controlling the shape of the reservoir and the main pathways for hydrothermal circulation. The most altered material is located at the intersection of two structural directions, mainly N-S and E-W. From this area, faults control propagation of the hydrothermal fluid far from the main reservoir. Hydrothermal processes alter the magnetized minerals of volcanic rocks hence the measurements of the magnetic susceptibility along outcrops show very low values of magnetic susceptibility for hydrothermalized samples (0.01 to 0.1·10-3 SI) while fresh materials have significantly higher values (around 10·10-3 SI). A new acquisition method is developed to acquire offshore magnetic data with high resolution. The magnetic map acquired around the island shows important variations of the magnetic susceptibility value with strong consistency with the onshore structural mapping. The magnetic study allows to understand the offshore extent of the reservoir. Within the project, airborne UAV magnetic data were also acquired over the island of Basse-Terre where the main geothermal target lies.