Characterization of Reservoir Properties of Andesitic Rocks on Basse Terre de Guadeloupe (French West Indies) in the Framework of Geotref Geothermal Program

Abstract

Active volcanic areas are prone to the development of geothermal energy. In particular, Guadeloupe (French West Indies) already produces electricity thanks to Bouillante geothermal power plant. Geotref program, funded by the French Agency for Environment and Energy Management (ADEME) aims to locate a new geothermal resource south of Bouillante, close to La Soufriere volcano. This zone being covered by abundant recent deposits, vegetation and buildings, few geothermal evidences are found at the surface. Hence, a natural analogous system representing a 2.8 Ma deep environment has been studied in the north of Basse Terre (Tillet Creek) in order to characterize fluid pathways. At Tillet Creek, andesitic rocks are strongly foliated and crosscut by fractures, thus showing a both vertical and horizontal network for fluid flows. This feature is found from the metric to the millimetric scales. A scan line performed on the outcrop (30m long, along Tillet Creek) provides fracture data that were analysed by classical means (orientation, density, spacing, …) and fractal analysis. Fractal analysis describes a fracture network of rather clustered fractures (D=0.4). The whole fracture network is scattered in terms of dip direction even though the 4 major orientations of regional fractures can be recognised and most of the fractures show a high dip ( more than 60°) consistent with an extensional stress regime. Similar clay minerals being found in both vertical and horizontal structures and being clearly injected from vertical fractures into horizontal schistosity, this means that fluid flow in such environments occurs not only thanks to vertical fractures as described at Bouillante but also with a horizontal component thanks to discontinuities such as schistosity and lava flow boundaries and occur from millimetric to metric scale (and even more) in the 3 dimensions of space.