Hydrogeological model of a high-energy geothermal field (Bouillante area, Guadeloupe, French West Indies)

Abstract

The Bouillante geothermal field presently provides about 8% of the annual electricity needs of the French West Indies island of Guadeloupe. It has been the subject of a large number of studies covering various disciplines. These results enable the proposal of a hydrogeological conceptual model of the field. The reservoir consists of two perpendicular sets of fractures and faults, related to major regional tectonic structures. The capacitive and transmissive functions of the field are assured by these deep faults and fractures, which have been clogged near surface by self sealing and clay fill phenomena. The heat exchanges of the reservoir with the outside are thus reduced, through thermal and moreover hydraulic blanketing, to conductive transfers. Convection cells are active within the reservoir, ensuring its thermal and geochemical homogeneity. Heat exchange with the magmatic chamber is only conductive. The Na-Cl geothermal fluid in the reservoir is composed of about 60% sea water and 40% fresh water of meteoric origin and has reached a chemical equilibrium with a mineralogical assemblage at 250-260°C. This equilibrium state, the absence of tritium in solution, the low ratio between water in and out fluxes and the large reservoir volume (estimated at more than 30 millions m 3 using tracer tests) suggest a relatively long (>> 100 years) residence time of the geothermal fluid in the reservoir. Three main factors, all essential, explain the existence and location of the Bouillante geothermal field: a heat source (cooling hypovolcanic intrusion), a network of permeable fractures at the origin of the geothermal aquifer, and an impermeable surface cover, limiting the loss of energy and ensuring the durability of the field.

Petrophysical properties of volcanic rocks and impacts of hydrothermal alteration in the Guadeloupe Archipelago (West Indies)

“…Three islands of the Guadeloupe archipelago belong to the recent volcanic arc (Bouysse and Westercamp, 1990): Terre-de-Bas and Terre-de-Haut in Les Saintes archipelago and Basse-Terre with the active volcano of La Soufrière (Figs 1b and c). The activity of the Grande Découverte-Soufrière Complex is a major human safety issue especially since the last major eruption in 1976 even if it was only limited to a seismic crisis and a phreatic eruption (Komorowski et al, 2005). Therefore a lot of studies brought out a large amount of geological, geochemical and geophysical data on this wellmonitored volcano (e.g.…”

“…It is made of sub-aerial lava flows and eroded volcanic edifices (Mathieu, 2010). The calc-alkaline Northern Chain, 1.81 ± 0.03 Myr to 1.15 ± 0.02 Myr (K-Ar ages, Samper et al (2007)), consists of voluminous lava flows and domes of andesitic and dacitic compositions (Komorowski et al, 2005). The Axial and Bouillante Chains composed of Piton Bouillante, Icaques, Capesterre and Sans Toucher volcanoes formed between 1.023 ± 0.025 Myr and 0.435 ± 0.008 Myr (K-Ar ages, Samper et al (2007)).…”

“…However, origins of fluid and heat are still under debate. Only few data have been published and conceptual models for fluid flow remain weak (Lachassagne et al, 2009;Lopez et al, 2011), as well as the geometry of the reservoir (Bouchot et al, 2010(Bouchot et al, , 2008).…”

Numerical modeling of the geothermal hydrology of the Volcanic Island of Basse-Terre, Guadeloupe

“…La Soufrière is an active volcano, and its inferred, underlying magma chamber likely provides a strong and active heat source (Lachassagne et al 2009). The composition of the magma is rather silicic, and the temperature of the magma chamber has been estimated as around 800 • C by Besson and Poirier (1994).…”

“…Geological context of the Guadeloupe archipelago. a Volcanic arc resulting from the subduction of the Caribbean plate under the North American plate at a rate of 2 cm/year (modified fromFeuillet et al 2002;Lachassagne et al 2009). b Lithology and fault network in the Guadeloupe archipelago, with volcanic units identified on Basse-Terre (modified fromKomorowski et al 2005;Lachassagne et al 2009) …”