| Abstract |
Volcanic edifices are recognized to represent a preferential target for geothermal play location. Geothermal reservoirs in this context are clearly composite and composed by lithological elements and structural patterns. Circulation of hot hydrothermal fluids induces strong transformation of the mineralogical content and of the structure and texture of the materials. These transformations control sharply the petrophysical properties and especially the transfer ones, for fluids and heat. Caribbean archipelago are an important target for development of high enthalpy geothermal resources to decrease the fossil resources dependence. In the objective of prospection in Guadeloupe islands (Basse-Terre and Les Saintes islands), the GEOTREF program is setting up with the support of the French government. To contribute to a characterization of these reservoirs, a large set of lithology is sampled from fresh and hydrothermalised facies. Fresh materials have mainly calc-alkalin composition with lava and ignimbritic-debris flow textures and are principally sampled on Basse-Terre Island while the hydrothermal facies of these both textures are sampled on Les Saintes Island. By this way, we suppose that Les Saintes system is an exhumed analogue for a potential active geothermal system on Basse-Terre Island like Bouillante system with its active power plant. For fresh facies, porosity ranges between 0 to 30 % and 10 to 65% for lava and ignimbritic-debris flow materials respectively, density ranges between 2.7 and 2 g.cm-3 and from 1.8 and 0.5 g.cm-3, permeability ranges between 10-19m2 and 10-12m2, and from 10-14 to 10-11m2, thermal conductivity ranges from 2 and 1.3 W.m-1.K-1 and from 1.4 and 0.4 W.m-1.K-1. A large variability of these properties is observed within the same lava flow. Other properties are also available : VP, VS, electric resistivity. Hydrothermallized facies, as well for lava and ignimbritic-debris flow protoliths, show intermediate values between the ranges of the both fresh facies. Hydrothermal alteration could be divided in two main groups of processes, dissolution and secondary minerals crystallization. Dissolution is the main process affecting lava and increases the porosity and the permeability, decreases density, thermal conductivity and propagation speed of the acoustic waves. Crystallization of secondary minerals is the main process affecting the porous network of ignimbritic-debris flow textures, it decreases weakly porosity and permeability and increases density, thermal conductivity. In relation with the local structural pattern, different facies, fresh and hydrothermallized, are localized in a conceptual geological model, their petrophysical properties are used to propose a first model usable for potential heat and fluid exchanges. |