Geothermal exploration in the Mount Pelée volcano-Morne Rouge and Diamant areas (Martinique, West French Indies) : Geochemical data
Résumé
The increasing demand for energy in the Caribbean islands and the aim to enlarge energy autonomy have resulted in starting again geothermal research on the Martinique island, in the Lesser Antilles. The first works, which were the drilling of three deep wells (1000 m) in the Lamentin plain in 2001, did not show the existence of high temperature geothermal resources. Consequently, new studies of geothermal exploration were carried out between 2001 and 2003 in two other areas which had already been noticed for their geothermal interest: Mount Pelée volcano-Morne Rouge and Diamant. These areas belong to the recent arc of post-miocene age. All the data obtained in water geochemistry during these studies have led to a better comprehension of the geothermal systems associated to the thermal springs known in these areas (less than 15). Sixteen soil gas profiles (CO2, CH4, O2, He) could be carried out in July 2002 in the studied areas, which represents approximately 260 analyzed gas samples. Among these samples, 11 were selected in order to perform additional chemical and isotopic (13C, 3He/4He) analyses. The geochemical results obtained during these studies made it possible to select three areas of high temperature geothermal interest: 1) the area of the thermal spring of Petite Anse du Diamant, whose the relatively saline water (TDS ≈ 20 g/l) and the CO2 gas emanations indicated high temperature conditions (around 180 °C) at depth, despite a low flow rate and temperature (35°C) at the emergence. 2) the area of the upper Chaude river, where the chemical and isotopic compositions of the waters collected from the thermal springs (TDS from 1.0 to 1.5 g/l, the hottest temperatures at the emergence in the island: from 38 to 65°C) suggested the existence of a deep geothermal reservoir at about 200 °C and the presence of primarily magmatic CO2 gas. The deep hot water is after diluted and cooled by superficial fresh waters, during its rise. 3) the area of the Desgrottes drill-hole, where the gas emanations testified of a magmatic origin but where no index of deep hot water was observed at the surface. For these three areas, results obtained from geological and geophysical studies supported the geochemical data. However, only the drilling of several deep exploration wells will be able to confirm the existence of workable high temperature geothermal resources in these areas.
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