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Geochemical investigations during a new geothermal exploration phase in the Lamentin Plain (Martinique, French West Indies)

Abstract : A new exploration phase was carried out in the Lamentin plain (Martinique, French West Indies) in 2001. On the three bore-holes drilled by CFG in this area, only the bore-hole located more at north (Californie bore-hole) and near the old LA-101 borehole, indicated the presence of inflows of hot fluid close to 90°C starting from 400 m of depth. The main inflow was observed at about this depth. A similar geothermal fluid was also found at a depth of about 400 m in the Carrère bore-hole, situated near thermal springs, but with a lower temperature (50°C). The geochemical monitoring carried out by BRGM made it possible to find some surface inflows of cold fluid with often a probable marine influence in the three bore-holes and to characterise the chemical and isotopic composition of the low temperature geothermal fluid. According to the geochemical data, this Na-Cl geothermal fluid (TDS ≈ 10-11 g/l; pH ≈ 6.2-6.3) is a mixing of 20-25% sea water and 80-75% fresh water, which could mainly come from the massif of the Pitons du Carbet, following a general structural NW-SE direction. This mixing has reacted with the surrounding volcanic rocks and with magmatic CO2 emanations (minimum CO2 partial pressure estimated to 1 bar). Relative to a diluted sea water, it is enriched in dissolved Ca, HCO3, SiO2, Sr, Ba, B, As, Fe, Mn, Li, Rb, Cs and Ge and depleted in SO4 and Mg. Additional chemical and isotopic analyses performed on the fluids of neighbouring thermal springs confirm these results and indicate that the geothermal fluid sampled in the bore-holes has been slightly diluted probably by the drilling fluid. When applied on all these fluids, numerous chemical geothermometers yield concordant values of deep temperature around 90-130°C, which are close to the measured temperatures and to those estimated using the gas geothermometers. The relatively high Mg and SO4 concentrations of these waters and their absence of a 18O enrichment from the rocks are rather in agreement with the moderate temperature estimated using the geothermometers. All the results suggest that the temperature gradient increases towards the north of the Lamentin plain where the geothermal reservoir would be rather located. According to the geochemical results, the geothermal fluid would derive from a reservoir situated near sea water, somewhere between the Lamentin plain and the Absalon-Didier area. This fluid would flow out laterally from NW to SE at relatively low depths. No major evidence of a high temperature geothermal reservoir was observed in the Lamentin and Absalon-Didier areas.
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Contributeur : Bernard Sanjuan <>
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Dernière modification le : mercredi 29 avril 2020 - 15:02:02
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Bernard Sanjuan, Hervé Traineau, Albert Genter, Herlander Correia, Michel Brach, et al.. Geochemical investigations during a new geothermal exploration phase in the Lamentin Plain (Martinique, French West Indies). Twenty-Seventh Workshop on Geothermal Reservoir Engineering, Jan 2002, Stanford, United States. 8 p. ⟨hal-00768758⟩

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