| Title | Recent Geological, Geochemical and Geophysical Survey of the Roseau Valley, High Temperature Geothermal Field, Dominica, West Indies |
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| Authors | Herve TRAINEAU, Eric LASNE, Nicolas COPPO, Jean-Michel BALTASSAT |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Dominica, Caribbean, exploration, high-temperature reservoir, conceptual model |
| Abstract | The most striking feature of the Roseau Valley geothermal field is the abundance of surface manifestations which are observed in two main spots distant of 4 kilometres: Wotten Waven in the Roseau Valley and the Boiling Lake – Valley of Desolation area. Several types of manifestations have been recorded including warm spring, hot spring, fumaroles and steam vent, kaipohan, solfatare, fossil alteration area, phreatic crater. Some of the hot springs discharge Na-Cl and Ca-rich Na-Cl waters which point out the existence of a high temperature geothermal reservoir. They have experienced interactions with reservoir rocks at high temperature as indicated by their oxygen-18 shift. Ca-rich Na-Cl waters discharged in the Valley of Desolation are thought to be the most representative of a deep parent fluid. Chemical geothermometers indicate deep equilibrium temperatures with reservoir rocks between 250-300°C. The Na-Cl waters discharged in the Roseau Valley area have lower deep equilibrium temperature (210-230°C). However all of these Na-Cl waters are thought to derive from a common deep reservoir. A magnetotelluric geophysical survey has been carried out at the scale of the inferred reservoir with 30 MT soundings. Despite the fact that the natural MT signal was low, this survey clearly images a shallow, highly conductive layer ascribed to the reservoir cap-rock. Some soundings also evidenced a slightly more resistive layer below which could be viewed at the top of the reservoir. These results support the existence of a large reservoir extending between Wotten Waven and the Boiling Lake – Valley of Desolation areas. The conceptual model which best fits in with the geological, geochemical and geophysical data is a deep Na-Cl reservoir extending below the Micotrin recent lava dome. Its temperature is estimated to be in the range of 250-300°C. Lateral fluid outflows directed to the southeast (Boiling Lake-Valley of Desolation) and to the southwest (Roseau Valley) are expected. The size of the reservoir area where high temperature fluids are thought to be encountered at depth is estimated to be about 15 km2. A preliminary assessment of the reservoir capacity has been done with a preliminary figure of 100-120 MWe. This is in accordance with the average power density of 10 MW per km2 observed for high temperature geothermal reservoir worldwide. |