| Title | A Revised Approach to the Hanlé - Gaggadé (Djibouti Republic): The Garabbayis Geothermal Site |
|---|---|
| Authors | Abdou Mohamed Houmed, Abrourahman Omar Haga and Jacques Varet |
| Year | 2012 |
| Conference | ARGeo |
| Keywords | Afar, Djibouti, ORC, Hydrothermal, graben |
| Abstract | The central part of Afar, between the active basaltic axial ranges of Asal and MandaInakir to the east and MandaHarraro to the west, is characterized by large horst-and-graben structures deeply affecting the stratoid series, 2.5 to 1 My old. This series is essentially made of trap basalts and a few rhyolitic centers outcrop in the upper part of the sequence. While the sequences in the hosts are essentially volcanic, the grabens are filled with sedimentary strata of detritic, lacustrine,and more recent evaporitic and eolian deposits. Hot springs and fumaroles are observed in several locations along the major faults limiting the grabens, both in Hanlé and Gaggadé, as in the whole area (both in Djibouti and in Ethiopia). Considering the fact that permeable reservoir could be well developed in detrital layers of the lower part of the sedimentary sequence, the exploration team considered the grabens as good geothermal targets. The Tendaho Graben in Ethiopia and the Hanlé Graben in Djibouti both looked as good candidates for geothermal feasibility studies. In the years 1980-1984, a significant geothermal exploration program was undertaken in Djibouti Republic under Italian and World Bank cooperation. The Hanlé and Gaggadé grabens were identified as targets of prime interest, with the hope of finding less salty fluids than in Asal, but at similar depth. Geological, hydro-geochemical and geophysical investigations, and a few gradient wells were used to locate deep exploration wells in the Hanlé gGaben. No sufficient geothermal gradient was however found, the Hanlé target was abandoned and the continuation of the drilling program was transferred to the Asal site. New site investigations, together with updates of the geological and geophysical data and geodynamic interpretations of this part of Afar led to the proposal of a new model for the geothermal fields, which the authors see as present in this area, not located in the graben but in the horsts. The Garabbayis site, where fumaroles above 100°C are present, appears to be located along a transverse fault, cutting through the whole horst along the southern limit of the Baba Alou rhyolitic massif. Fumaroles, hot grounds, deeply altered basalts and hydrothermal deposits (silica, zeolites, calcite) are found along this fault line, and are particularly well developed when crossing the cliffs bordering the Gagagdé and Hanlé grabens. Significant seismic activity characterizes these NE trending faults, which are much less spectacular in the landscape but most active at present. The geodynamic model proposed for the area by Barberi and Varet (1977), précised by more recent geophysical considerations including geomagnetism and summarized by Tapponier et al. (1990), shows that the whole area is subject to dextral rotation. This allows the development of intense bookshelf faulting and transverse fracturing. At Garabbayis, we observe that this rotation produces a significant extensional movement along the transverse faults. The following interpretation and propositions are made in terms of the geothermal model: - A high temperature geothermal reservoir was produced by transverse fractures, rejuvenated by present seismicity, with a significant extensional component at Garabbayis; - The heat source is uncertain, but could be magmatic and related to the presence of an important magma chamber underneath Baba Alou volcano, presently fossil but eventually still hot. It could equally be just produced by hydrothermal fluids circulating from deep in the mantle. Gas compositions in fumaroles show significant magmatic component; and - Further geochemical, geophysical, detailed geological surveys and civil engineering studies on the upper part of Garabbayis hydrothermal site should be conducted to locate places for geothermal exploration and production wells. They should be located on the volcanic horst and not in the sediments, and be drilled with the correct inclination in order to cross the NE trending faults at the suitable depth for high temperature steam production. |