| Abstract |
The Obock and Rouéli geothermal sites, Djibouti Republic Abdourahman Omar Haga, Hamoud Souleiman Cheik, Jacques Varet Ministry of Energy, in charge of Natural Resources, Djibouti Republic Bilane24@yahoo.fr ;hamoudsoulei@yahoo.fr ;j.varet@brgm.fr The western extension of the Aden ridge penetrates the African continent through the Gulf of Tadjourah, an active oceanic ridge elongated in an East-West direction, but made of NW-SE rift segments and NE-SW transform faults, emerging in the Asal-Ghoubbet westernmost segments where two geothermal sites have been identified (see Abdourahman et al. this volume). While looking at the bathymetric or seismicity maps of the area, a striking feature appears: the active ridge is not located in the middle of the Gulf of Tadjourah, but on the northern part, very close to the coast, which itself appears as paralleling the ridge. This means that an important heat source is available from the shallow anomalous mantle and active volcanism on the spreading sea floor (1500m deep in Tadjourah trough) very near to the coast. Near to the town of Obock, the dominantly coralline coastline, uplifted and affected by E-W faults, is very close to the ridge itself, whereas in Rouéli - a recent basaltic plateau (“gulf basalts”, emitted 1 to 2My ago, at the early stage of opening of the Gulf of Tadjourah) forming a promontory above the sea level and deeply affected by both WNW and NE fault systems - gets very close to the transverse faulting linking at depth the Obock and Tadjourah troughs. While the surface geology is rather well known (Marinelli& Varet, 1973, Richard & Varet, 1979, Manighetti et al. 1998), themarine side was studied by at least two oceanographic cruises SUDMEROUAD in 1977 (Choukroune et al. 1986), and TADOURADEN in 1995 (Hebert, 1998). From the latest studies, it was shown that the direction of spreading is oblique with respect to the rift as well as to the transform faults, and that both have hence an opening component (Dauteuil et al. 2001). It means that a shallow hot, anomalous,and magma generating mantel, is also present along transverse structures. This geophysical interpretation corroborates the field observations at Rouéli where both WNW and NE faults look extensional. As a matter of fact, several hot springs are encountered at both sites, and steam vents are also present at Rouéli. Available hydro-geochemical data indicate temperature of 210°C for the geothermal reservoir at Obock(Houssein et al. 1993, Sanjuan, 2010). The high degree of recent faulting and the currently important seismicity let envisage a good permeability at depth, also linked with the expected geological nature of this talus continental made of detrital formations interbedded with coral reefs and faulted basalts. Of course, the geothermal reservoir should be looked for by deviated wells, towards south in Obock and towards South-east in Rouéli. The presence of an attractive geothermal resource in the area is of interest at two stages: at present to answer the need of the local population, presently (only very partly) answered by diesel engines in Obock and Tadjourah, and in the future in order to serve the railway line and port linking central-northern Ethiopia to the sea through central Afar. In addition to such promising perspectives for geothermal power production, these sites appear also particularly appropriate – and eventually unique in the world to this respect – for attempting to drill deep (with ICDP) in an oceanic ridge from the continent, an approach to future exploitation of the huge energy potential of mid-oceanic ridges in general. |