| Abstract |
Due to a regional consideration of the whole Afar Depression, for ranking the best geothermal sites combining geodynamic and hydrogeological approach (J.Varet, Argeo meeting, 2011), the area located to the north of the Ghoubbet rift appeared as most promising for geothermal energy developments in the Djibouti Republic. This site within 10 Km long and 6 Km large, covered at the surface essentially by the recent basalts of the gulf (i.e. emitted during the early stage of penetration of the Aden ridge through the Gulf of Tadjourah, 4 to 2 My ago, O.Richard & J.Varet, 1979) benefits from unique geological conditions: - The immediate vicinity of the Ghoubbet active spreading segment, with a shallow anomalous (partially melting, 1300°C) upper mantle at 7 Km depth, providing an important and safe (regularly renewed, hence renewable) heat source for the geothermal field. - A tremendous fracturing of the whole block (made at the surface of recent basalts of the gulf) with at least 3 direction of open faulting with transverse components (NW-SE, NNW-SSE and NE-SW), providing a good fracture permeability of the reservoir. This particular tectonic setting is of course due to the vicinity of the transform faulting linking Ghoubbet and Tadjourah oceanic rift segments. But it also results (as shown by Manighetti et al. 2011) from the fast rotation of this brittle block. Numerous fumaroles and hot springs, some with important silica deposits, affecting the whole block, testify of the leakage of the geothermal reservoir due to the fairly active tectonics of the site. - Good reservoir conditions are also expected from the geology itself, as very important detrital deposits accumulated there, for the last 5My, underneath the more recent basalts, due to the up-rise and concomitant faulting and erosion of the whole area located North before and since the early stages of opening of the Gulf (up to 1000m high Dalha basaltic plateau and the famous Day mountain). - This reservoir should also benefit from relatively low salinity fluids – certainly the best for Djibouti Republic - due to the long lasting flow of water from the Day mountain downstream towards the Gulf for the last few My. Within this specific location site, a preliminary geochemical and geophysical study were undertaken in the area by BRGM (French geological survey) and chose several high and low gravity anomalies not distributed uniformly. The CERD (Center of research of Djibouti) has undertaken in 2011, geochemistry and geophysical surveys, to complete the early exploration studies. Reservoir model for the site have been proposed and wells site location for exploratory wells have been proposed using MT and TDEM methods. However the investigation, presently limited to the immediate surroundings of the major surface hydrothermal manifestations should be extended to the whole area down to the sea shores. |