| Abstract |
The aim of this paper is to give an approach for geothermal system in Hanle prospect including previous data of 1980s’ and recent survey by JICA. Hanle geothermal prospect is located at approximately 110 km west of Djibouti city, away from the urban area. Hanle prospect is a plateau located between the Hanle Plain and the Gaggade Plain, extending NW-SE direction with an altitude of between 300 and 500 m. This topographic structure is delineated by faults oriented in the NW-SE direction, which is concordant with the direction of spreading axis of the Red Sea – plate boundary between Africa and Arabia. The geodynamic model proposed for the previous study in the area, specified by more recent geophysical considerations, shows that the whole area is dextral rotated. Hot springs and fumaroles are observed in some locations along the major faults limiting the grabens, both in Hanlé and Gaggadé. In early 1980s’, three gradient wells at an average of 450m depth (Teweo-1, Garabayyis-1 and Garabayyis-2) were drilled in Hanlé prospect to confirm the geothermal model and the precise temperature at depth before undertaking deep drillings. Measurements gave up to 121,7°C at Garabayyis-1 at 445m and a maximum temperature of 87°C on Garabayyis-2 at 75m. In late 1980s’, two deep wells were drilled, named Hanlé-1 (1623m) and Hanlé-2 (2038m) respectively. Hanle-1 recorded a maximum of 72°C at 1420m while only 124°C at 2020m in Hanle-2, 3D of these wells was done in Rockworks 17 to evaluate the permeable zone, correlate the formation and alteration distribution and have an approach of evolution of the reservoir. In Hanle plateau, basaltic lava of Afar Stratoid Series is distributed as lava flow which can be divided in to 3 layers, namely lower, middle and top basalt layer. Aqueous sedimentary tuff is often intercalated in basalt layers. In the northern part, underlying the basalt layer a thick rhyolite lava layer is distributed (Baba Alou Rhyolite). Besides, around the boundary of Hanle plateau and Hanle plain, widespread travertines of unknown age have been confirmed. Among the volcanic rocks in the survey area, altered minerals of zeolite, calcite and quartz are identified by XRD analysis (JICA survey 2014/2015). Geo-thermometer indicates that reservoir temperature may range from 180°C to 260°C (Daher and al WGC 2020). Considering the fact that permeable reservoir could be exist along NW-SE fault/fracture system that is located around the fumaroles according to the geophysical investigation. Based on those Data, the conceptual geothermal model was created, temperature and the volume of geothermal reservoir were estimated, and drilling target for test well drilling was determined. |