| Abstract |
Ministry of Energy in Charge of Natural Resources, Djibouti Republic An update of available geophysical geochemical and geophysical date, together with new field works and research undertaken by the Ministry of Energy in Charge of Natural Resources of the Djibouti Republic and CERD allow for an attempt for proposing new geothermal models for several sites identified for their geothermal potential from previous general surveys and specific studies for the upcoming geothermal plans. While other contributions describe in detail the characteristics of each site, this paper aims at proposing a synthetic view of the different components and an overall concept, mainly based on the geodynamic and hydrogeologic environment of this eastern portion of the Afar triangle. Concerning the heat source, two major cases occur: - The geothermal systems located at the top or in the immediate vicinity of the oceanic ridge, whether submarine or emerged. That is the case of Asal, Nord-Ghoubbet, MandaInakir, Rouéli, Obock and lastly Arta. - The geothermal systems relying upon deep faulting, frequently transverse to the dominant rift system (NW-SE) without evidence of active volcanic heat sources. This is the case of Garrabayis and eventually Abhé. For the geothermal reservoir, we also consider it relies directly upon the geodynamic environment of the site. It may either result from simple extension and be produced by normal faulting and open fissures, that is the case in Asal, Obock and MandaInakir, or result from transverse faulting with the eventual association of block rotation, as in Nord-Ghoubbet, Rouéli, Garrabayis and Abhé. The composition of the geothermal fluid in the reservoir will directly relie upon its hydrogeological environment. From marine brines at Asal, it will vary up to meteoritic water in MandaInakir, Abhé, or Nord-Ghoubbet, to dominant sea water component in Obock, Rouéli and Arta. Continental brines will probably dominate in other sites of endoreïc basins and strong evaporation (such as Hallol-Sakhalol, or the fumarole Qiqleh site of Arta), not yet retained as geothermal sites at present due to insufficient knowledge. As a whole, our approach helps to provide a first hierarchy and structural plan of the geothermal potential of Djibouti in terms perspective objective for future power production. Ridge located sites have the best heat source and therefore the largest quantitative potentials and were proven by the result of researches. But transverse fracturing and block rotation will allow for development of the best reservoirs, while water composition will directly influence the costs of systems and maintenance. Of course, real quantitative figures will be possibly approached only after feasibility drilling will begin at the end of last quarter of 2014 and production tests will have been undertaken at the following years. But we also need to develop an overall prospective and strategy for the country geothermal plan, that the present approach, further works and confrontation of experts views will help to finalize the progress of geothermal drilling that will be held through the next years by the result of the study research. |