Auxiliary Chemical Geothermometers Applied to Waters from some East African Rift Areas (Djibouti, Ethiopia, Kenya) for Geothermal Exploration

International audienceIf the temperature values of geothermal reservoirs given by classical chemical geothermometers applied on thermal waters are relatively divergent, it is often difficult to estimate the temperatures of these reservoirs with sufficient accuracy for geothermal exploration, before drilling operations. In this case, some auxiliary chemical geothermometers such as Na-Li, Mg-Li, Na-Rb, Na-Cs, K-Sr, K-F, K-Mn, K-Fe, K-W, etc., existing in the literature can be useful tools to help estimating these temperatures. However, previous studies have shown these geothermometers are not only dependent on temperature, but also on other parameters such as the nature of the reservoir rocks and minerals, the fluid salinity, etc. Consequently, they must be used with caution. Another problem for the use of these geothermometers is that Li, Sr, Cs, Rb, Mn, Fe and W are under the form of trace elements in numerous geothermal waters and that it is often difficult to find analyses of these elements in the literature. In this study, we test some of these auxiliary geothermometers on several waters selected from literature data relative to East African Rift (EAR) geothermal areas, in Republic of Djibouti, Ethiopia and Kenya, where temperature values of deep geothermal reservoirs have been measured into wells or estimated with certainty using chemical classical geothermometers applied on thermal waters. The comparative results are discussed and allow bringing conclusions and recommendations. Among the tested auxiliary geothermometers, this work shows that the Na-Li thermometric relationship defined for the dilute waters from hightemperature volcanic geothermal areas of Iceland is one of the most relevant to estimate the temperatures of deep reservoirs for waters from several geothermal areas from the Republic of Djibouti, Ethiopia and Kenya. In previous studies, this Na-Li thermometric relationship had also given good estimations of reservoir temperatures for high-temperature (T ≥ 300°C) borehole dilute waters from the Los Humeros geothermal area, in Mexico, in volcanic environment. The use of other auxiliary geothermometers such as K-F, K-Sr, Na-Rb and Na-Cs (when F, Sr, Rb and Cs have been analysed) can be also relevant in some cases