Geochemical Interpretation of Thermal Water and Gas Samples from Lake Abbe, Djibouti

Abstract

The Lake Abbe geothermal field is located in the south-western region of the Republic of Djibouti, on the border with Ethiopia. It is one of the twenty two geothermal potential sites identified in Djibouti. Surface geology is characterized by stratoid basaltic rock and the geothermal surface manifestation in this area include hot springs with surface temperatures of more than 90°C, fumaroles and travertines. The surface water consists hot springs, groundwater and a lake. Eight hot springs, one fumarole, one groundwater and one lake water were collected and analysed in this study with the purpose of estimating the reservoir temperature by using geothermometers and interpreting the chemical compositions of the water by using ternary diagrams. Based on different geothermometers, the reservoir temperature is estimated between 107-160°C and hydrothermal data shows that the thermal waters have a high concentration of Cl which classifies them as volcanic waters and that these waters are equilibrated with the rock. To determine the origin of the water, the stable isotopic (δD and δ18O) composition and Cl/B ratio were used and based on these results, the thermal waters are local meteoric waters from the same source and they are not mixed with Lake Abbe’s water. The saturation state of the hot springs with respect to calcite, amorphous silica and anhydrite at the surface temperature was also assessed using the WATCH speciation program to assess the scaling potential. Based on this program, calcite deposition may occur during geothermal production. The estimated reservoir temperatures suggest that the Lake Abbe geothermal area is a medium-temperature geothermal system.