Geochemistry of the high-salinity geothermal field of Asal, Republic of Djibouti, Africa

In 1987-88 deep exploratory geothermal wells were drilled in the Asal Rift in the Republic of Djibouti. The salinity of the brines produced in the Asal field exceeded 100 000 mg/kg, with a low gas content of the order of 0.6 mmol/mol. Measured temperatures in the producing zones and at wellbottom vary from about 260 to 359°C. The sphalerite and galena scaling observed in producing wells starts at the flash level located at about 850 m depth. The calculated geochemical temperatures are close to 260°C, which is the average temperature of the main reservoir located at depths between 1000 and 1300 m. As a result of sulphide precipitation, about 90% of HzS is removed from the original fluid before discharge. The computed composition of the brine at reservoir conditions is compatible with the presence of stable phases of epidote, (Mg-Fe)-chlorite, albite, K-feldspar, K-mica, pyrite and anhydrite. The main recharge water is seawater. © 1998 CNR. Published by Elsevier Science Ltd.