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 th e order of 0.6 mmol/mol. Measured temperatures in the producing zones and at wellbottom vary from about 260 to 359 degrees C. The sphalerite and galena scaling observed in producing wells starts at the flash le vel located at about 850 m depth. The calculated geochemical temperatu res are close to 260 degrees C, which is the average temperature of th e main reservoir located at depths between 1000 and 1300 m. As a resul t of sulphide precipitation, about 90% of H2S is removed from the orig inal 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 a nhydrite. The main recharge water is seawater. (C) 1998 CNR. Published by Elsevier Science Ltd.