Borehole geology of well Asal-5, Asal geothermal field, Djibouti

Abstract

This report describes the borehole geology of the well Asal-5 in the Asal geothermal field in Djibouti. This well was sunk down to 2105 m depth in the "inner rift" in order to explore there the temperature, permeability and the fluid characteristics of the area. The well penetrated a lava succession of olivine-, ferro- and trachybasalts as well as trachytes. Intrusive rocks are inferred at four depth intervals. The well showed a very low permeability and could not be discharged. Three aquifers were located above the production casing, the largest and the hottest one being at 500-550 m. Three minor aquifers were found in the production part of the well below 1600 m depth. The rocks show an extensive water/rock interaction where the intensity as well as rank of alteration increases with depth. The hydrothermal assemblage above ca. 1700 m depth indicates a much higher temperature condition than presently measured which clearly shows that the geothermal system has cooled down by as much as 200 C at 1000 m depth. A vague indication of retrograde mineralization, by the common occurrence of smectite down to 1000 m depth and laumontite presence at 600 m and 800 m depth, may indicate a lowering of temperature to below 200 C.