Title: Borehole geology of well ASAL-5, Asal geothermal field, Djibouti

Abstract

This report describes the borehole geology of the well Asal-S 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-, ferroand
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.