Title: Sub-surface geology, hydrothermal alteration and 3D modelling of Wells LA-9D and LA-10D in the Aluto Langano geothermal field, Ethiopia

Abstract

Aluto Langano geothermal field is located in the Aluto volcanic complex (AVC), within the Central
Main Ethiopian Rift System (CMERS). AVC is a silicic peralkaline complex, which is affected by
Wonji fault belt (WFB), trending NNE- SSW. Ten exploratory wells (LA-1 to LA-10D) have been drilled
in Aluto Langano geothermal field, with a maximum depth of 2500 m and maximum measured temperature
of 335°C.
This study focused on sub-surface geology and hydrothermal alteration of wells LA- 9D and LA-10D.
The wells were drilled directionally on the top of AVC. Data form neighbouring wells (wells LA-3 to
LA-8) are used for comparison purposes. Lithologies that are found in the study wells, identified
with the aid of binocular microscopy and petrographic microscopy, are pyroclastics, silicic tuff
and breccia, sediments, rhyolite, trachyte, basalt and ignimbrite, while ICP-OES analyses also led
to the identification of very scarce and thin layers of trachyandesite, basaltic trachyandesite,
trachydacite, basaltic andesite and andesite units. Hydrothermal alteration minerals are found as
replacement of primary minerals and as fillings of veins and vesicles. Four alteration zones are
recognized in both study wells, and include an unaltered zone, smectite zone, illite/chlorite
zone and illite/chlorite/epidote zone.

Eight feed zones (with one major and seven minor) were identified in well LA-9D and seven (one
major with six minor) feed zones in LA-10D are observed. Most of the feed zones are located in
fractured basaltic units. The highest temperature recorded was more than 300°C in well LA-9D below
1560 m depth, and below 1600 rded temperature indicated is more than 300°C. Both wells
appear to be in the upflow zone of the system.