The southern sector of the Ethiopian Rift Valley hosts the Aluto Geothermal Field, located between Lakes Ziway and Langano. Initial exploration studies until 1978 by the Ethiopian Geological Survey outlined two separate large areas (> 30 km2 each) with active thermal manifestations. One area, exhibiting fumaroles and steaming ground, occupies the greater summit region (> 2000 masl) of the inactive Aluto volcano, a strato-volcano comprising a complex pile of rhyolitic domes and pyroclastic flows. The second area with significant surface discharges covers the lower southern flanks near the N shores of Lake Langano where hot springs and weak fumarole activity discharge up to 20 MW anomalous heat around the greater Oitu Bay area (c. 1600 masl).
A UNDP- sponsored mission in 1978 proposed deep exploration drilling of several geothermal prospects in the Southern Rift Valley and obtained pledged financial support by the Ethiopian Govt, the EEC, and UNDP. During 1980 a second review meeting selected the Aluto prospect as its first target. A conceptual model was used to locate the first two deep well sites assuming that a convective, liquid dominated, high-T system occurs beneath Oitu Bay. The model was based on observed low resistivity values of rocks at intermediate depths and the geochemistry of discharged fluids. Deep drilling was supervised by a NZ consulting team (GENZL). Results and progress were reviewed by a small group of project engineers and earth scientists with the first author (MPH) acting as consultant on secondment from the University of Auckland.
Drilling of the first deep well (LA-1) near the lake shore started in 1981 and reached a final depth of c 1.3 km; a bottom hole T of 85 deg C was encountered. T-logs showed that fluids with elevated temperatures (up to 72 deg C) were only found in the upper 150 m. The results refuted the original model of a high T-system with convective upflow beneath Oitu Bay but indicated shallow lateral (advective) outflows from a high-T reservoir beneath the Aluto strato-volcano.
The 2nd well (LA-2) was shifted to the western foothills of Aluto dome , c. 10 km to the NW of LA-1 , close to a large fumarole where coherent low resistivity rocks at intermediate depth had also been found throughout the area.. Well LA-2 was completed in late 1982 and encountered 105 deg C at bottom hole. Mineralogical studies of cuttings and cores showed that high-T, alteration minerals (epidote for example) can be found in the well. This finding confirmed that the local low resistivity structures, used in part for locating the first two wells, can reflect some old (palaeo-fluid) alteration effects not only recent thermal alteration. The palaeo-temperature structure in LA-2 could be interpreted in terms of a conductive heating-and cooling cycle lasting over a period of c. 0.14 Ma (using a one-dimensional analysis).
The project proceeded with the search for a third well site within the high standing region around Aluto dome where scattered steaming ground had been observed although repeat- resistivity surveys had not found a coherent, low resistivity pattern in the upper 300 m. The project opted to use c. 50 to 70 meter temperature-gradient (TG) holes to explore high- standing hot grounds. Using an intersection pattern of hot grounds and fault-controlled NNE trending steam vents, well LA-3 (1920 masl) could be sited. Another logistic problem had to be overcome, namely pumping (drill-) water via a 7 km long pipeline from Lake Ziway (1640 masl) to the upper slopes of Aluto Dome. After its completion, drilling of LA-3 well was started in February 1983. It was drilled to c. 2.1 km depth, encountered 320 deg C fluids at the bottom, and discharged freely – it became the discovery well of the Aluto prospect.
The Aluto study has shown that siting of exploratory wells over inferred permeable fault structures of volcanic geothermal systems, based in part on resistivity surveys, becomes an uncontrolled exercise if targets are not associated with coherent manifestations. Thermally altered volcanic rocks at Aluto are caused by present-day convective and advective hot fluids but also by palaeo-thermal outflows through now cooler rocks. The Aluto example shows that its likely hot and productive area cannot be assessed from the extent of low resistivity anomalies at intermediate depths if palaeo-resistivity structures are present. The work has been published in Geothermics.
Manfred P. Hochstein, Befekadu Oluma, Hagen Hole. Early exploration of the Aluto geothermal field, Ethiopia (History of discovery well LA-3). Geothermics volume 66 (2017) pages 73–84.Go To Geothermics