Early exploration of Aluto geothermal field, Ethiopia (History of discovery well LA-3)

Significance Statement

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.

About The Author

Manfred P Hochstein

Dipl. Geophysiker, TU Clausthal, Germany
Dr. rer. nat. Münster, Germany (PhD)
Fellow of the Royal Society of New Zealand

Born in 1932 in Germany, I started my career by specialising in Geophysics, Geology and Physics when receiving my diploma and doctoral degrees in 1959 and 1962 respectively at German universities. Post-graduate studies followed (1962-1964) in the US and Antarctica (Antarctic Service Medal 1963) prior to joining the Department of Scientific and Industrial Research (DSIR), Wellington (NZ) as a geophysicist in 1964.

I continued to work as:

  • Superintendent of the Geophysical Survey, Geophysics Division (DSIR) from 1966 until 1972.
  • Associate Professor at the University of Auckland teaching Applied Geophysics at the Geology Department (1973-1997).
  • Founding member of the UNDP- sponsored Geothermal Institute (1978) to provide graduate courses for earth-science and engineering students (mainly from developing countries). Director of Geothermal Institute (affiliated with the Geology Dept. and School of Engineering, University of Auckland) from 1980 until 1994; continuation with part- time teaching at the Institute from 1998 to 2008.
  • Invited as consultant to assess UNDP sponsored geothermal projects (Chile, El Salvador, Nicaragua, Kenya, Ethiopia, P.R of China and Tibet) between 1967 and 1988; participation in NZ – Indonesia bilateral geothermal aid projects (1972-1978); consultancy work for UNEP projects (Kenya, Rwanda, Tanzania) since 2012.
  • Consultant since 1988 for geothermal projects in New Zealand, Indonesia, El Salvador, Philippines, and Zambia; involvement in sub-contracted studies for Castlerock Consulting (World Bank-sponsored), assessment of geothermal prospects in Indonesia and Vanuatu (2010 and 2011).

Since my retirement in 1998, I continued with consulting work for the NZ Dept of Conservation (monitoring thermal manifestations), undertaken through UniServices, the commercial arm of the University. I became a sub-contracted consultant for the AECOM Group (Auckland and Jakarta) and  continued with own research work assisted by  IGNS (Wairakei) staff.

During the next decade (up to 2010), I received invitations to visit geothermal projects in Indonesia and China. My visit of Indonesian prospects on  Banda Arc Islands (2008) was sponsored by PGE (Pertamina, Jakarta) – it was followed by teaching of a refresher course at the 2009 annual Geofisika Indonesia Conference in Yogyakarta and a Peer Review of the Wayang-Windu  Field  (Java), contracted through the AECOM (Auckland ) group. I visited China as invited speaker at the Intern. Geothermal Conference in Beijing (2002) and the UNESCO sponsored Conference on Coal Fire Research (Beijing 2006). During this period, I became Guest Editor of a special issue of ‘Geothermics’, covering Indonesian geothermal developments (2008) and joined the IESE research group at the Univ. of Auckland as a ‘research scientist’.

Consultancy work in 2011 involved a review of the Jailolo prospect (Moluccas) for StarEnergy, reviews of heat-loss studies at Muaralabuh, Rantau Dedap, and Rajabasa (Sumatra) for Supreme Energy involving also ‘hands–on’ teaching. Similar work in 2012 covered a Peer Review of low T systems in Zambia (for Kalahari GeoEnergy Ltd), a review of the Karisimbi project (N Rwanda) for IESE/UniServices  Auckland, and due diligence assessments of the Ijen Caldera prospect (Java) for AECOM (Jakarta) and a small, high T prospect in NZ (Reporoa) for AECOM (Auckland).

Consulting in 2013 was taken up by a review of the Tampomas geothermal prospect (Java) for IESE, a review of the Silali prospect in Kenya (for UNEP, Nairobi) and a review of the Tangkuban Perahu project for Indonesian Power (IP) under an IESE/UniServices contract. PGE (Pertamina Jakarta) invited me and other IESE staff to conduct a 2-week professional refresher course for their geothermal staff at Bandung in August. Professional work in 2014 involved continuation with the IP (Tangkuban Perahu) project and tendering for a Tendaho (Ethiopia) project. During the year, I completed a review of the Kinigi (Rwanda) project for UNEP (Nairobi) and an appraisal of the first phase of the Baturraden (G.Slamet) project in Java under contract with AECOM (Jakarta).

In 2015 I undertook a review of the Banten (Java) geothermal prospect for a PIP (Indonesian  Govt.) loan assessment, contracted by Castlerock Consulting. I became a member of the IGA Geothermal Resources and Reserves Committee and reviewed old assessments of Nicaraguan geothermal resources by private consultants. Later during the year, exploration reports of two Mexican geothermal prospects, a Univ. of Mexico project, were assessed under contract with the newly formed IESE (Auckland) group. A review of two Ethiopian prospects (Tulu Moye and Gedemsa) for the Auckland AECOM group completed the year’s activity. Work on the Mexican projects and reviews of Baturraden reports continued into 2016 until May when I reviewed several Tanzanian geothermal projects for UNEP (Nairobi). Recently, I was invited as guest speaker and (short course) lecturer at the  Universitas Indonesia, Jakarta (Oct 2016). In 2016 I was awarded the annual ‘Patricius Plakette’ by the German Bundesverband Geothermie in Essen (Germany).

My own research studies during my professional activities during the last 20 yrs involved the assessment and delineation of natural resources (geothermal, ground water, and mineral resources) and studies of regional geological/geophysical topics (sedimentary basins, volcanic and tectonic structures, glaciology, seismological research). I have maintained good contact with many ex-students both in NZ and overseas who still ask me to review some of their manuscripts. My research has led to the publication of c. 100 papers and book articles in refereed journals and c. 100 articles in the Proceedings of geothermal conferences and workshops (references listed in ‘Google Scholar’ and ‘Research Gate’). I am still a member of the Editorial Board of the Geothermics journal, a life honorary member of the NZ Geothermal Association, and since 1992 a Fellow of the Royal Society of NZ.

Reference

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