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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. GRC Transactions, Vol. 30, 2006 Status of Geothermal Energy Exploration in Uganda Godfrey Bahati Department of Geological Survey and Mines, Entebbe, Uganda [email protected] • [email protected] Keywords methods. Drilling of shallow boreholes to a depth of 200-300m Geochemistry, resisitivity, geothermometry, Katwe, Buranga, and subsequent temperature gradient measurement on the Kibiro, Uganda anomalies will prove the existence of the resource and update the geothermal model that will be a basis for drilling of deep exploration wells. ABSTRACT Other geothermal areas are at preliminary level of investiga- tion and the results will soon be available that will be a basis Exploration for geothermal energy in Uganda has been in for their prioritisation for future development. progress since 1993. The studies have focused on three major geothermal areas namely Katwe, Buranga and Kibiro. Katwe and Kibiro are in an advanced stage of surface exploration and Introduction will soon be subjected to exploratory drilling that will pave way The exploration for geothermal resources in Uganda is still for the feasibility study. The overall objective of the study is at the reconnaissance and exploration stage. Reconnaissance to develop geothermal energy as an alternative to hydro and surveys on Ugandan hot springs started in 1921 by the geo- other sources of power to meet the energy demand of rural logical survey of Uganda and the first results were published areas in sound environment. in 1935. In 1973, as a result of the oil crisis, an attempt was The methodology of the study has focused on geology, geochemistry, hydrology and geophysics with the aim of elucidating subsurface tempera- tures and the spatial extent of the geothermal systems. The results indicate that the geo- thermal activity in the three areas is related to the volcanic and tectonic activities of the Rift Valley, which has a higher heat flow than the surrounding Precambrian crust. Subsurface temperatures of approximately 140-200˚C, 120-150˚C and 200- 220˚C for Katwe, Buranga and Kibiro respectively have been predicted by geothermometry. Anomalous areas have been delineated in Katwe and Ki- biro prospects using geological, hydrological and geophysical Figure 1. a) East African Rift System (EARS), b) Locations of the geothermal areas of Uganda. 377 Bahati made to initiate a geothermal project with the United Nations support, but this did not materi- alise due to the political turmoil in the country. Geothermal resources were estimated at about 450 MW in the Ugandan Rift System and three areas Katwe-Kikorongo (Katwe), Buranga and Kibiro identified as promising areas for geother- mal exploration (McNitt, 1982). Various surface exploration methods have been used with the aim to establish the subsur- face temperature of the geothermal systems, the source of heat, the flow characteristics of the geothermal fluids and the spatial extent of the geothermal reservoirs. Two areas Katwe and Ki- biro are in advanced stages of surface exploration and will soon be subjected to drilling of the first deep exploration wells. The third area Buranga is still being investigated and more information is still needed before the area is recommended for exploratory drilling. Other geothermal areas are at preliminary level of investigation and the results will soon be available that will be a basis for their prioritisation for detailed surface exploration. This paper is the current status of the explora- tion for geothermal resources in Uganda. Study Areas The main geothermal areas are Katwe- Figure 2. The geology of the Katwe-Kikorongo volcanic field and surroundings. Kikorongo (Katwe), Buranga and Kibiro. They are both located in the Albertine graben that runs along the deposit is greyish, generally coarse-grained and calcareous. border of Uganda with the Democratic Republic of Congo Travertine deposits have been found in Lake Katwe, Lake (Figure 1). The Albertine graben is part of the western branch Nyamunuka, Lake Kasenyi, and Lake Kikorongo (Groves, of the East African Rift System in Uganda, commonly known 1930) and in the vicinity of Lake Kikorongo at Kikorongo as the Western Rift Valley. junction (Kato, 2003). The lava flows, craters and extinct hy- The three areas were chosen for study because of their vol- drothermal deposits give an indication of a heat source for the canic and tectonic features that indicate a powerful heat source geothermal activity. The prospect stretches from Lake Katwe and high permeability. to Lake Kikorongo and occupies an area of approximately Other areas are located within or on the outskirts of the 150 km2. Outside the crater area, the geology is characterised Rift Valley in southwest, west, and north and in isolated places by surficial deposits to the east and the west, and to the north in east and northeast Uganda (Figure 1). lie the Rwenzori Mountains whose geology is dominated by gneisses and in some places granites and quartizites along the Kikorongo - Kasese road. Current Status Katwe Geothermal Prospect Geochemistry The geothermal surface manifestations in the Katwe pros- Geology pect are hot springs located in the Lake Kitagata crater, and The results of geological surveys in the Katwe prospect warm springs and travertine deposits that have built up tufas indicate that the system is dominated by explosion craters, in the Lake Katwe crater, which is located 12 km southwest of ejected pyroclastics, tuffs with abundant granite and gneissic Lake Kitagata (Figure 2). The maximum surface temperatures rocks from the basement (Figure 2). The volcanic rocks, in the hot springs in Lake Kitagata crater is 70°C, while in composed mainly of pyroclastics and utramafic xenoliths, are Lake Katwe it is 32ºC. deposited on the extensive Pleistocene lacustrine and fluvial The geothermal fluids are characterized by high carbonate Kaiso beds and in some places directly on Precambrian rocks. and sulphate contents and salinity of 19,000 to 28,000 mg/kg Minor occurrences of lava are found in the Lake Kitagata and total dissolved solids. High carbonate and sulphate in the geo- Kyemengo craters. The age of the volcanic activity has been thermal waters tends to invalidate solute geothermometer results estimated as Pleistocene to Holocene (Musisi, 1991). The in Katwe. Subsurface temperatures are estimated at 140-200˚C 378 Bahati using solute geothermometers (Armannsson, 1994). The thermal Geophysics fluids from Lake Kitagata and Lake Katwe craters are charac- The results of the geophysical surveys (Gislason, et. al., terised by the presence of high levels of hydrogen sulphide of 2004) indicate the existence of two low resistivity anomalies about 30-40 ppm which, suggests the source of the geothermal (Figure 3). The first one is located around Lake Katwe and the water to be volcanic and hydrothermal (Bahati, 2003). second stretches from Lake Kitagata to Lake Kikorongo. The TEM surveys were limited to the volcanics to avoid interfer- Isotope Hydrology ence of the highly conductive surficial sediments. The gravity A number of water samples from hot springs, cold springs, data agrees with the resistivity data and indicates that the low rivers and lakes as well as rock samples were collected and resistivity anomalies are controlled by a N-S fault east of Lake analyzed for isotopic compositions. The objective was to pro- Katwe and a NNE-SSW fault in the Lake Kitagata –Lake vide hydrological information essential to the exploration for Kikorongo area (Figure 3). geothermal resources using isotopes. 2 Isotopes analyzed for included those of hydrogen (δ HH2O, Kibiro Geothermal Prospect 3 18 18 34 HH2O), oxygen (δ OH2O, OSO4), sulphur (δ SSO4), and stron- 87/86 87/86 Geology tium ( SrH2O, SrRock). The results indicate that the Katwe geothermal system The Kibiro geothermal prospect is divided into two entirely is most likely recharged from high ground in the Rwenzori different geological environments by the escarpment, which Mountains. The tritium concentration in the thermal waters cuts through the field from SW to NE. To the east of the es- from the Katwe area is minimal suggesting that there is pos- carpment, the geology is dominated by an ancient crystalline sibly no mixing of hot spring water with cold groundwater. basement, characterized by granites and granitic gneisses. To The isotope composition of sulphur and oxygen in sulphates the west, lie an accumulation of thick sequences of rift valley suggests a magmatic and hydrothermal source for the geother- sediments of at least 5.5 km thickness, but without any volcanic 87/86 mal water. Strontium isotopes in water and rock ( SrH2O, rocks on the surface (Figure 4). 87/86 SrRock) indicate an interaction between the rocks sampled Recent geological and geophysical studies show anomalies and the geothermal fluids. The rocks interacting with the that can be traced along faults in the block faulted granites fluids, reservoir rock types, in Katwe are basalt (leucites and to the east and away from the Rift escarpment (Gislason, et.