| Abstract |
Uganda is endowed with geothermal energy resources related to extensional tectonics and high heat flow areas. It lies in area on stretched and fault-broken rocks (rift valley). The intra-continental rift is favourable target for geothermal resources. Timeline for geothermal exploration has stretched far dating back as far as 1954 when first swallow wells were drilled in Buranga. It appears without a good understanding of the geology of an area, exploration is merely guesswork. You have recognize the importance of having and following a strategy to minimize cost and maximize success in exploring for and evaluating geothermal resources. It is important to determine which of the techniques used for geothermal exploration did not succeed and which was successful. We have reviewed the geology of geothermal resources in Uganda and come out with an exploration strategy. The Western rift, the western branch of the East African rift system (EARS), is bordered by high angle normal fault systems bounding one side of spoon-shaped basins (Ebinger, C.J. 2015). Depth-to-detachment estimates of 20-30 km, the rollover geometry of asymmetric basins, and seismicity throughout the depth range 0-30 km suggest that planar border faults along one side of rift basins penetrate the crust (Ebinger C.J, 2015). Geophysical evidence for crustal thinning across the 1,300-km-wide East African Plateau is restricted to 40- to 75-km-wide zones beneath the Western (Rykounov and others, 1972; Bram and Schmeling, 1975; Maguire and Long, 1976; Hebert and Langston, 1985; KRISP, 1987). On the basis of seismic refraction data, crustal thinning beneath the northern part of the Western rift system is less than 25% (Ebinger, C.J. 2015). The western rift arm of the EARS is in initial stages of continental rifting (early continental rifting). According to Corti G (1022), in these initial phases of continental rifting, widespread magmatism may encompass the rift, with volcanic activity localized along major boundary faults, transfer zones and limited portions of the rift shoulders (off-axis volcanism). Major boundary faults are ideal exploration targets. Moeck I (2013) classifies the rift valley geothermal system as Extension Domain play type CV3. In an Extensional Domain Geothermal Play (CV3) the mantle is elevated due to crustal extension and thinning (Moeck, 2013). The elevated mantle provides the principal source of heat for geothermal systems associated with this Play Type (Moeck, 2013). The resulting high thermal gradients facilitate the heating of meteoric water circulating through deep faults or permeable formations. According to Moeck (2013), these are fault controlled extensional domain play with elevated mantle due to active crustal extension. Glassley W (2014) classifies the rift system as Fault-bounded Extensional (Horst and Graben) complexes Brophy Type E. Extension and thinning of crust give rise to fracturing and faulting resulting in formation of steeply dipping faults. These high angle faults bound the graben (Glassely, 2014) and can extend to considerable depth. Such setting are places were magma often rises into the crust. According to Glassley (2014), as a result of the presences of the heat sources, numerous geothermal reservoirs can be present. Permeability and controlling structure is restricted to fault-controlled zones in the vicinity of horst and graben boundaries as evidenced by alignment of geothermal features. Uganda main geothermal prospects (Kibiro, Panyimur, Katwe-Kikorongo and Buranga) are deep-circulation geothermal systems. In many respects, they typifies other fault-controlled geothermal fields that are driven by deep circulation of ground waters into the thermal zone beneath the crust. Here fluid movement is controlled by the main fault zone that bounds rift valley. Joint Magnetotelluric (MT) and TEM profiling across these geothermal field is expected to reveal a deep, sub-vertical conductor (Kibiro case). Gravity and magnetic data may be useful and cost effective in defining the reservoir’s structural setting and fault locations. The utility of the gravity data is due to the large displacement between the escarpment and the rift valley. Reflection seismic is good in defining deep basement involved faults. One has to note that individual geothermal prospects, even in the same geological setting may differ hence no single technique can be recommended. Nevertheless you must have a strategy. “There are no bad wells”. Plan for failure, i.e have a plan for what to do next if a well in not successful and have an exit strategy. |