Ihimbo Geothermal Exploration Approach

Ihimbo is a geothermal resource area located in Rukungiri, South West Uganda. Geothermal surface manifestations include hot springs (70C), warm springs, travertine domes and gaseous emissions. The resource area is located in a sedimentary basin. Early studies in this generally involved surface water sampling and analysis. This was undertaken by ISOR, JICA and Directorate of Geological Survey and Mines. Ihimbo is presumed to be a deep circulation extensional amagmatic system. In many respects, it typifies other fault-controlled geothermal systems in Uganda like Panyimur, Kibiro, Buranga and Katwe that are driven by deep circulation of ground meteoric water. Fluid movement is controlled by an internal fault parallel to the main rift bounding fault. According to Moeck classification of geothermal system, Ihimbo is Extensional Domain play type CV3, where mantle is elevated due to crustal thinning and extension. The elevated mantle is presumed to be the principal source of heat at Ihimbo. The resulting high thermal gradient facilitate the heating of meteoric waters circulating through deep rift faults. According to Glassley, W. E’’s description of geothermal systems, Ihimbo is a typical fault-bounded extensional (horst and graben) geothermal system. The western rift is still in intermediate phase of evolution of continental rift development, where by rift bounding faults are major players and are characterized by volcanism and high thermal gradients (Corti et al, 2011). This is called boundary fault stage of rift development. In the initial phases of rift development widespread magmatism may encompass the rift with volcanic activity localized along major boundary faults and transfer zones (Corti et al, 2011). Recent exploration approach included literature survey, geological survey, geochemical survey (gas sampling and analysis, gas flux measurements), geophysical survey (swallow temperature Vincent Kato; Geraldine Paula Babirye measurements and Magnetotelluric MT surveys). This data is being supplemented by oil and gas exploration data. MT measurements have revealed a sub-vertical conductor (low resistivity signature) presumed to be a sub-surface geothermal reservoir. Soil gas and gas flux measurements have revealed anomalous gas concentration (near surface environment) ascribed to concealed fault that act as conduit for geothermal fluids. Identification of high concentration of gases is presumed to be related to enhanced permeability (critically stressed faults). Reflection seismic data from oil and gas exploration revealed deep penetrating faults presumed to control geothermal activity. The area is seismically active which keeps the fracture permeable. Chemical analysis indicate mixing of meteoric and geothermal waters as expected since recharging water and geothermal water use the same pathway. Planned work include Time Domain Electromagnetic (TDEM) survey to correct for static shifting in the acquired Magnetotelluric Data. Static shifting is inherent issue due to near surface inhomegenities and distorts the magneto telluric response. Detailed structural mapping to characterize the structural stetting of Ihimbo is also planned. Geological conceptual models will be developed through data integration. Initial models will be tested, supplemented and refined by further field work. The process will continue until hopefully a reliable model is achieved. Thermal gradient drilling is recommended prior to committing deep expensive exploration wells. LiDAR mapping is recommended to delineate faults, create high resolution DEMs, quantify fault kinematics and develop linear maps. Shallow temperature measurements should be conducted more so where conductive anomalies have been revealed.