The Katwe Geothermal Prospect in Western Uganda: A Deep Circulation, Fault Zone-Hosted Geothermal System?
Edward M. ISABIRYE, William CUMMING, Thomas ASKEW, Nick HINZ, David SUSSMAN, Peter MAWEJJE and Isa LUGAIZI
[Geothermal Resources Department, Directorate of Geological Survey and Mines, Uganda]
Motivated by a revised expectation that the Katwe geothermal prospect is more likely to host a deep-circulation rather than a magmatic type of geothermal system, new structural geology and Magnetotelluric (MT) resistivity surveys have been acquired and integrated with previous data including spring geochemistry, TEM resistivity and temperature-geology logs from six shallow wells, supporting the development of a geothermal resource conceptual model for a more than 125°C deep-circulation resource. The Katwe geothermal prospect is situated in the western branch of the East African Rift System (EARS) which has recently been recognized to more likely host 230°C magmatically-heated geothermal reservoirs that have been drilled and developed in the eastern branch of the EARS. The Katwe prospect is characterized by 32.5° – 72.1°C hot springs in the Kitagata crater and warm springs in the Katwe crater within the Katwe-Kikorongo Volcanic Field (KKVF). Geochemistry at Katwe is complicated by evaporites and saline lakes in the basins adjoining the KKVF. Based on the evidence of volcanic influence from the 140 maar craters and cinder cones of the KKVF and more than 200°C cation geothermometry of the hot springs that had seemed consistent with a volcano-hosted geothermal model, in 2006/2007, six temperature gradient holes (TGHs) of depths ranging from 200 to 300 m were drilled but with disappointing temperature gradients of 1000 m deep well to encounter a more than 125°C deep-circulation resource. However, the location of the fault-hosted upflow remains ambiguous and so two additional shallow TGH wells have been recommended prior to considering any deeper drilling. Such a geothermal resource would be potentially suitable for pumped production for binary electricity generation and/or direct use opportunities that may be consistent with the special land use status of the Katwe area.
|        Topic: Exploration||Paper Number: 11115|