Rationalization of Conceptual Models for Targeting Temperature Gradient Drill Holes Using Structural Data at the Panyimur Geothermal Prospect, Uganda

Abstract

Panyimur geothermal prospect is located near the northern end of the Lake Albert Basin within the western branch of the East African Rift System. It has several hot springs, the maximum surface temperature of all them is 61.4oC.Other surface manifestations are: travertine and amorphous silica. Substantial surface exploration has been conducted at this prospect, including regional geological mapping (1961), geological investigations in 2011 and 2012, a gravity survey in 2011-2012, a follow up gravity and ground magnetics study 2015, MT/TEM data in 2016 acquisitions and additional MT in 2017as well as petroleum exploration wells, 2D seismic profiles. In this study, fault traces were mapped in detail to refine the conceptual model for this prospect, which has been characterized as a likely fault-related geothermal system. The fluid flow pathways in such systems are typically controlled by normal fault step overs, fault terminations, and major fault intersections. The recency of rupture was also evaluated for each major fault segment. Exposed fault surfaces were analyzed to determine the dip of the faults, the sense of motion along these faults and the local stress field. These surficial data has been combined with existing and new acquired MT/TEM, seismic geophysical studies in the past several years to update the conceptual model for this geothermal prospect. The Panyimur geothermal area appears to be a fault-controlled system that is associated with a 1.5 to 2 km-wide step-over linking two NNE-striking faults. The local and regional geomechanical data indicate that these NNE-striking faults are nearly pure dip-slip, making this setting a step-over between pure normal faults in contrast to a pull-apart between oblique- or strike-slip faults. Normal fault step-overs are the most common structural setting for geothermal systems in the Basin and Range region in the United States. Two models have been derived; a fault-hosted resource model and a sedimentary resource option with two isothermal patterns on a MT resistivity cross-section. Both models, which have been used to target 15 Temperature Gradient Holes (TGHs), assume that the resource is fault-hosted (with no direct magmatic contribution of heat or fluid) and heated by deep circulation to 3000 or 4000m depth.