Status of geothermal energy exploration at Buranga geothermal prospect, Western Uganda

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1 Status of geothermal energy exploration at Buranga geothermal prospect, Western Uganda ARGeo-C6, Addis Ababa 2016 James Francis Natukunda and Godfrey Bahati Ministry of Energy and Mineral Development Directorate of Geological Survey and Mines, P.O Box 9, Entebbe, Uganda. s: 1

2 OUTLINE Objective Location Regional structural setting Geological mapping of Buranga Geochemistry Geophysical surveys Conclusions Way forward 2

3 OBJECTIVE To explore geothermal energy for electrical power production and direct use. 3

4 LOCATION (Source: James Wood and Alex Guth; Space Shuttle radar topography image by NASA). 4

central & Southern fault domains Area is seismically active

5 REGIONAL STRUCTURAL SETTING OF BURANGA Main faults strike NE- SW Buranga is close to the central accommodation zone between central & Southern fault domains Area is seismically active with deep seated Earthquakes up to 40 km Buranga Source: PEPD, Uganda 5

6 GEOLOGICAL MAPPING OF BURANGA Local geology General Buranga is located NW of Rwenzori massif near the base of Bwamba escarpment that strikes 45 o & dips 65 o Hot springs emerge through Quaternary sediments Crystalline basement Consists of Archaean, (TTG gneiss and granite), Palaeoproterozoic Buganda Group (amphibolites and Mica schist with quartzitic interbeds) Highly faulted (faults strike NE-SW & nearly E-W) and rock outcrops have complex joint systems 6

7 GEOLOGICAL MAPPING cont d Sediments Quarternary Epi-Kaiso beds and variable sands & gravels with irregularly distributed boulders Kisegi formation: Underlies Kaiso beds; fine to medium-grained, poorly consolidated sands & clays, some coated with calcareous material. Geothermal manifestations 3 main manifestation areas Mumbuga, Nyansimbe and Kagoro Occur in a linear structure striking & oblique to Bwamba rift escarpment 7

8 1. Hot springs Occur at Mumbuga, Nyansimbe & Kagoro Bubble out colourless gases Surface temp. is 98 C and Flow rate is l/s 2. Water pools Have numerous hot springs e.g. Nyansimbe water pool Manifestations cont d Mumbuga erupting spring Nyansimbe pool 8

9 Manifestations cont d 3. Travertine cones (calcareous tufa) The tufas rise from less than 1m to 1.5m high at Mumbuga and Kagoro Some tufa cones are still active with hot springs emerging from them At Nyansimbe, tufa forms terraces around the main water pool Kagoro 9

10 Manifestations cont d 4. Hot grounds Patches of hot, dry and whitish grounds occur close to hot springs as at Mumbuga area. Characterised by hydrothermally altered crystalline rock boulders, cobbles and pebbles and rift sandy-clayey sediments. In some parts the sediments have completely altered to clays 10

11 Manifestations cont d 5. Hydrogen sulphide and sulphur A smell of H 2 S gas is common in areas around the hot springs. During the dry season when the GW table is low some gas vents (fumaroles) form at Mumbuga and Kagoro areas. Sulphur occur at Mumbuga and Kagoro thermal areas 11

12 Manifestations cont d 6. Secondary minerals Crystalline halite and other secondary precipitate around the gas vents and on hot grounds 7. Geothermal grass Green geothermal grass grows around the hot springs At Nyansimbe, GG forms a big swampy area with several hot and warm springs in it 12

13 GEOCHEMISTRY Buranga fluids are neutral with a ph of 7-8 and Salinity of 14,000 17,000 mg/kg TDS Plausible solute geothermometers tested for Buranga hot springs and pools predict a subsurface temps. of C The gas composition is dominated by CO 2 and no H 2 which suggests subsurface temp. is <200 C (Ármannsson, 1994) 13

14 d 2 H ISOTOPE HYDROLOGY Hot Springs Rivers Groundwater Kibenge-HS d 18 O LMWL The δd and δ 18 O data plot close to LMWL (δd = 8*δ 18 O ) No signs of oxygen shift, low to medium subsurface temp is inferred Depletion in both δd and δ 18 O compared to local GW suggests recharge is from high ground (Rwenzori Mt) No tritium in Buranga thermal waters implying no mixture of hot water and cold GW Strontium ratios: 87/86 SrH 2 O, 87/86 SrRock indicate that Buranga thermal waters interacts with granitic gneisses Source of sulphate in water is minerals or rock (terrestrial evaporates) with magmatic contribution Isotopic ratio 3 He/ 4 He in gaseous discharged from hot springs suggests a magmatic source of solutes (BGR-GoU,2007) 14

15 ISOTOPE HYDROLOGY cont d Reservoir temps. of Buranga 200 C Sulphate-water (S 18 O 4 -H 2 18 O) isotope geothermometer 15

flow/outflow zone Micro-seismic surveys located a subsurface anomaly in

16 GEOPHYSICAL SURVEYS 1. Micro-seismic surveys ( BGR- MEMD, 2007) Conc. of EQs around Buranga hot springs suggests area of high permeability up flow/outflow zone Micro-seismic surveys located a subsurface anomaly in the vicinity of hot springs which could be source of heat Source: BGR-MEMD, 2007

17 GEOPHYSICAL SURVEYS 2. TEM/MT surveys TEM survey was aimed at mapping shallow structures and for static shift correction on the MT soundings made on the same site MT survey was used to map deeper resistivity structures of the area. 108 MT and 94 TEM soundings were made at approx m interval 17

18 Geophysical surveys cont d Iso-depth resistivity map at 600 m depth (at sea-level) Extensive low resistivity due to conductive minerals in clays in the sediments Conductivity is high around the hot springs possibly due to geothermal fluids Courtesy: GIDS Consult Ltd,

19 Iso-depth Resistivity map at 1500 m depth ( 1 km b.s.l) Extensive low resistivity anomaly possibly due to conducting minerals in the sediments Geophysical surveys cont d Courtesy: GIDS Consult Ltd,

20 Additional MT/TEM surveys (2015) Iso-depth Resistivity map at 3,000 m depth ( 2.4 Km b.s.l.) Extensive low r resistivity due to minerals in rift sediments High conductivity at the hot springs and area towards SW possibly indicating a reservoir Courtesy: GIDS Consult Ltd,

21 Geophysical surveys cont d Cross sections NW-SE perpendicular to the rift strike Conductive layers due to conductive sediments Resistive body at about 3,000m b.s.l possibly a reservoir/source of heat Courtesy: GIDS Consult Ltd,

22 Geophysical surveys cont d Cross sections NE-SW along the rift strike Conductive layers due to conductive sediments Resistive body at about 3,000-4,000 m b.s.l possibly representing reservoir/heat source High resistive body at 5,000 m due gaps in data Courtesy: GIDS Consult Ltd,

23 Conclusions Impressive geothermal surface manifestations occur at Buranga Buranga is a basin range, deep fracture/fault extensional circulating system Subsurface temperatures of C are inferred by geothermometry;, if confirmed, these temperatures are good enough for electricity production and for direct use in industry and agriculture Isotope hydrology results indicate the recharge (source) of the geothermal fluids at Buranga to be from high ground in the Rwenzori Mountains; subsurface temperatures of 200 C have been predicted; source of heat is magmatic and reservoir rock types are granitic gneisses. The faulted and fractured basement of the Rwenzoris provides adequate permeability necessary for recharge and & flow of thermal fluids at Buranga. TEM and MT surveys at located a low resistivity anomalies up to 3000 m.b.s.l; that could represent reservoir/heat source 23

$Way forward Carry out infill TEM/MT surveys (500m interval), to close gaps and delineate the geothermal reservoir Carry out refraction seismic surveys to locate structures/faults that could be$

24 Way forward Carry out infill TEM/MT surveys (500m interval), to close gaps and delineate the geothermal reservoir Carry out refraction seismic surveys to locate structures/faults that could be conduits for the geothermal and groundwater Carry out CO 2 & Rn surveys to trace structures that constitute up-flow zones Update geothermal models and locate of exploratory drill sites; and Collect baseline data for sociological and environmental impact assessment Drill 2-3 exploration wells in the delineated geothermal anomalies to confirm existence of geothermal reservoir 24

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Geothermal Energy Exploration at Buranga Geothermal Prospect, Western Uganda

Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 Geothermal Energy Exploration at Buranga Geothermal Prospect, Western Uganda James Francis NATUKUNDA and Godfrey BAHATI