CATHERINE NYAKECHO

GEOLOGIST

Department of Geological Survey and Mines, Uganda

Email: ncathy.geology@gmail.com

World Geothermal Congress 2010, Bali, Indonesia. 25th-30th April, 2010

ACKNOWLEDGEMENT

� Funders of the project, UNDP/UNDESD co-funded by OPEC and Iceland

� Gestur Gíslason and Halldór Ármannsson, UNU-GTP Iceland.

� All the trained geothermal team in Uganda.

CONTENT

1.Introduction2.Field activities3.Results, chemical analysis and discussion4.Conclusion4.Conclusion

LOCATION

Kibiro

BurangaBuranga

Katwe

�The Prospects are all located in the Albertine graben that runs alongthe border of Uganda with the DRC.�The three areas were chosen for study because of their volcanic andtectonic features that indicate heat sources and high permeability.

OBJECTIVES OF THE STUDY

�The principal purpose of this geochemical survey in thethree propects was to predict subsurface temperatures, obtaininformation on the origin of the geothermal fluid andunderstand subsurface flow directions.

�The basis philosophy behind geochemicalprospectingof�The basis philosophy behind geochemicalprospectingofgeothermal resources is that the concentration of manycomponents in the geothermal fluid, i.e. neutral aqueoussolutions and gaseous steam, reflect thermal conditions atdepth.

FIELD ACTIVITIESThe sampling was done in 1993-1994 (UNDP/UNDESD project co-funded by OPEC and Iceland).

The main activities:�Chemical sampling of geothermal surface manifestations(hot springs);�Surfaceandgroundwaterstemperature;�Surfaceandgroundwaterstemperature;�Flow rates;�PH field and conductivity measurements;�Preparation of samples (Filtering, sample for CO2/H2S);�Analysis of the samples for major and minor componentsand stable isotopes;�Sample analysis and interpretation;

•pH

•Filtering

Field activities continued…………•Hot springs with flow over 0.5 L/s were preferentially sampled since these were less likely to have been affected by dilution and evaporation.

•Water samples were collected in a jug (on a rod, at Nyansimbe pool) so that they could be obtained from the center of the spring (its hottest part, presumed to be the point where hot water enters the pool).

•The waters were transferred to a rinsed 100ml plastic bottle.•The waters were transferred to a rinsed 100ml plastic bottle.•The untreated H2O samples were to be analyzed for PH-lab, SO4, HCO3.

•Another set of H2O samples were treated at the time of collection (filtered-0.4µm filters, acidified with HNO3-1ml of acid/100ml of H2O) & were collected into a rinsed 50-ml polythene bottles. This was to be analyzed for Cl, SiO2 & major cations.

•All samples were then bottled at the sampling temperature to minimize errors arising from evaporation.

Katwe Kigorongo Geothermal, surface and ground water sampling points.

Buranga Geothermal surface and ground water sampling points.

Kibiro Geothermal surface and ground water sampling points.

RESULTS CHEMICAL ANALYSIS AND DISCUSSION

Laboratory analysis was carried out at BGR, Germany.

High salinity, high CO3 ,SO4contents tend to invalidates solutegeothermometer results. Subsurface temperatures are estimated at 140-200°C using plausible solute geothermometers (Ármannsson, 1994).Lake Kitagata and Lake Katwe craters have high levels of H2S of about30-40 ppm which, suggests the source of the geothermal water to bevolcanic and hydrothermal (Bahati, 2003).

�Neutral PH and salinity of 14,000 – 17,000 mg/kg TDS.� After a good agreement from earlier study by Ármannsson (1994) forall solute geothermometers tested for several hot springs and pools, itwas concluded that the subsurface temperature was 120-150°C.�No H2S from gas analysis indicating that the subsurface temperaturecannot exceed 200°C.�There are no indications of mixing of the geothermal and cold water.

•Neutral pH, salinity of up to 4,000 - 5,000 mg/kg TDS. •High Cl compared to the SO4 and HCO3 concentration and is classified as mature waters suitable for application of geoindicators.

•Relatively low B values compared to Cl and Li suggest that the fluids are more likely to originate from volcanic basement rocks rather than from the young overlying sediments.

Discussion Kibiro Results Continued:

•Results by Ármannsson (1994) for different geothermometers for hotwater samples from Kibiro fell into two groups ; Temperature of about150 C and another 200 - 220 C.

• One component solute geothermometers, e.g. quartz, andgeothermometers based on ratios of components that equilibrate fast, e.g.K-Mg gave the lower temperatures . The higher temperature wasobtained by geothermometers based on ratios between components thatequilibratemoreslowly, e.g. Na-K, andgasgeothermometers.equilibratemoreslowly, e.g. Na-K, andgasgeothermometers.

•These observations suggest that the geothermal fluid is a mixture of ahot water component at 200 - 220°C and cold water producing a secondequilibrium at 150°C.

• A subsurface temperature of 200 - 220°C is therefore inferred bygeothermometry and mixing models for Kibiro.•The fluids maybe dilute and would probably be hard to utilize.

CONCLUSIONS1. Katwe –Kikorongo

•The chemistry of the waters from the surface manifestationsand hot springs indicate high subsurface temperatures.•Geothermometry has predicted reservoir temperatures in therange of 150 - 200 C.

•The fluids in the crater lakes and springs are saline and•The fluids in the crater lakes and springs are saline andalkaline and are probably concentrated by evaporation and/ormodified by near surface processes, but the reservoir fluidsmay be more dilute. This is supported by the low salinity ofthe geothermal waters sampled from the hot springs in theLake Kitagata crater as compared to its lake water.

•The fluids would be suitable for direct power production.

2. Buranga Geothermal prospect

•The reservoir temperatures for Buranga are estimated at 120-150 C by geothermometry.

•There is plentiful supply of fluid, which is fairly saline withtotal dissolved solids of up to 14,000 - 17,000 mg/kg.

•This fluid would be suitable for binary turbine power•This fluid would be suitable for binary turbine powerproduction and industrial use.

3. Kibiro Geothermal prospect

•Chemistry and stable isotope data interpretation has indicatedmixing of geothermal water and cold groundwater in Kibiro.

•A subsurface temperature of 200 C and above is predicted bygeothermometry and mixing models.

•The geothermal fluid is less saline, neutral, with total•The geothermal fluid is less saline, neutral, with totaldissolved solids of approximately 4,000 - 5,000 mg/kg.

•This fluid would be suitable for direct power production.

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