Geothermal Provinces of KENYA

By Skúli Jóhannsson 23.09.2009

This is an automated transcription from the original report; some formatting (figures, tables) lives only in the PDF. The original is available as a PDF, from the full archive.

Annað veldi ehf Geothermal Provinces of Kenya David Köndgen and Skuli Johannsson September 29, 2009

OUTLINE

• Overview Geology of Kenya • Excursus: The Wilson Cycle • How do rifts form? • The world of Platetectonics and Hotspots • Hotspots and heat anomaly – 1. How is the heat obtained? – 2. Mantle transition zone structure – 3. Edge-driven convection model – 4. Rift geothermal model

• Geothermal regions of Kenya and temperature gradient in the Rift Valley • Southern Rift Valley – Olkaria volcanic complex – Eburru geothermal field – Longonot geothermal field – Suswa volcano • Central and Northern Rift Valley – More potential and projects • Conclusions • References

Overview Geology

• The Kenyan Rift, which is part of the East African Rift System (EARS), is the most important geological feature of Kenya: – Classic example of a continental rift – modern, actively spreading – Volcanic activity related to the rift started ca. 30 Ma (Million years ago) in northern Kenya, moving southwards, reaching northern Tanzania around 8 Ma – Volcanism especially in northern Kenya continues to the present

Volcanoes

Fig. 1: The East African Rift System, modified after Wood and Guth

Excursus: The Wilson Cycle Fig. E: The Wilson Cycle, showing stages of rifting and ocean formation, from futura-sciences.com

How do rifts form?

• The Kenyan Rift, which is part of the East African Rift System (EARS), is the most important geological feature of Kenya:

– Popular model assumes elevating heat flow from the mantle

Triple junction

– Causing a pair of thermal “bulges”in central Kenya and Arfar (Kenya and Ethopian Dome, respectively)

– “Bulges” are initiated by plumes heating the overlying crust

Volcanoes

– Streching and fracturing  Horst and Graben structure

– Ideally three branches occur from one point with an angular separation of 120°  that point is called a triple junction

Fig. 12: The East African Rift System, showing different

branches

modifiedfaroftmer Wood and Guth

How do rifts form?

• The Kenyan Rift, which is part of the East African Rift System, is the most important geological feature of Kenya: – Popular model assumes elevating heat flow from the mantle – Causing a pair of thermal “bulges”in central Kenya and Arfar – Streching and fracturing  Horst and Graben structure – “Bulges” are initiated by plumes heating the overlying crust – Ideally three branches occur from one point with an angular separation of 120°  that point is called a triple junction

Fig. 3: Textbook horst and graben formation,

from Wood and Guth

The world of Platetectonics and Hotspots

Kenya

Divergent plate boundaries Transform plate boundaries Convergent plate boundaries Plate boundary zones Selected Hotspots

Afar Hotspot

Fig. 4: World overview of selected Hotspots and plate boundaries,

after wikipedia.org

Turkana Plume South African Superplume

Ingredients of a geothermal field • An Aquifer and a cap rock • Aquifer – Porous rocks that can store water • Cap rock – Prevents fluid escaping upwards – Mudrocks, clays and unfractured lavas are ideal – Given time, many geothermal fields develop there own cap rocks  not always the youngest volcanic areas are the most productive for geothermal » rocks are such good insulators  magmatic intrusions stay hot for millions of years » Such intrusions, therefore, act as a focus for „hot fluid“

Fig. 5: Conceptual model of a typical volcanic geothermal system,

Boyle (2004)

Hotspots and heat anomaly Fig. 6: Hotspot, David Köndgen • Arfar Hotspot • Plume under Turkana Depression • South African Superplume

1. How is the heat obtained?

• Situation in northern Kenya (Turkana depression)

– Distinctive crustal extension

leads to a progressive

thinning of the lithosphere

6°N

and a 150 km wide fault

zone

– Underlying and hot Asthenosphere is now closer to the surface

– Creation volcanoes and rising of geothermal gradients

Key: SCLM = Subcontinental

Lithospheric

Mantle

Fig. 7: Linked evolution of Turkana rifting and magmatism, after T. Furman

2. Mantle transition zone structure

• Seismic studies of wavetraveltimes have delineated velocity anomalies beneath Kenya • Localized 350°C thermal anomaly at the 410 km discontinuity • Discontinuity beneath the rift in Kenya and northern Tanzania depressed 30-40 km  increase in temperature • Thermal anomaly down in the mantle is much broader than at the top, extending all over east Africa (1000-1500 km wide)  Diffusion of heat connected to thermal upwelling (which will be explained next)

Fig. 7: Derivation from normal mantle temperature in Kenya and northern Tanzania, from Huerta et. al (2009)

Fig. 8: Schematic cross-section showing receiver function

stacks of mantle transition zone and cartoon of the

associated thermal upwelling,

Huerta et. al (2009)

3. Edge-driven convection model

• Occurence in small-scale along Cratons (regions of continent, which have been stable for >1 billion years)

Outline of Precambrian Cratons

• Estimated seismic velocity in the picture shows regions of downwelling (cold thermal anomalies, blue) and upwelling (hot thermal anomalies, red)

• Hotspots (located in Arfar

and underneath Turkana),

the South African

Superplume and volcanism

in Eastern Africa might be

connected to this down- and

upwelling

Fig. 9: Heat generation due to edge-driven convection, depth: 600 km,

showing high- and low-velocity regions (blue and red,respectively),

after King (2000)

4. Rift geothermal model

Fig. 10: Rift geothermal model,

Mwangi (2006)

Geothermal regions

Northern Rift Valley

Ethiopia

Uganda Somalia

Central Rift Valley

KENYA

Southern Rift Valley

Nairobi

100 km Fig. 11: Kenyan geothermal regions, after www.commons.wikimedia.org

Geothermal gradient

Uganda Somalia

> 200°C/km > 200°C/km > 200°C/km > 200°C/km > 200°C/km

Ethiopia KENYA Nairobi

> 200°C/km 100 km Fig. 12: Kenyan geothermal gradient in Rift Valley, after www.commons.wikimedia.org

Uganda Somalia

Southern Rift Valley Ethiopia KENYA Nairobi 100 km Fig. 13: Southern Rift Valley, after www.commons.wikimedia.org

Olkaria volcanic complex

• Most interesting geothermal area: – Thermal gradient: > 200°C/km – Reservoir depths: 750-900 m (steam dominated) and 11001300 m (richer in water) – Reservoir temperature: 340°C – Covered area: about 80 km2 – 110 wells drilled so far • Exploration started 1956: Olkaria I • Increasing interest in 1970´s leads to initial production start in 1981 • Situation in 2009: 129 MW installed – Olkaria I producing 45 MW – Olkaria II producing 70 MW – Olkaria III producing 12 MW – Planned: 576 MW until 2025

Fig. 14: Overview of geothermal prospects within the Rift

Valley of Kenya,

after J. Were, KenGen (2009)

Olkaria volcanic complex

• Most interesting geothermal area:

– Thermal gradient: > 200°C/km

– Reservoir depths: 750-900 m (steam dominated) and 11001300 m (richer in water) – Reservoir temperature: 340°C

Oserian

Olkaria II Olkaria I

– Covered area: about 80 km2

– 110 wells drilled so far

Olkaria IV

• Exploration started 1956: Olkaria I • Increasing interest in 1970´s leads

Olkaria III

to initial production start in 1981 Fig. 15: Olkarian fields in the area of Olkaria volcanic • Situation in 2009: 129 MW installed complex (about 80 km2), Oserian is a green house complex

– Olkaria I producing 45 MW

(2 MW Binary power plant), growing flowers to be shipped by

air to Europe,

after J. Were, KenGen (2009)

– Olkaria II producing 70 MW – Olkaria III producing 12 MW – Planned: 576 MW until 2025

KenGen = Kenya Electricity Generating Company, Government Company, licensed in 1982

Olkaria volcanic complex

Fig. 16: Schematic cross section through Olkaria

reservoir,

after Bhogal (2000)

Eburru geothermal field

• Interesting geothermal area: – Thermal gradient: > 200°C/km – Reservoir depths: 2400-2700 m – Reservoir temperature: 131-156°C, max. 279°C – High enthalpy area: about 2 km2 – Wells drilled in six thermally anomalous areas • Proven geothermal resource: 20 MW • Current situation: 5 MW binary geothermal power plant, KenGen • Previous use: drying pyrethrum and domestic water supply via condensers

Fig. 14: Overview of geothermal prospects within the Rift

Valley of Kenya,

after J. Were, KenGen (2009)

Eburru geothermal field

Fig. 17: Piezometric map of Southern Kenya Rift, showing Eburru and Olkaria geothermal fields, after J. Lagat (2003)

Longonot geothermal field

• Interesting geothermal area: – Thermal gradient: > 200°C/km – Reservoir depths: about 1500 m – Reservoir temperature (under the volcano): > 250°C – Prospect area: about 60 km2 – Well drilled to 55 m, half a kilometer south of Longonot recorded a bottom hole temperature of 200°C • Geothermal resource (KenGen): approximately 200 MW • Joint Geophysical Imaging study in 2004, African Rift Geothermal initiative (Argeo) plans pilot plant this year

Fig. 14: Overview of geothermal prospects within the Rift

Valley of Kenya,

after J. Were, KenGen (2009)

Suswa volcano • Good prospect: – Thermal gradient: > 200°C/km – Reservoir temperature: >250°C (caldera), range between 270°C and >300°C (fumarole geothermometry studies) – Two calderas area: 120 km2 • Geothermal resource (KenGen): approximately 200 MW • Detailed scientific studies by KenGen in 1992-1993, possibly shallow heat source under the caldera

Fig. 14: Overview of geothermal prospects within the Rift

Valley of Kenya,

after J. Were, KenGen (2009)

Uganda Somalia

Central and Northern Rift Valley Ethiopia KENYA Nairobi 100 km Fig. 18: Central and Northern Rift Valley, after www.commons.wikimedia.org

More potential and projects

Fig. 19: Overview of geothermal projects, Rift Valley of Kenya,

after J. Were, KenGen (2009)

Conclusions • Kenya has a really huge potential of geothermal regions due to ist geological situation (EARS): – Very good potential • Shallow lithosphere-asthenosphere boundary • General high heat flow • Anomalous geothermal gradient of >200°C/km • Higher temperatures at shallower depths • A lot of research and drilling done yet • Successfully operating powerplants at Olkaria 27/28

References

Bhogal, P. S. (2000): Reservoir models of the Olkaria geothermal project (1975-1985), Proceedings World Geothermal Congress 2000, Kyushu-Tohoku, Japan.

Boyle, G. (Edt.) (2004): Renewable Energy – Power for a sustainable future, Oxford University Press with the Open University, Milton Keynes.

Furman, T. et. al (2006): Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-m Volcanism in Africa, Journal

of Petrology, 47 (6): 1221–1244, doi:10.1093/petrology/egl009.

Furman, T., et. al (2004): East African Rift System (EARS) plume structure: insights from Quaternary mafic lavas of Turkana, Kenya. Journal of Petrology, 45, 1069-1088.

Huerta, A. et. al (2009): Mantle transition zone structure beneath Kenya and Tanzania:more evidence for a deep-seated thermal upwelling in the mantle,

Geophys. J. Int. (2009), doi: 10.1111/j.1365-246X.2009.04092.x.

Karekezi, S. and Kimani, J. (no year): Renewable Energy Technology Transfer in Africa: The Value of Demonstration, The Case of Bagasse-based Cogeneration and

Geothermal, Powerpoint Presentation, Nairobi ,Kenya.

King, S. and Ritsema, J. (2000): African Hot Spot Volcanism: Small-Scale Convection in the Upper Mantle Beneath Cratons, Science 10 (290), 5494, 1137–1140, doi:

10.1126/science.290.5494.1137.

Lagat, J. (2003): Geology and the geothermal systems of the southern segment of the Kenya Rift, International Geothermal Conference (September 2003, session #4),

Reykjavík, Iceland.

Lin, S-C et. al (2005): Thermal plume models and melt generation in East Africa: A dynamic modeling approach, Earth Planetary Science Letters.

Mwangi, M. (2006): Geothermal development in Kenya: The indegenous, renewable, green option, Kenya Electricity Generating Company Ltd. (KenGen), Nairobi, Kenya.

Ni, S., et. al (2004): Three-dimensional structure of the African superplume from wavefront modelling, Geophys. J. Int. (161) 283-294, doi: 10.1111/j.1365-

246X.2005.02508.x.

No names, (2006(?)): Geothermal potential in Kenya.

Nyblade, A., et. al (2000): Seismic evidence for a deep upper mantle thermal anomaly beneath East Africa, Geology, 28, 599-602.

Were, J. and Mwangi, M. (2009): An overview of geothermal development in Kenya, Kenya Electricity Generating Company Ltd. (KenGen), Nairobi, Kenya.

http://geology.com/articles/east-africa-rift.shtml (James Wood and Alex Guth - Michigan Technological University)

http://www.futura-sciences.com/fileadmin/Fichiers/images/Terre/cyclewilson.jpg

http://www.worldbank.org/html/oed

http://www.kengen.co.ke/

First picture (Mount Kenya) from: http://www.myeverestchallenge.com