PROCEEDINGS, Kenya Geothermal Conference 2011 Kenyatta International Conference Centre Nairobi, November 21-22, 2011

GEOTHERMAL SURFACE EXPLORATION APPROACH: CASE STUDY OF MENENGAI GEOTHERMAL FIELD, KENYA

John Lagat

Geothermal Development Company Ltd,

P. O. Box 17700-20100,

Nakuru, Kenya jlagat@gdc.co.ke

ABSTRACT

Geothermal exploration in a new prospect involves multi-disciplinary

approach where surveys involving different geoscientific methodologies are conducted so as to be able to accurately develop a working model that will

be used to site exploratory wells. These methods include geological

mapping of the prospect area to study the volcanological evolution of the

volcano and be able to model the heat source(s). Hydrogeological surveys and structural analyses are also carried out to relate their association with

the development of the geothermal systems. Geophysical techniques

normally employed included transient electromagnetic (TEM),

magnetotellurics (MT), seismics, gravity and magnetics. Integrated

geophysical surveys assist in imaging the subsurface to identify the geothermal reservoirs, the heat source and the depth, buried structures and

the size and extend of the geothermal system. Geochemical surveys include

sampling of gas and steam condensate from fumaroles, carrying out ground

radon and CO2 traverses and hot spring and ground water sampling. Analyses and interpretation of geochemical data provide information on the

subsurface temperatures, nature of geothermal reservoirs, origin of

geothermal fluids and to map permeable zones. Surface heat loss surveys are

carried out to determine the amount of heat lost by conduction and convection, hence understand the nature and size of the heat source.

Integration of all the geoscientific data assists in the development of a

geothermal model where the heat source and the size, the recharge of the

system, the structures that control the geothermal system and the depth to the reservoir(s) are envisaged. This paper therefore describes the approach

used to carry out surface exploration in Menengai geothermal field to the

development of a conceptualized geothermal and siting of exploration wells.

Keywords: Exploration, conceptual model

INTRODUCTION

Geothermal Energy

Geothermal energy is the natural heat stored within

the earth. The resource is manifested on the earth’s

surface in the form of fumaroles, hot springs,

steaming grounds and altered grounds. The economically usable geothermal energy is that

which occurs close to the earth’s surface where it

can be tapped by drilling wells up to 3,000 m

below the earth’s surface. Such shallow heat sources are in most cases attributed to volcanic

activity, which in many cases are associated with

plate boundaries. The East African Rift is a good

example of this. Thus, potential areas for the

development of high temperature geothermal systems are associated with young Quaternary

volcanoes similar to those that occur within the Rift

valley.

As in the search for any natural resource, a strategy

for geothermal energy exploration must be defined and followed as stipulated. Once a geothermal

prospect area has been identified, the next step is to

use the various exploration techniques to locate the

most interesting geothermal area and identify suitable targets for resource exploitation.

EXPLORATION METHODS

To reduce the cost of exploration, it is normally

approached in a prescribed sequence of steps, altering the order from time to time depending on

prior knowledge of the area in question. In some

cases, high costs will lead to the elimination of

some steps in the sequence. This normally

involves multi-disciplinary approach whereby all the geoscientific methods are applied. In Menengai,

the following multi-disciplinary approach was

followed:

Geological and hydrogeological surveys,

Geochemical surveys,

Lagat

Geophysical surveys,

Heat Loss surveys and finally

An Environmental and Social Baseline Social

Impact Assessment (ESIA) was also carried before

drilling commenced so as to determine expected

impacts and their mitigations.

The discussions of the results from the

multidisciplinary surveys carried out in Menengai

geothermal prospect and the development of the conceptualized model to siting of the exploratory

wells is described in the subsequent sections below.

Geoscientific Surveys Results

Geological mapping

Geological surveys provided information on the evolution of Menengai volcano, hydrogeological

controls and also the stratigraphic and structural

framework of the area. Menengai geothermal

reservoir is associated with a caldera volcano and therefore the heat source is associated with the

partially emptied magma chamber below the

volcano. Continued intra and post caldera eruptions

estimated to be a few hundred years indicate that the magma body is still active. The hydrogeologic

regime comprises of recharge from the higher rift

scarps and the intense rift floor fracture/faulting

resulting from extensional tectonics of continental rifting, provide for a good structural set-up that

allows water from the rift scarps to penetrate deep

into the crust and thus up flows into shallow

reservoirs under Menengai. The regional TVA’s

are important conduit of deep fluids thus an

important geothermal controlling feature in the

area.

The main structures in Menengai caldera volcano

include the 88 km2 caldera, Solai and Ol Rongai

TVA (Figure 1). The floor of the Menengai

geothermal prospect depicts extensional tectonics with the main trough trending N-S over north of

Menengai and NNW-SSE for section south of

Menengai. The Ol Rongai structural system

represents a part of the larger Molo TVA that has had a lot of volcanic activity including eruptions

resulting to a buildup of NNW trending ridge

referred to as Ol Rongai volcanoes. The Solai

tectonic axis is a narrow graben averaging 4 km wide that runs on an N-S direction from the eastern

end of Menengai caldera, through Solai. It is

comprised of numerous fault/fractures all-trending

in N-S direction.

Geophysical mapping

The rocks within the earth's subsurface have

physical properties that vary from place to place.

The common properties include electrical

conductivity, seismic energy transmission, magnetic acquisition and gravity attraction. The

geophysical methods employed in exploration for

geothermal energy in Menengai are the electrical

conductivity techniques, which include transient electromagnetic (TEM) and the magnetotelluric

(MT), gravity, magnetic and seismic methods.

0

9920 S

0

2000 N

L. Bogoria

L. Baringo

Satima fault

L. Nakuru

Kilombe volcano

Eburru

L. Elmentaita

L. Naivasha

170000 m E

Volcanic center

Fault

Inferred fault

Lake

LEGEND

Caldera wall

20 km0 Scale

Olbanita

Menengaicaldera

Prospect area

Figure 1: Map showing the structural set-up of the Kenya rift floor.

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Gravity Surveys

To examine gravity distribution beneath Menengai

caldera several cross-sections were constructed along A-A´, B-B´, C-C´ and D-D´ as shown in

Figure 2.

The trend for all these sections (Figure 3) indicates low gravity within the caldera into which a high

density body is superimposed. Peaks are also

observed on this high density body suggesting

shallow dykes or intrusions that form the heat sources.

Electrical Resistivity Surveys

Electrical Resistivity methods play a significant role in the investigation for geothermal energy

since the methods probe deep into the subsurface.

The geophysical techniques that were employed in

Menengai geothermal field included TEM and MT.

Joint TEM and MT inversions were carried out to image the subsurface for the existence of

electrically conductive zones that form the

geothermal reservoirs and the results used to

interpret the data.

Figure 4 shows MT resistivity cross -section E-W

passing through the caldera. This profile shows

generally higher resistivity near the surface which is probably due to un-altered rocks in the near sub-

surface. Underlain is a low resistivity layer about 1

km thick which run accross the entire cross -section.

This shallow low resistivity layer on this profile

defines the clay layer formed due to hydrothermal

alteration at the upper zone of the geothermal system in Menengai prospect and the outflow

zones. A localized low resistivity anomaly is also

observed at a depth of about 4 km. This low

resistivity body could be associated with magmatic intrusion which is a probable source of crustal

fluids for this prospect.

Seismic Surveys

Seismic studies (Young et al., 1991; Simiyu and

Keller, 2001; Tounge et al, 1992) indicate that most

of the activity is above the depth of 6-7 km (Figure

5), as shown by the seismic attenuation trends coinciding with the principal direction of the faults .

Figure 5 shows depth event distribution along a

NW-SE profile through Ol Rongai hills and

Menengai caldera.

Events are shallower below Ol-Rongai hills and

Menengai caldera. Interpretation of these

observations indicates that a geothermal system

exists in the area and is shallower below Ol Rongai hills and Menengai caldera. Seismic events show

attenuation below 4 km at Menengai caldera and

below 4-5 km at Ol Rongai hills indicating the

brittle-ductile transition zone, confirming the magmatic bodies forming the heat sources for the

geothermal system occur below those depths .

168000 170000 172000 174000 176000 178000 180000

Grid Eastings

9972000

9974000

9976000

9978000

9980000

9982000

Gri

d N

ort

hin

gs

-1890

-1865

-1840

-1815

-1790

-1765

-1740

-1715

-1690

-1665

-1640

-1615

B B'

C C'

D

D'

A A'

Figure 2. Profiles of the gravity cross-sections through Menengai caldera (after Mungania et al, 2004).

PROCEEDINGS, Kenya Geothermal Conference 2011 Kenyatta International Conference Centre Nairobi, November 21-22, 2011

168000 170000 172000 174000 176000 178000

Grid Eastings

Profile BB'

-1860

-1820

-1780

-1740

-1700

-1660

Grav

ity

168000 170000 172000 174000 176000

Grid Eastings

Profile DD'

-1860

-1820

-1780

-1740

-1700

-1660

Gra

vity

168000 170000 172000 174000 176000 178000

Grid Eastings

Profile CC'

-1860

-1820

-1780

-1740

-1700

-1660

Grav

ity

West

East

West

West

East

East

NW SE

Caldera

Caldera

Caldera

Caldera

168000 170000 172000 174000 176000 178000

Grid Eastings

Profile AA'

-1860

-1820

-1780

-1740

-1700

-1660

Grav

ity

Figure 3. Gravity cross-sections through Menengai caldera as shown in Figure 1 (after

Mungania et al, 2004)

PROCEEDINGS, Kenya Geothermal Conference 2011 Kenyatta International Conference Centre Nairobi, November 21-22, 2011

Menengai Caldera

Figure. 4. 2-D East-west MT resistivity cross-section (Lagat, et al., 2010)

.

Men. CalderaNorthWest SouthEast

165000 167500 170000 172500 175000 177500 180000

Distance (M)

-8

-6

-4

-2

0

Depth

(Km

)

Ol Rongai Men. CalderaNorthWest SouthEastMen. CalderaNorthWest SouthEast

165000 167500 170000 172500 175000 177500 180000

Distance (M)

-8

-6

-4

-2

0

Depth

(Km

)

Ol Rongai

Figure 5. Depth distribution of micro-seismic events through Menengai caldera (after Simiyu et al., 1997)

Geochemistry Mapping

Geochemical observations are particularly important for geothermal resource assessment in

the stages of exploration. Prior to drilling, chemical

geothermometers provide the only information on

estimated reservoir temperatures. Similarly, geochemical surveys are also important to delineate

the areal extent of the geothermal.

Geothermometry

One of the most important contributions of geochemistry to geothermal resource assessments

is chemical geothermometery. Geothermometry is

the application of geochemistry to infer reservoir temperatures from the composition of geothermal

fluids. Geothermal fluids that are found at the

surface above Menengai geothermal systems are

fumarole and steaming ground. Some geothermal systems do not have any fumarole or any other

visible surface geothermal activity and in such

areas soil gas mapping (degassing) which normally

comprises radon and carbon dioxide is used. Following are the computed results for both the

gases in the fumaroles and well discharge.

The temperatures calculated using TH2S using the

Arnorsson and Gunnlaugsson (1985)

Lagat

geothermometer gave relatively high values over

270oC ranging between 271-300

oC (Table 1). The

calculated TCO2 temperatures ranges between 324-340

oC. From this well discharge geothermometers

Menengai geothermal reservoir is estimated to be

over 270oC (Table 2).

Table. 1. Gas geothermometers of fumaroles

Fumarole No. Gas Geothermometers (°C)

TH2S TH2S-CO2

MF-1 280 276

MF-2 293 304

MF-6 296 302

MF-8 295 299

MF-9 279 274

MF-12 298 299

Table. 2. Gas Geothermometry of MW-01 Arnorsson and Gunnlaugsson (1985)

Sample ID TCO2

(ºC) TH2S (ºC)

TCO2/H

2 (ºC) TH2S/H2

(ºC)

MW-01-1 339 271 285 328

MW-01-2 340 275 285 325

MW-01-3 339 283 297 335

MW-01-4 334 275 299 341

MW-01-5 333 273 293 334

Radon/CO2 ratios

The source of Rn-222 can be either magmatic

where the uranium accumulates at the late stages of differentiation or from any other source containing

radium, the immediate precursor of Rn-222. When

Ra-226 is present in the hydrothermal system, it

indicates that the hot water percolates through the host rock thus dissolving Rn-222, which is

produced from alpha decay of Ra-226. Upon

boiling of the water, the Rn-222 partitions into the

steam phase and is transported to the surface through permeable zones. Due to its short half-life,

Rn-222 has to travel for long distances within a

relatively short period to be detected on the surface.

Similarly, CO2 has to travel through a relatively permeable zone to avoid dispersion and subsequent

dilution for it to be detected in high concentrations.

CO2 may also originate from other sources like

organic sources, which are likely to give false

impressions of a geothermal source. The ratio of the two gases would be a good indicator of the

magmatic source of the gases since the ratio is not

expected to change if they are from the same

source. High values of these ratios are found inside the caldera and towards the north and north western

parts of the caldera. High values of Rn and CO2

ratios are found inside the caldera with a circular

pattern like that of radon (Figure 6).

Figure 6: Radon/CO2 ratios distribution map

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Heat Loss Surveys

Conductive heat loss and convective heat loss

measurements were carried out in the prospect (Mungania et al., 2004). Heat flow data in the

prospect covered an area of close to 900 km2,

where Shallow 1 m temperature gradient holes

were drilled and temperatures obtained on the surface and at 25 cm, 50 cm and 1 m depths below

the surface. Figure 7 shows the distribution of

temperatures at 1 m depth in the Menengai

geothermal prospect. From this figure, it is observed that areas with high temperatures at 1 m

depth occur at the central parts of the caldera and

around Ol Rongai and Ol Banita regions. Results

indicate that total conductive heat loss from Menengai prospect is estimated to be in excess of

1,060 MWt with over 250 MWt being lost in the

caldera. The convective heat loss is estimated to be

more than 2,476 MWt with 2,440 MWt being lost

in the caldera and this indicates that the heat source in Menengai is huge.

GEOTHERMAL MANIFESTATIONS

Geothermal activity is manifested in this area by

the occurrence of fumaroles (Plate 1), warm springs, steaming/gas boreholes, hot/warm water in

boreholes, Fimbristylis exilis ‘geothermal grass’

(Plate 2) and altered rock/grounds (Plate 3).

Fumaroles are located mainly inside the caldera floor. Three groups of active fumaroles found in

the caldera have aerial extent ranging from a few

m2 to less than a km

2. The two groups in the

central and western portion of the caldera floor are located within fresh lava flow and close to their

eruption centres. The steam emission has a mild

H2S smell. Some sulfatara deposition is evident on

the surface. The other group of fumaroles located in the central

eastern part of the caldera floor is found at the

young lava/pumice contact and has extensively

altered the pumiceous formation. The structural

controls for these groups of fumaroles appear to be the eruption craters that may be the source of the

pyroclastic deposits. The caldera floor is, however,

almost covered by young lava flows.

Figure. 7. Temperature distribution at 1 m depth (modified from Munganiaet, al,. 2004)

Lagat

Plate. 1. Fumaroles on the caldera floor

Plate 2. Fimbristylis exilis ‘geothermal grass’

Plate 3. Altered ground

GEOTHERMAL MODEL

The heat source, the reservoir, the recharge area

and the connecting paths through which cool

superficial water penetrates the reservoir and, in

most cases, escape back to the surface, compose the geothermal system. The multi-disciplinary

surveys conducted in Menengai reveals that all

these conditions have been met and a commercial

geothermal system exists under the caldera and immediate surroundings. These factors are

described in details below and summarized in

Figure 8.

Heat Source

The presence of a caldera beneath Menengai

volcano represents a collapse directly above an 88

km2 partially emptied vast magmatic chamber.

Seismic surveys indicate a ductile brittle zone

below 6 km indicating presence a molten body below that depth. Shallow intrusives conduct the

heat to shallow levels, thus heating the geothermal

fluids The continued post and intra caldera

eruptions that are as young as several hundred years indicate that the body is still active.

Hydrogeology

The location of Menengai prospect on the rift floor

is lower than high rift scarps that form the recharge areas. Meteoric waters flow from the flanks and

the intense rift floor fracture/faulting, provides a

good structural set up that allows water to penetrate

deep into the crust towards the magma bodies.

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Figure. 8: Geothermal conceptualized model of Menengai geothermal prospect (Lagat et al., 2010)

SYSTEM CAPPING

Menengai volcano eruption included eruptions of

pyroclastics and lavas. Glassy component

pyroclastics is usually very susceptible to alteration forming hydrated clays that are cause of self-

sealing. Repeated eruptions of thick lavas also

form very good capping for geothermal systems.

Reservoir Rocks

The rocks that occur at subsurface of Menengai geothermal field prospect where the geothermal

reservoir is hosted are made up of faulted Pliocene

flood lavas, which include mainly trachytes. The

combination of the local and regional structures has enabled fracturing in the reservoir rocks to allow

for permeability and storage of the geothermal

fluid.

WELL SITING

Drilling of exploratory wells represents the final

phase of any geothermal exploration programme

and is the only means of confirming the

characteristics and potential of a geothermal reservoir. Based on results of the surveys carried

out in Menengai indicate a high potential. The

prospect has 4 drilled wells currently and the

exploration wells have proved presence of a viable

geothermal resource. Accurate well siting is very important especially in a new field, which is still

under exploration because it saves costs.

The exploratory well siting involves multi-disciplinary surveys where all the results are

incorporated for accurate results. The exploration

wells sites were placed within the caldera floor

close to the young volcanic centres in the centre of the caldera and within a local gravity high that

trends coincident with the Molo TVA. The area has

Shallow intrusive, which act as the heat source for

the geothermal system. The location being along

the Molo TVA, high permeability is expected arising from fractures associated with the structure.

Low resistivity anomaly in this locality indicates

the presence of a geothermal system. High

geothermal potential of the area is also indicated by

high radon-222 radioactivity and CO2 values both in the soil gas and in the fumarole steam. Gas

geothermometry from a fumarole close to the site

gave temperatures of more than 270oC. Heat flow

measurements indicate highest heat loss around this area indicating high subsurface temperatures at

depth.

Menengai Well MW-01, which was the first deep exploration well to be drilled in the Menengai

geothermal field, is drilled to a depth of 2206 m.

The well discharged on test therefore confirming a

geothermal reservoir exists in Menengai geothermal field. Measured formation temperatures

confirmed the results estimated from the fumaroles

(Mibei, 2011).

CONCLUSIONS

• Successful exploration requires multi-disciplinary approach incorporating all

geoscientific disciplines

• Geothermometry temperatures from results of

the already discharged MW-01 correlate very

well with estimated fumarole geothermometers

REFERENCES

Arnorsson, S. and Gunnlaugsson, E., (1985)., New

gas geothermometers for geothermal exploration–

calibration and application . Geochim. Cosmochim.

Acta, 47, 567 – 577pp.

Lagat

Lagat, J., Mbia P., and Muturia, C., (2010).,

Menengai Prospect: Investigations for its

geothermal Potential. A GDC Geothermal Resource Assessment Project Report, Second

Edition.

Mariita, N.O. (2003). An integrated geophysical study of the northern Kenya rift crustal structure:

implications for geothermal energy prospecting for

Menengai area. A PhD dissertation, University of

Texas at El Paso, USA.

Convine, O., (2011)., Borehole Geology and

Hydrothermal alteration mineralogy of well MW-

01 and MW-02, Menengai Geothermal Field, Central Kenya Rift. Report 30 in: Geothermal

UNU-GTP training in Iceland 2011.

Mungania, J and Lagat, J. K., (2004)., Menengai

Volcano: Investigations for its geothermal

potential. Report prepared by KenGen and the Ministry of Energy.

Simiyu, S. M. and Keller, G, R. (1997)., An

integrated analysis of the lithospheric structure across the East African plateau based on gravity

analysis and recent seismic studies.

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