Download Geothermal Surface Exploration Approach- Case Study of Menengai Geothermal Field, Kenya Oyedele PDF

TitleGeothermal Surface Exploration Approach- Case Study of Menengai Geothermal Field, Kenya Oyedele
TagsVolcano Petroleum Reservoir Geothermal Energy Geophysics Types Of Volcanic Eruptions
File Size1.3 MB
Total Pages10
Document Text Contents
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PROCEEDINGS, Kenya Geothermal Conference 2011
Kenyatta International Conference Centre Nairobi, November 21-22, 2011

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

G
ri

d
N

o
rt

h
in

g
s

-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).

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

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 km
2
,

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

m
2
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)

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