| dc.description.abstract | The comparative study of resistivity models from Menengai and
Olkaria geothermal prospects was carried out in two stages. The
first was the re-evaluation of the sub-surface geoelectric
structure of Menengai prospect with a view to determining layers
that might be associated with geothermal fluid migration, the
probable extent of the field and the structural discontinuities.
The study was carried out using the results of the symmetrical
Schlumberger array with a maximum current electrode spacing of
8000m. The second part dwelt on the comparison of the above with
the well documented Olkaria geothermal field model based on the
same array. This comparison was done in an attempt to describe
the resistivity characteristics of the two geothermal prospects
located in the same major structural regime: The Kenyan Rift
Valley.
The Menengai resistivity data analysis was carried out by curve
matching and fitting and 1-0 forward and inverse computer
modelling. Geophysical and limited geological data analysis has
identified four geoelectrical units with contrasting thicknesses
and depths. These are the laterally heterogeneous surficial
unit (100 to 200m thick); the resistive caprock (200 to 800m);
a conductive unit (5 - 10 Ohm.m, 500 to 1500m) and an electrical
substratum (50 - 300 Ohm.m). The deep conductive unit underlying
the whole field is envisaged to contain the potential geothermal
reservoir with a general discharge of fluids north to north-west.
Geoelectric structural models on line profiles have identified
horst-graben structures bound by normal NS and EW trending
discontinuities. Some of these coincide with fault zones which
are permeable zones of complex fluid migration and sometimes leak
to the surface leading to surface geothermal manifestations.
Geological correlation in Menengai area is limited due to lack
of deep drill holes.
The two prospects show comparatively similar sub-surface
resistivity patterns within the limits of survey procedures and
instrumentations. Both areas are underlain by doming or horst
structure deep conductive zones (5 - 10 Ohm.m) which are overlain
by intermediate to high resistivity zones which in Olkaria also
constitute the present East Olkaria production reservoir. The
thicknesses and resisti vities of the above mentioned intermediate
to high resistivity zones generally increase westwards in both
prospects. Lack of the electric substratum in Olkaria could be
attributed to shorter depth of probe in the surveys done,
however, in some soundings done this zone is evident.
From this study, it is evident that the conductive unit in
Menengai which is covered by resistive 'cap' rock offers good
prospect for further geothermal exploration.
It is further noted that both the Menengai and Olkaria prospects
exhibit comparable resistivity patterns within the earlier
mentioned limits. | en |
