TITLE:
Structural Controls on the Geochemistry and Output of the Wells in the Olkaria Geothermal Field of the Kenyan Rift Valley
AUTHORS:
Ruth N. Wamalwa, Christopher M. Nyamai, Willis J. Ambusso, Josephat Mulwa, Aaron K. Waswa
KEYWORDS:
Structural Controls, Geochemistry, Productivity, Buffering, Olkaria Geothermal Field, Kenya
JOURNAL NAME:
International Journal of Geosciences,
Vol.7 No.11,
November
8,
2016
ABSTRACT: The Olkaria geothermal
field is located in the Kenya Rift valley, about 120 km from Nairobi.
Geothermal activity is widespread in this rift with 14 major geothermal prospects
being identified. Structures in the Greater Olkaria volcanic complex include: the ring structure, the
Ol’Njorowa gorge, the ENE-WSW Olkaria fault and N-S, NNE-SSW, NW-SE and WNW-ESE
trending faults. The faults are more prominent in the East, Northeast and West
Olkaria fields but are scarce in the Olkaria Domes area, possibly due to the
thick pyroclastics cover. The NW-SE and WNW- ESE faults are thought to be the oldest and are
associated with the development of the rift. The most prominent of these faults
is the Gorge Farm fault, which bounds the geothermal fields in the northeastern
part and extends to the Olkaria Domes area. The most recent structures are the
N-S and the NNE-SSW faults. The geochemistry and output of the wells cut by
these faults have a distinct characteristic that is the N-S, NW-SE and WNW-ESE
faults are characterized by wells that have high Cl contents, temperatures and
are good producers whereas the NE-SW faults, the Ring Structure and the
Ol’Njorowa gorge appear to carry cool dilute waters with less chloride
concentration and thus low performing wells. Though the impacts of these faults
are apparent, there exists a gap in knowledge on how wide is the impact of
these faults on the chemistry and performance of the wells. This paper
therefore seeks to bridge this gap by analysis of the chemical trends of both
old wells and newly drilled ones to evaluate the impacts of individual faults
and then using buffering technique of ArcGis estimate how far and wide the
influence of the faults is. The data was obtained after the sampling and analysis
of discharge fluids of wells located on six profiles along the structures
cutting through the field. Steam samples were collected with a stainless steel Webre
separator connected between the wellhead and an atmospheric silencer on the
discharging wells whereas the analysis was done in house in the KenGen
geochemistry laboratory. The results indicates that Olkaria field has three
categories of faults that control fluid flow that is the NW-SE trending faults
that bring in high temperature and Cl rich waters, and the NE-SW trending Olkaria fracture tend to
carry cool temperature waters that have led to decline in enthalpies of the wells it cuts through. The faults
within the Ol Njorowa gorge act to carry cool, less mineralized water. Though
initially, these effects were thought to be in shallow depths, an indication in
OW-901 which is a deeper at 2200 m compared to 1600 m of OW-23 well that proves otherwise. This is, however, to be proved later as
much deeper wells have been sited.