Title: Borehole geology and sub-surface petrochemistry of the Domes area, Olkaria Geothermal Field, Kenya, in relation to Well OW-922

Abstract

Olkaria is a high temperature geothermal system located within the central sector of the Kenya Rift
System, and associated with a region of Quaternary volcanism. Recharge into the system is
exclusively by meteoric water, derived primarily from the high-altitude rift escarpments. Tectonic
structures play without a doubt a pivotal role in enhancing fluid flow within the geothermal
system. For this study, data and samples from four wells, OW-905A, OW-910, OW-917 and OW-922, have
been used. Wells OW-905A, OW-910 and OW-917 were drilled within and along the proposed caldera
structure. Well OW-922 is a step-out vertical well, drilled to the east of the proposed caldera
structure. This study largely focuses on well OW-922, while, the other three wells have been
included for comparison purposes. The study presents a comprehensive look at the borehole geology
and hydrothermal mineralisation, sub-surface petrochemistry and microprobe analysis. Lithologies
intersected by the study well include pyroclastics, tuff, rhyolite, basalt, trachydacite, basaltic
trachyandesite, trachyandesite and trachyte. No intrusions are found in this well. Hydrothermal
products in the study well occur both as replacement of primary components or glassy matrix and as
open space filling in veins, fractures and vesicles. Evidence of a significant cooling process (up
to over 110ᵒC) in the geothermal system around the study well is provided by the difference between
measured formation temperature (127ᵒC) and inferred hydrothermal alteration temperature (epidote
240ᵒC). Cases of calcite overprinting epidote further points towards a cooling process.
Five feed zones have been deduced from the well and all are categorised as small. Generally, the
well has low permeability and could not sustain discharge after 49 days of heat-up. Observations
from temperature distribution across the field indicates that the study well is located outside the
Olkaria Domes heat source, and consequently, outside the main Olkaria volcanic zone. Whole-rock
chemistry displays a range of compositions of the samples of the study well, from basalt to
trachyte or rhyolite, with the prevalence of the rocks being the highly evolved derivatives. Major
oxides and trace element systematics are dominated by crystal fractionation, even though other
differentiation processes cannot be ruled out. Evidence of a common differentiation mechanism for
the surface and sub-surface rocks seems likely on the basis of comparison between the two groups of
samples. Microprobe analyses have been carried out on epidote and chlorite in order to assess the
chemical composition and the compositional range of the two minerals. It is found that most of the
epidote are aluminium-rich, whereas, the chlorites are iron- rich varieties. Limited number of
epidote and chlorite has however, impeded correlation of the stratigraphy in the four wells based
on major and minor
element contents.