Borehole geology and sub-surface petrochemistry of the Domes Area, Olkaria geothermal fiele, Kenya, in relation to well OS-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 110o C) in the geothermal system around the study well is provided by the difference between measured formation temperature (127o C) and inferred hydrothermal alteration temperature (epidote 240o C). 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 ironrich varieties. Limited number of epidote and chlorite has however, impeded possible correlation of the stratigraphy in the four wells based on major and minor element contents.