Structural Controls on the Geochemistry and Output of the Wells in the Olkaria Geothermal Field of the Kenyan Rift Valley

The Olkaria geothermalfield is located in the Kenya Rift valley, about 120 km from Nairobi.Geothermal activity is widespread in this rift with 14 major geothermal prospectsbeing identified. Structures in the Greater Olkaria volcanic complex include: the ring structure, theOl’Njorowa gorge, the ENE-WSW Olkaria fault and N-S, NNE-SSW, NW-SE and WNW-ESEtrending faults. The faults are more prominent in the East, Northeast and WestOlkaria fields but are scarce in the Olkaria Domes area, possibly due to thethick pyroclastics cover. The NW-SE and WNW- ESE faults are thought to be the oldest and areassociated with the development of the rift. The most prominent of these faultsis the Gorge Farm fault, which bounds the geothermal fields in the northeasternpart and extends to the Olkaria Domes area. The most recent structures are theN-S and the NNE-SSW faults. The geochemistry and output of the wells cut bythese faults have a distinct characteristic that is the N-S, NW-SE and WNW-ESEfaults are characterized by wells that have high Cl contents, temperatures andare good producers whereas the NE-SW faults, the Ring Structure and theOl’Njorowa gorge appear to carry cool dilute waters with less chlorideconcentration and thus low performing wells. Though the impacts of these faultsare apparent, there exists a gap in knowledge on how wide is the impact ofthese faults on the chemistry and performance of the wells. This papertherefore seeks to bridge this gap by analysis of the chemical trends of bothold wells and newly drilled ones to evaluate the impacts of individual faultsand then using buffering technique of ArcGis estimate how far and wide theinfluence of the faults is. The data was obtained after the sampling and analysisof discharge fluids of wells located on six profiles along the structurescutting through the field. Steam samples were collected with a stainless steel Webreseparator connected between the wellhead and an atmospheric silencer on thedischarging wells whereas the analysis was done in house in the KenGengeochemistry laboratory. The results indicates that Olkaria field has threecategories of faults that control fluid flow that is the NW-SE trending faultsthat bring in high temperature and Cl rich waters, and the NE-SW trending Olkaria fracture tend tocarry cool temperature waters that have led to decline in enthalpies of the wells it cuts through. The faultswithin the Ol Njorowa gorge act to carry cool, less mineralized water. Thoughinitially, these effects were thought to be in shallow depths, an indication inOW-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 asmuch deeper wells have been sited.