Olkaria is a high-temperature geothermal system located within the central sector of the Kenya Rift Valley and associated with an area of late Quaternary rhyolitic volcanism. The geology is dominated by Pleistocene-Holocene comenditic rhyolite flows on the surface and basalts, trachytes and tuffs in the subsurface. The Olkaria field can be separated into east and west stratigraphic zones with the divide through the Olkaria Hill. The reservoir characteristics also follow this zonation. The geothermal reservoir in the east is hosted within Pleistocene Plateau Trachytes, while in the west it is within the Pliocene Mau Tuffs. Structural, geochemical, and hydrothermal alteration patterns indicate that the west field is at the margin of the larger Olkaria system. The anomalous bicarbonate enrichment in the west sector is due to additional adsorbed carbon dioxide from the mantle.