||Hydrothermal alteration minerals have been used successfully to observe the changes in a geothermal reservoir. The temperature dependent minerals are used as indicators to determine the entrance into the reservoir and the production casing depths. The alteration intensity shows a quantity of fluid flow through the rock; therefore can be used as an indication of porosity and/or permeability. Isograds give a general picture of the temperature distribution in the geothermal system as they delineate the first appearance of an index mineral, although the distribution of hydrothermal mineral isograds may not be distinctly parallel to the isothermal contours indicating that some thermal changes could have taken place in a geothermal reservoir over time. Drilling in the Olkaria Domes geothermal field is currently ongoing in quest to explore and appraise the field as the boundaries to the geothermal reservoir is still unknown. Four geothermal wells OW-911A, OW-912B, OW-916 and OW-914A were drilled in Olkaria Domes field to gather steam for the 140 MW power project and to also appraise the field both to the east and south east. The wells were drilled to an average depth of 3000 m and all encountered a high temperature system with alteration mineralogy indicating a reservoir temperature of above 250°C. All wells apart from OW-912B discharged during testing. Rock types identified in the wells include pyroclastics, rhyolites, tuffs, trachytes and basalts. Intrusions in the wells were of rhyolitic, basaltic and syenitic compositions occurring as dykes from as shallow as 520 m depth in well OW-916. Hydrothermal alteration indicates that the geothermal system is grading from low to high temperature with increasing depth, although certain wells show an evidence of cooling probably due to cold fluids incursion in the well. The wells display an evolution history indicated by the deposition of minerals like zeolites, opal and chalcedony with their stable temperatures of below 150°C at shallower depths grading into a zone dominated by minerals like quartz, wairakite, chlorite, epidote, prehnite, wollastonite and actinolite which are stable above 180°C occurring deeper in the wells. Epidote was first noted in well OW-916 at 556 m depth and occurred deeper in OW-912B at 950 m depth. Other minerals noted to be occurring shallower at well OW-916 are quartz and actinolite. Alteration mineral assemblages identified in the wells include zeolite-illite, chlorite-illite, epidote-chlorite-illite and actinolite-epidote-chlorite-illite zone. Permeability is relatively low in wells OW-911A and OW-912B while good permeability was encountered at wells OW-916 and OW-914A. From the assessment of hydrothermal mineralogy, it is evident that well OW-916 is located at or near the upflow zone of the Olkaria Domes geothermal field and the resource is deeper towards OW-911A with the extent to the east remaining unknown. This paper therefore discusses the successful use of hydrothermal alteration mineralogy in mapping the geothermal resource in the Olkaria Domes field and evaluating it for further production.