IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG) e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 9, Issue 1 Ser. II (Jan. – Feb. 2021), PP 12-20 www.iosrjournals.org DOI: 10.9790/0990-0901021220 www.iosrjournals.org 12 | Page Evaluation of Reservoir Sub Surface Structures from Temperature Heat Up and Alteration Minerals in Olkaria Domes Geothermal Field, Kenya Solomon Namaswa 1, 3 , John Githiri 1 , Nicholas Mariita 2 , Maurice K’Orowe 1 , 1 Physics Department, Jomo Kenyatta University of Agriculture and Technology, Kenya 2 Geothermal Training and Research Institute, Dedan Kimathi University of Technology, Kenya 3 Physics Department, Multimedia University of Kenya Abstract The Olkaria Domes Geothermal field is situated in the south of Lake Naivasha, in southern sector of the Kenya Rift system, approximately 127 km northwest of Nairobi in Kenya. Geothermal reservoir evaluation involves various kinds of tests, data interpretation and modelling. In this paper, heat-up temperature profiles tests during well recovery for fourteen wells and the first occurrence of high temperature alteration minerals such as Illites and Quartz data from five wells were analysed for the purpose of obtaining temperature distribution, permeability structures, feed zone locations as well as the up-flow and down-flow zones. From the temperature distribution results, two main heat sources were inferred; one to the Northwest and the second to the Eastern side of the field. The two heat sources are separated by a SW-NE oriented fault that is believed to control the fluid flow. The natural recharge to the reservoir comes from the SW direction. The reservoir had two major feed zones at depths of (900-1300) masl and (250-0) masl. The Illites and Quartz contour distribution indicate the minerals first appear shallower on the Eastern region as compared to the Western region of the field hence elevated temperatures to the eastern side. This indicates that the heat source on the Eastern side is closer to the surface compared to the west side and therefore SW-NE oriented fault could be as a result of differential upward movement of the two magma chambers with more upthrow to the east as compared to the west. It is recommended that more production wells be drilled in the eastern region of the geothermal field. Key Words: Geothermal Reservoir, Temperature, Well Testing, Alteration Minerals ------------------------------------------------------------------------------------------------------------------------------------- Date of Submission: 26-01-2021 Date of Acceptance: 11-02-2021 --------------------------------------------------------------------------------------------------------------------------------------- I. Introduction The greater Olkaria geothermal area is situated in the southern part of the Kenyan rift, nearly 127 Km from Nairobi (0° 53’S; 36° 18’E) as shown in Figure 1. The Kenyan rift forms part of the structure of the East African rift that extends to Mozambique region in the south from Ethiopia (Kandie et al., 2016). It forms a section of the eastern arm that extends to Lake Natron from Lake Turkana, (Omenda et al., 2016).