Analysis of the geothermal potential of the North West Olkaria prospect (oserian sanctuary) in Kenya, using transient electromagnetic method and Magnetotelluric method

Abstract

The Northwest Olkaria prospect, Oserian Sanctuary, is part of the Greater Olkaria Geothermal Area located within the Kenyan rift valley in Nakuru County. It is bounded by the longitudes 360 15′E and 360 12′E with an estimated area of 30 km2. Studies previously done did not adequately cover the area to the North West hence no geophysical data is available on the resistivity of key concealed geothermal structural features like thin massive heat sources and shallow faults based on subsurface resistivity distribution as most methods were largely DC. This study aimed at using the Magnetotelluric method and the Transient Electromagnetic Method to determine the electrical resistivity distribution of the subsurface and locate possible structural features controlling geothermal distribution in the prospect area by developing a 1D joint inversion of Magnetotelluric and Transient Electromagnetic data of the Olkaria North West prospect, 1D Iso-resistivity maps of the Olkaria Northwest prospect and a 1D Magnetotelluric model of the Olkaria Northwest prospect. A total of 50 Magnetotelluric soundings were analysed in this research as well as 47 corresponding central loop Transient Electromagnetic soundings. To allow the static shift correction in the 1D inversion, the Magnetotelluric data were jointly inverted with Transient Electromagnetic data. The results and interpretations of the Joint 1D inversion of Magnetotelluric and central loop Transient Electromagnetic data of Olkaria Northwest field were presented inform of resistivity Iso-maps and cross sections. Results of the interpretation show four main resistivity zones below the prospect area: a shallow superficially high resistivity zone (> 30Ωm) interpreted as unaltered rock formations and a thick pyroclastic cover, a thick dome shaped conductive layer (< 6Ωm) that extends to about 1900 m.a.s.l dominated by low temperature alteration minerals such as smectite and zeolite interpreted as the clay cap, a resistive layer (>15-50) Ωm associated with the high temperature secondary minerals such as epidote, chlorite and biotite present in the reservoir and another low conductivity layer extending to about 6,000 m.b.s.l. which is interpreted as a heat source of the Olkaria geothermal system. The study revealed that the main reservoir exists to the western side and it appears to be deeper than that in the eastern field. The conclusion characterised the Northwest Olkaria prospect as a geothermal potential having a heat source, a reservoir and a recharge system which are the main features of a geothermal system. The main reservoir to the western side appeared to be deeper than the eastern field, therefore it is recommended that the outlet wells should be to the west and the reinjection wells to the east of the prospect. Resistivity results from North West also indicated a resource boundary to the west at 1000m.a.s.l.