PROSPECTS FOR GEOTHERMAL ENERGY IN MENENGAI AREA, KENYA: A REINTERPRETATION OF GEOPHYSICS DATA

Newly collected geophysics data from Menengai geothermal prospect has been interpreted along with existing data. The heat source for the prospect is probably associated with a hot magmatic body that underlies the caldera structure at Menengai and a similar but probably older heat source is likely to be associated with the Olrongai/Olbanita geothermal areas. Positive magnetic anomaly that is coincident with low resistivity indicates the presence of a magmatic body at shallow depth. Seismic data indicates that the magmatic body is at about 12 km depth. Two low resistivity areas exist to the northeast and southwest of the caldera. They show similar characteristics to the proven geothermal fields of Olkaria and Eburru and therefore are good prospects. However, Olbanita area shows very high resistivity thus pausing doubts on the potential of this part of the prospect as a high temperature resource. The geothermal reservoir at Menengai is hosted within the fractured/faulted brittle trachytic lavas of the rift floor and welded pyroclastics that underlie the volcanic pile. Although there are few surface geothermal manifestations, shallow boreholes in this area have encountered steam with a gas geothermometry indicating equilibrium reservoir temperature of 170-190ºC in the Emining – Olrongai area. Calculated gas geothermometry of fumaroles along the Molo River gave temperatures of 200ºC. The main recharge is associated with the adjacent rift with high rainfall scarps along Njoro - Timboroa area to the west and Marmanent – Subukia in the east. The extensive fault/fracture network that is evident in the rift flanks possibly provides hydraulic connection and flow towards the rift floor. The prospect is estimated to cover an area of more than 90 km2 which, if proven, can generate more than 200 MWe. The top of the reservoir is projected to be at 1000 masl.