Heat source mapping and evaluation of geothermal resource potential in Lake Bogoria basin, Kenya

Abstract

The Lake Bogoria basin, here in referred to as the study area, is located in the greater Baringo-Bogoria basin (BBB), about 250 km to the north of the city of Nairobi and about 100 km to the north of Menengai geothermal prospect on the floor of Kenya Rift Valley (KRV). It is bound by latitudes 0o 00’ and 0o 30’N and longitudes 35o45’E and 36o15’E within the rift graben. The study area is characterised by geothermal surface manifestations which include hot springs, spouting geysers, fumaroles/steam jets and mud pools. The area is overlain by Miocene lavas lavas, mainly basalts and phonolites, and Pliocene to recent sediments and pyroclastics such as tuffs, tuffaceous sediments, superficial deposits, volcanic soils, alluvium and lacustrine silts. The terrain is characterized by extensive faulting forming numerous N-S ridges and fault scarps.

Gravity survey was undertaken in the study area to map the possible heating source and evaluate the geothermal resource potential of the basin. Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north- South direction and interpreted to be due to a series of dyke injections and hence the heat source in the basin. The interpretation of Bouguer anomaly has been constrained by using results from previous seismic surveys undertaken in the Kenya rift valley by Simiyu and Keller (2001), Keller et al., (1994a) and Braile et al., (1994). P-wave velocities have been converted to densities using the equation derived from Gardner et al. (1974).

Microseismic studies in Lake Bogoria basin by Young et al. (1991), Tongue (1992) and Tongue et al., (1992, 1994) show that the basin is characterised by high frequency of low magnitude (< 3) seismic events which are correlated with surface faulting and multiple episodes of dyke injections. The gravity results in this study are therefore fairly consistent with results of previous microseismic studies undertaken in the basin.

On the basis of the gravity results, the heat source in Lake Bogoria basin is due to cooling dyke injections occurring at depths of ~3 – 6 km. Since a heat source, in addition to fluid circulation, is an integral component of a geothermal system, it is evident that a geothermal reservoir exists in Lake Bogoria basin and the fluids can be tapped for generation of geothermal power.