Integrated geophysical study of Lake Bogoria basin, Kenya: Implications for geothermal energy prospecting

Citation:
Mulwa JK. Integrated geophysical study of Lake Bogoria basin, Kenya: Implications for geothermal energy prospecting. Mulwa JK, of Prof. Justus Barongo(University of Nairobi DG), of Prof. Jayanti Patel(University of Nairobi DP), Prof. Derek Fairhead(Leeds University and MD GETECH), of and Prof. Greg Houseman(Leeds University IGT), Dr Nicholas Mariita(Kenya Electricity Generating Company OGP), eds. Nairobi/Leeds: University of Nairobi/Leeds University; 2011.

Thesis Type:

PhD

Abstract:

The Lake Bogoria basin, herein referred to as ‘the study area’, is located in the greater Baringo-Bogoria basin (BBB), about 250 km from the city of Nairobi 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 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 which forms numerous N-S ridges and fault scarps.

Gravity and magnetotelluric (MT) surveys were undertaken in the area in order to determine the heat source and evaluate the geothermal resource potential of the basin for generation of geothermal power. The gravity data used was from the University of Texas at El Paso and Leicester University gravity data bases. New gravity measurements’ comprising 260 data points was undertaken for the purpose of this study. In addition, magnetotelluric data comprising about fourty sites was also acquired in the study area.

Gravity survey results indicate Bouguer anomaly having an amplitude of ~40 mGals aligned in a north-south direction and this has been interpreted to be due to a series of dyke injections and hence the heat source in the basin. The dyke injections occur at depths of 3-6 km on average, but at 1 km depth at the shallowest. The gravity models show a north-south gradual variation in thickness of the uppermost low density layer comprising rift-fill volcanics from 1-4 km on average. The variation in thickness of this layer from south-north suggests that these volcanic deposits are as a result of volcanic eruption(s) outside Lake Bogoria basin such as Menengai to the south.

The MT survey results show three distinct relatively thick layers in the basin. The first of these layers, which is overlain by high resistivity (50-1000 m) thin (100-500 m) layers, is ~3 km thick and has resistivities ranging between 4-30 -m. This layer is interpreted as the geothermal reservoir and the low resistivities are due to a combination of circulating hot mineralized geothermal fluids, hydrothermal alteration and saline sediments. The second layer is ~10 km thick and resistivity values range between 85-2500 -m and is interpreted to be a fractured and hydrothermally altered basement metamorphic rock. The relatively high degree of fracturing has considerably enhanced circulation of water which gets heated by the underlying dyke injections and thus inducing convective heat transport to the geothermal reservoir. The substratum is characterized by resistivities ranging between 0.5-47 -m and is interpreted as hot dyke injections at depths of about 6-12 km, which are the heat sources for the geothermal system.

Consequently, a heat source and a geothermal reservoir exist in Lake Bogoria basin. The heat source(s) is/are due to cooling dyke injections occurring at depths of 3-6 km on average, but 1 km depth at the shallowest near Arus where steam jets and fumaroles occur. The magnetotelluric method, however, favours depths of 6-12 km for the heat source and this may be attributed to lack of significant resistivity contrast between the dyke injections and the basement rocks where the former have intruded the latter rocks.
More gravity data is warranted so as to precisely define the geometry and areal extent of the heat source in Lake Bogoria basin. However, based on the results of this study, it is recommended that:- 1) exploratory drilling be undertaken in the area near Arus steam jets, 2) even though the study area is not prone to any pre-historic eruptions, microgravity and seismic monitoring be undertaken so as to help in tracking possible magma migration and variations in magma input. Such data could, in turn, play an important role in predicting future eruptive events in Lake Bogoria basin.

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