The influence of geology and geological structures on the occurrence and distribution of geothermal systems in the Kenyan rift

Abstract

Geothermal is the internal thermal energy of the earth generally employed to denote systems in which the earth’s heat is sufficiently concentrated to form an energy resource. Kenya is endowed with vast geothermal resource potential along the Kenyan Rift that transects the country from the north to the south. Explorations reveal that geothermal potential exceeds 7000MWe and is capable of meeting Kenya`s electricity needs over the next 20 years. Out of this potential, only 167MWe and 18MWt are being utilized for indirect and direct uses respectively. The Kenyan Rift is a graben and during the formation of the geothermal sites hot rocks were brought closer to the surface and with intensive faulting the geothermal systems were formed. The concentration of thermal energy requires besides high porosity (storage coefficient) also high permeability (hydraulic conductivity) of the reservoir rocks. The concentration of thermal energy in geothermal systems relative to adjacent terrain is characterized by the existence of positive geothermal anomalies - young magmatic heat sources at high crustal levels, hydrothermal circulation of meteoric water in fault/fracture systems and irregularities in conductive heat transfer (differences in regional heat flow from deep crustal levels, contrasts in thermal conductivity and /or radiogenic heat production of the rocks in question). The most significant geothermal anomalies are related to upward moving magma. The combination of locally variable factors like enthalpy/temperature, permeability distribution, and reservoir depth leads to a great variety of different geothermal systems. Different types of geothermal systems exist in the Kenyan rift such as the high temperature vapor dominated like Olkaria, or low temperature, low enthalpy water dominated such as Lakes Bogoria and Baringo. High temperature systems possibly exist at Menengai, Paka and Korosi also. The geology and the geological structures within the rift affect the occurrence and the distribution of these geothermal systems, for instance Olkaria is a major geothermal system due to the underlying silicic magma. Other factors include; rock types, hydrogeology, permeability, faults and presence of calderas and volcanoes.