| Abstract |
The Olkaria Geothermal Complex is a high temperature field in the East African Rift System. The litho-stratigraphy of the Olkaria geothermal area as revealed by data from geothermal wells is characterised by pyroclastics, rhyolites, basalts, trachytes and intrusive rocks. Recent drilling activities within the Olkaria East and South East fields encountered intrusives in various geothermal wells from as shallow as 2200m below ground level. The need to characterise the nature and distribution of the intrusives is significant in understanding their influence to the geothermal system. Microscopic examination of drill cuttings in thin section, indicate the intrusions are mainly microgranites with a granophyric texture; composed of coarse grains of quartz interlocking with feldspars and pyroxenes (augite). Syenitic intrusions are also encountered in some wells but occur intermittently with trachytes as thin dykes. From this study, it is evident that the intrusives have a positive influence on the reservoir temperatures either by directly heating reservoir fluids or heating the overlying rocks which in turn heat up the reservoir fluids. However, more studies need to be carried out to ascertain this phenomenon. |