Untitled - Orkustofnun

GEOTHERMAL TRAINING PROGRAMMEMwangi, M.: Geothermal in Kenya and AfricaReports 2005Number 4, 15-27LECTURE 2HISTORY OF DEVELOPMENT OF THE CONCEPTUAL MODELOF OLKARIA GEOTHERMAL FIELD BY USE OFGEOPHYSICS AND OTHER METHODSMariita Bw’Obuya and Peter OmendaKenya Electricity Generating Company Ltd,Olkaria Geothermal ProjectP.O. Box 785, Naivasha 20117KENYAABSTRACTOlkaria Geothermal field is a high temperature geothermal resource in the KenyaRift Valley, which has been used for electricity generation since 1981. Thegeothermal resource is associated with an area of Quaternary volcanism in whichrhyolites dominate. Geophysical exploration for the resource during the earlystages of development included dipole, Schlumberger, electromagnetic, head-on,gravity, seismic and magnetics and various levels of success were achieved. It wasnoted that whereas resistivity was the most important in identifying the reservoirs,depth of penetration was low for dipole and Schlumberger while interpretation ofhead-on data was ambiguous. Latest investigations at Olkaria have involved the useof transient electromagnetics (TEM) and magnetotellurics (MT) due to their ease ofdeployment and better depth of penetration than the other resistivity methods. Theresults of the studies indicate that the Olkaria geothermal system is controlled byrift structures where upflow zones are at fault intersections while N-S and NWtrending faults are mainly recharge paths. Gravity, seismic and MT data indicatethat the heat sources for the system are discrete shallow magma intrusions locatedunder the main upflow zones.1. INTRODUCTIONOlkaria Geothermal field is located within the central Kenya segment of the East African Rift System.The geothermal area is characterized by Quaternary volcanism of silicic composition of which theyoungest is of Holocene age. The rock outcrops are dominated by comendite rhyolites and pyroclasticswhile in the subsurface are trachytes, basalts, rhyolites and tuffs. Geothermal manifestations includefumaroles, hotsprings and hot grounds. Exploration for geothermal resources in Kenya started in1950’s with mainly geological investigations in the region between Olkaria and Lake Bogoria in thenorth rift. The exploration resulted in the drilling of two wells X-1 and X-2, which encountered hightemperatures at depth. The exploration then gained momentum with support of the United NationsDevelopment Programme (UNDP), which saw more extensive geophysical investigations undertakenand additional wells drilled between 1973 and 1980.The geophysical studies included gravity, various resistivity techniques (dipole, Schlumberger,electromagnetic, head-on), magnetics and seismics. The activities resulted in the construction andcommissioning of Africa’s first geothermal power plant at Olkaria with 45 MW capacity between1981-1985. Changes in technology saw the deployment of modern geophysical techniques thatincluded transient electromagnetics (TEM) and magnetotellurics (MT), which made it possible forshallow and deep conductors to be accurately imaged and thus better geothermal models developed.15