Geology and Structural Control of the Geothermal System in Olkaria Northeast Geothermal Field, Naivasha Kenya
[Kenya Electricity Generating Company. Plc, Kenya]
ABSTRACT The general subsurface strata of Olkaria Northeast field is divided into four main series as depicted from the analysis of cuttings from the drilled wells. Pyroclastics form the top most layer and it is believed to be from Olkaria, Longonot, and Suswa eruptive products. It overlies a series dominated by eruptives of rhyolitic composition underlain by a series of basaltic lavas intercalated with trachytes. The upper basalt units at Olkaria have been used as a marker horizon in the Greater Olkaria field. The bottom most series is dominated by trachytes and form the main reservoir rock. Intrusives are first encountered as dykes within the trachytic series. The trends in the occurrence of alteration minerals with reference to to high temperature minerals indicate that Olkaria fault and Gorge farm fault had high paleo-temperatures. Delineation of the field by use of temperature models into upflow and downflow clearly indicates that cooler fluids are recharged into the system along gorge farm fault. Well OW-737 that has been drilled approximately 1 km to the east of the Gorge farm fault also indicates cooling within the vicinity of the well. This cooling trend extends from the Gorge farm fault to well OW-737 indicating that the fault could be marking the boundary of the geothermal resource to the east of the Northeast production field. The veining observed from the analysis of cuttings from the wells comprising of hydrothermal minerals indicates fracturing and exemplify movement hydrothermal fluids along these fractures. The permeability in the Northeast field is structurally controlled. Major feed zones are mainly controlled by large fractures and the interconnection of the smaller fractures within the rocks.
|        Topic: Geology||Paper Number: 12153|