Characterizing Permeability from Geological and Geochemical Data in the Olkaria Domes Field in Kenya

Abstract

Olkaria geothermal field is located in the Kenyan Rift Valley that is a part of the Great East Africa Rift System (EARS). The geothermal field continues to be associated with a high geothermal gradient that arises from shallow magmatic activities which are ongoing in the enormous igneous province. Exploration and drilling of wells that were undertaken in the past revealed the existence of exploitable geothermal steam. The Olkaria field is divided into seven sections namely; Olkaria East field, Olkaria North East field, Olkaria North West field, Olkaria South East field, Olkaria South West field, Olkaria Central field, and Olkaria Domes field. The productivity of the geothermal wells continues to be influenced by factors such as subsurface permeability. Permeability is one of the parameters used for the characterization of geothermal fields. Other parameters used for characterization are associated with geotechnical weak zones and include features such as; fractures, vein bodies, and deformational fault systems. The research work involved geoscientific characteristics of the Olkaria Domes field based on the geological and geochemical factors to characterize the permeability of the field. The research involved studying rock types in the area by analysing drill cuttings obtained from six drilled wells in the Olkaria Domes field. Three of the six drill wells were considered for correlative description for the purpose of this paper. Correlation of the main lithologies and zones for loss of circulation in the field was also undertaken as well as the creation of mineralogical maps to capture the distribution of the minerals that were derived from hydrothermal weathering processes. The depths and formation for major loss circulation zones in the reservoir section of the field were identified and included in the description. Analysis of soil gas survey using radon as a geochemical tool in the Domes field was also carried out successfully. The relatively high levels of the soil gas ratios that were analysed captured the ratio distribution of carbon dioxide to radon at various reservoir depths. The detection of the two gases at the surface showed the existence of permeable zones which facilitated the movement of the gases through the fault-controlled structural systems of the studied Olkaria Domes field.