Rock Types and Alteration Mineralogy Occurring In Olkaria Geothermal Field, Kenya

Abstract

Olkaria Geothermal Field is a high temperature geothermal system; the system is characterized by Quaternary volcanic centers in the central Kenya Rift Valley. The surface geology of Olkaria Geothermal Field is mainly dominated by commenditic lavas and pyroclastic. The subsurface geology is mainly characterized by basalt, tuffs, rhyolite, trachyte and intrusion. The aim of this study was to describe the lithology and alteration mineralogy of OW 924A, OW 923A, 917A, OW917A, OW914B, OW731C, OW710C and OW39B wells.The lithology and alteration mineralogy in geothermal system is used to solve the following; 1) Trachyte formation and epidote mineral are indicators of entry into the reservoir and hence must be considered when setting up production casing during drilling. 2) It also helps in understanding the reservoir by modelling the inflow, up-flow and outflow of geothermal systems. 3) The fluid chemistry can be studied so as to predict future problems in the fields. For example, mineral sequence can indicate cold-water incursions into the well while abundant calcite and quartz in a well may suggest casing corrosion problems in the future.Drill cuttings were collected at a two meter intervals at the rigs site and analyzed in preparing lithological and alteration logs by KenGen rig geologists. We obtained permission from KenGen management to use the analyzed data. The data was analyzed using computer software LogPlot 8 to interpret the lithology, alteration mineralogy and temperatures heat up profiles.