Application of alteration minerals and thermal fluid geochemistry in geothermal conceptual modeling, case study of Olkaria geothermal field in Kenya

Abstract

The Olkaria geothermal field is located within the central sector of the Kenyan Rift Valley in Naivasha basin. It is a high-temperature geothermal system and is associated with late Quaternary rhyolite volcanism. The surface geology of Olkaria is mainly characterized by comendite lavas, trachyte lavas and pyroclastics while low lying areas are mainly masked by Holocene sediments especially in the Ol'Njorowa Gorge. Shallow rhyolitic intrusions are thought to be the heat source for the geothermal system. This study considers geothermal conceptual modeling as an important tool in exploration and exploitation of geothermal resources. The main objective is to use thermal fluids and hydrothermal alteration mineral geothermometry to develop a conceptual model of the Olkaria geothermal field. Hydrothermal alteration mineralogy and thermal fluid chemistry are important sources of useful information in geothermal conceptual modeling. Alteration minerals can be used to infer subsurface formation temperatures and also reconstruct the thermal history of a geothermal field. Chemical and solute geothermometers from thermal fluid analysis on the other hand help in the calculation of subsurface temperatures. The approach pursued in the study included; (1) The chemical analysis of major elements in the thermal fluids from Olkaria Northeast, Olkaria East and Olkaria Domes subfields, in order to calculate subsurface temperatures using geothermometric equations by Fournier (1979), Arnosson et al. (1983) and Giggenbach (1988). (2) The use of lithological logs from wells OW710, OW-23 and OW-910A drilled in the aforementioned subfields to reconstruct lithostratigraphy and hydrothermal alteration zonation. The key index minerals are identified and temperature isotherms mapped. The temperature results from thermal fluids and alteration minerals are analyzed and correlated to identify the inflow, upflow and the outflow. The data is analyzed with the help of high definition softwares such as Rockworks, Surfer and ArcGIS. In this study, minerals such as epidote and actinolite are identified, they infer temperatures of >250°C and 280°C respectively and are markers for an upflow zone in the system. Results from calculated temperature on fluid chemistry and inferred temperatures from alteration minerals, indicate an anomalously relatively high subsurface temperature in Olkaria Northeast compared to Olkaria East and Olkaria Domes. This indicates that the Northeast field is one of the upflow zones for the Olkaria geothermal system.