Fluid-rock interaction and initial aquifer geochemistry in the Olkaria geothermal system

Abstract

The Olkaria geothermal field is one of the quaternary volcanic centres in the Kenyan Rift valley. Geochemical methods have been applied in studying the elemental origin and reactions at depth. Fluid-rock interaction is one of the key reactions of interest in geothermal systems. Two models have been postulated in the evaluation of aquifer fluid composition. The first model assumes a single-phase liquid aquifer, while in the second model one assumes a two-phase fluid aquifer: vapour and liquid. The concentration of non-volatile components is unaffected by the choice of model except for fluid discharges approaching dry vapour. However, the concentration of volatiles is greatly influenced by the selection of a model for calculating aquifer fluid composition. As a consequence, solute geothermometers like quartz and Na/K have similar temperatures for the two models, whereas gas geothermometers like H2S show distinctively lower values when assuming two-phase aquifers. Many common minerals observed within the Olkaria geothermal system are observed to be saturated, including calcite, fluorite, epidote-clinozoisite, prehnite, feldspars and pyrite. Based on this, the mineral-fluid equilibrium is considered to control the fluid chemical composition. Mineral buffer reactions are considered to control the H2S and H2 volatile concentrations, whereas the concentration of CO2 may either or both be controlled by mineral equilibria in the geothermal system or by an external source.