| Abstract |
The chemical composition of geothermal fluids provides critical information about the nature, geology, chemistry and hydrology of that geothermal system. One of the major chemical constituents that is applied in the understanding of the geothermal system is the concentration of the gases in steam. Geothermal discharges from wells have in their vapour phase some amount of gases including CO2, H2S, CH4, N2, H2 and NH3. The partial pressure of gases like CO2 are controlled or related to the aquifer temperature. The partial pressure of H2 and H2S are fixed by temperature dependent mineral equilibria. The mineral buffer Clinozoisite-prehnite-quartz-calcite could control the aquifer partial pressure of CO2 while H2 and H2S are buffered by the mineral assemblage pyrite-pyrrhotite-magnetite. The partial pressure of CO2 is in equilibrium with the mineral buffer Clinozoisite+ quartz+ calcite+ prehnite for the wells OW 916 and OW 904 A while OW 905A and OW 910 are tenths of magnitude far away. The partial pressure of H2S for OW 916 is in equilibrium with the mineral buffer Pyr + Pyrr + Mag, while OW 904, OW 904A are slightly off. However, they are all off the mineral buffer Pyr + Pyrr + Pre + Epi, which clearly is not contributing to the concentration of these gases. The partial pressure of H2 has similar representation as that for H2S only that in this case, the data for OW 905A is slightly off the equilibrium line unlike in the H2S plot where it was on the line. NCG content of selected wells in the Olkaria domes wells over a period of time gives two different clusters. The first cluster is represented by OW 910 and OW 916. This represents wells in the eastern side of the domes area. These are characterised by lower NCG contents of averagely 0.5 – 0.6 %. The other wells OW 905A, OW 904 and OW 904A are characterised by higher NCG contents of more than 1% up to 1.8%. |