Geochemical exploration of geothermal prospects : a case study of Menegai, Kenya

Abstract

Regional exploration for geothermal resources in Kenya indicates that the Quaternary volcanic complexes of the Kenya rift valley provide the most promising prospects for geothermal exploration. A number of geoscientific studies have been conducted to assess the geothermal potential of this area. This paper reports the results of a survey of fumaroles gas and condensate analysis, alongside CO2 soil fluxes and concentrations of thoron (Rn220) in soil gases. The procedures employed in the study are divided into: fumarole steam condensate and gas sampling and soil gas sampling to determine mainly the concentrations of carbon dioxide and radon radioactivity. For fumaroles, steam and condensate samples were collected for various analyses. Soil CO2 concentrations measurements were performed using an Orsat apparatus whereas Rn220 soil gas concentrations were measured with a portable radon detector (emanometer). A total of 275 sampling points were measured. The obtained values ranged between zero to 12% of CO 2 in total gas and zero to 6425cpm of Rn220. The TH2S geothermometer gave temperatures ranging from 279ᵒC-296ᵒC while TH2S-CO2 gave temperatures ranging from 274ᵒC-304ᵒC. The two geothermometers correlate well and from the temperature estimation, it is clear that the reservoir temperatures beneath Menengai caldera are over 270°C. Peak levels of soil CO2 concentrations and thoron concentrations were identified to be located on major faults and within the caldera floor where hydrothermal fluids are rising and the rocks are highly fractured allowing the release of CO2 and thoron locally marked by fumaroles. Areas identified with the highest anomalies for CO2 and thoron soil gas concentrations can be seen in the contour maps presented. The caldera, Molo TVA and also Solai graben are the important geological structures associated with these anomalies.