MENENGAI CALDERA GEOTHERMAL FIELD - EASTERN UPFLOW

In Menengai increased production drilling and subsequent modeling has greatly improved the understanding of the geothermal system. Menengai hosts a structurally controlled geothermal system and the focal drilling targets are thus structures of proven permeability and high temperature to yield productive wells. Some studies and improvement of ideas have been on-going as more data becomes available, and various misconceptions have been rectified. Remarkably, the area to the east of the caldera was never a high priority for exploratory drilling based on two assumptions; (1) that the area lacked permeability due to absence of obvious surface structures and (2) there was the possibility of a cooling effect by cold meteoric water channeled by the Solai structure, both of which are now obsolete. Subsurface data show that the major reservoir begins at the quartz-illite-epidote zone; this zone exhibits two distinct doming features suggesting 2 upflow zones. One is in the vicinity of MW-15 and the other within the central caldera area. This zone indicates alteration temperatures of over 230°C. Accordingly, gas chemistry of fumarole MF-2 situated to the immediate north of MW-15 yields the highest equilibrium temperature estimate for the reservoir and a low CO2/H2S ratio suggesting that it could be positioned within or near an upflow area in Menengai. Well MW-18A fluid discharge chemistry results appears to be benign and shows relatively low concentrations of carbonates compared to other wells in Menengai.

Key words: Menengai, upflow, geothermal system