Correlating Resistivity with Temperature and Alteration Mineralogy in Menengai Geothermal Field: Case Study of Menengai Well MW-01

Abstract

Resistivity surveys is one the methods used in identifying deep subsurface structures indirectly from the surface at relatively low cost, in comparison to probing by drilling. The geophysical techniques employed are aimed at mapping electrical resistivity of the earth’s matrix beneath the surface to determine geothermal potential areas from the resulting resistivity anomalies. Magnetotelluric (MT) and Transient electromagnetic (TEM) images of the subsurface were applied in Menengai to map the anomalous geothermal reservoirs and electrically conductive heat source and cap rocks. Resistivity cross-section across Menengai geothermal field indicates four distinct anomalies of very resistive top layer of approximately 200m from the surface, followed by low resistivity layer (cap) overlying moderately resistive core (reservoir) underlain by deep low resistivity dense bodies, interpreted to be a heat source. Comparison between the resistivity structure and the subsurface well data, mainly alteration mineralogy and temperature profiles were carried out at the Menengai geothermal field. Results indicate that conductive minerals such as zeolite dominate in the temperature range of 80-200°C representing low resistivity cap between the depths of 400-800m. Between depths of 900-2200m, at temperatures of 200-240°C, zeolites diminishes and is replaced by more resistive clays. Above 250°C epidote dominates the relatively resistive reservoir core, where conductivity is controlled by pore fluids rather than clay minerals. Thus from this correlation it is clear that resistivity data can be interpreted and used in the prediction of pre-drilled well temperature and alteration mineralogy distribution with caution.