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

Abstract

Resistivity measurements are one of the methods of finding deep sub-surface structures indirectly from the surface at relatively low cost, in comparison to direct method of drilling forgeothermal prospect. The geophysical techniques employed aim at mapping electrical resistivity of theearth matrix beneath the surface to determine geothermal potential areas from the resulting resistivity anomaly. Magnetotelluric (MT) and Transient electromagnetic (TEM) imagesubsurface geology in search for anomalous geothermal reservoirs and electrically conductiveheat source and cap rock zones. Resistivity cross-section from Menengai indicates four distinct anomalies of very resistive top layer 200m from the surface, followed by low resistivity layer (cap) overlying moderately resistive core (reservoir) underlain by deep sited low resistivity dense bodies interpreted to be a heat source. Comparing the resistivity a structure with well data, mainly alteration minerals and temperature, at depth is the principal objective of this paper. The conductive minerals such as zeolite dominatein the temperature range of 80-200°C representing low resistivity clay cap rock between the depths of 400-800m. Between depths of 900-2200m, at temperatures of 200-240°C, zeolites diminishes replaced by more resistive epidotes. 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’s clear that resistivity data can be interpreted and used in the prediction of predrilled well temperature and alteration mineralogy distribution with caution.