Characterization of Geothermal Resource in the Olkaria South East Field using Multidimensional Magnetotelluric Data Inversion

Abstract

One of the seven blocks that make up the Greater Olkaria Complex is Olkaria South East field. There are few wells that have been drilled, therefore the area is continuously being explored. Olkaria south east field is characterized by numerous surface manifestations such as fumaroles and altered ground, faults and structures that trend in the NE-SW direction. Most of the drilled wells are not in production and they experience temperature reversal at depth. Therefore, it is important to understand the physical properties of the Olkaria south east geothermal system. This information would be key in delineating the presence of caprock, reservoir, heat source and other geological structures controlling the system. To achieve this 23 MT data stations were used. The purpose of these geophysical data was to facilitate imaging of the subsurface geothermal structure with a view of characterizing and delineating the extent of the geothermal resource. MT data analysis was done to derive a dataset suitable for defining the resistivity model of the Earth from the observed MT data. The spatial median filter approach was used to correct the static shift effects of the MT datasets. Swift-skew, bahr-skew, ellipticity, and phase tensor dimensionality investigations revealed reduced skews at short periods and higher skews at long periods, revealing 1D and 2D characteristics at shallow depth and significant 3D structures at depth. 1D and 2D inversion were performed to generate resistivity models of Olkaria south East field. 3D MT inversion of Olkaria South East datasets using the ModEM program was performed to obtain the resistivity structure of the field. Generally, the inverted resistivity model fits the MT data very well, as demonstrated by the data misfit curves. Resistivity profiles were obtained from the 3D model to cut across the known geological structures in order to characterize the resistivity structure in the field. Since the profiles pass through the same soundings as in 2D inversion, a joint interpretation was made based on the two models. From the 2D and 3D resistivity models three layers have been revealed. The first layer has a high resistivity (~80 Ωm) which could be interpreted as the unaltered volcanic rock formation possibly from the pyroclastic cover. The second layer has a low resistivity (5- 10 Ωm) interpreted as the cap rock. The low resistivities are attributed to alteration minerals such as smectites and illites. The third layer has a relatively high resistivity (> 80 Ωm) and is interpreted as the geothermal reservoir. The resultant 3D resistivity models are generally similar to the two-dimensional (2D) inversion models; however, the deeper portion of the 3D model seems to be more realistic than that of the 2D model. From the conceptual model, Olkaria geothermal field reservoir is characterized by high resistivities and a drop in temperatures at the intercept of the intrusion. It can therefore be concluded to be as a result of cold inflow from the two faults transversing the field, majorly the Olobutot fault.