Paolo Chiozzi

Based on geophysical, geological and geochemical investigations carried out in the last decade, we reviewed three major geothermal plays that well represent the different structural, volcanological and hydrogeological realms that can be encountered in the East African Rift System (EARS). Alalobeda (Ethiopia) and Menengai (Kenya) are examples of typical geothermal plays of the Eastern Branch of EARS. The former is a fault-leakage-controlled geothermal play located in a graben structure. The heat source is likely deep-seated, widespread magmatism, associated with the lithosphere thinning that regionally affects this area. The reservoir temperature of the water-dominated system ranges from 185 to 225 °C. Menengai can be classified as a convection-dominated magmatic play type. The heat source could be a magmatic intrusion located beneath a caldera. A shallow, liquid-dominated reservoir (with temperatures of 150–190 °C) and an intermediate-deep reservoir, hosting steam and liquid (with temperatures of 230–340 °C), were detected. The Kilambo-Ilwalilo play (Tanzania) is an example of geothermal play of the Western Branch of EARS. It is in a half-graben realm where a regional fault controls the ascending groundwater flow. Reservoir temperatures are about 110–140 °C, and the heat source is provided by lithosphere thinning. The results of this study provide helpful guidelines for future studies on the geothermal resources in the rift.