Energy plays a fundamental role in societies impacting everything from basic human needs like lighting, cooling and heating to complex industrial processes that significantly influence social development, economic growth, and national security particularly in terms of access and affordability for all citizens. The world’s electricity demand grew by 2.2% in 2023 and is expected to rise at a faster rate by an average of 3.4% in 2026 (IEA, 2024). With the global climate warming, it is important to reduce our societal impact on Earth by using clean energy sources. One of the clean sources is geothermal energy which has a great potential to reduce dependency on the fossil fuels. Geothermal energy sources can deliver both, heat and electricity. Tanzania has a huge geothermal potential that has not yet been used and has only been explored to a limited extent. This thesis attempts to identify the geothermal potential of southwest Tanzania with appropriate drilling zones by assessing the main components of a geothermal system reservoir which are temperature, permeability and fluid flow at Songwe Tanzania. It is one of the identified potential development areas with very little information available.
The work flow begins with chapter 2 where a geological and thermal numerical model is set up to simulate the temperature at depth. The thermal model considers pure conductive heat flow to achieve a first idea of the temperature distribution in the different geological layers.
Chapter 3 describes the conducted field study at the thermal spring areas as well as the laboratory analysis of the samples in order to understand the geothermal fluid source by using geochemical modelling, obtain the reservoir temperature through geothermometer calculation and locate the up-flow and the outflow zones of the geothermal field.
In chapter 4 numerical simulations of fluid and convective heat flow are performed to allow understanding the heat transport by fluids and the hydrogeological behaviour of the geothermal reservoir by varying thermal and physical parameters.
Chapter 5 shows possible drilling locations for a first geothermal well in the study area, whereby areas of different productivity are distinguished.

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The Songwe geothermal prospect is situated in western Tanzania in the Rukwa Rift of the western branch of the East African Rift System. Thermal springs discharge along NW–SE oriented fracture zones in two separate areas: in the main Songwe graben (Iyola, Main springs, Rambo and Kaguri) and eastern Songwe graben (Ikumbi). Lithologies forming and filling the Songwe graben are metamorphic gneiss and shist as basement rocks, overlain by the Karoo sandstones, and Red sandstones, both silt- and sandstones with a carbonatic matrix. In some areas of the graben, volcanic rocks intruded these formations forming basalt outflows. The discharge temperatures of springs are between 37 and 85 °C with Na-HCO₃ type fluids. Carbonate deposits surround most of the springs. Using previous geophysical, geological studies and historical fluid geochemical data and mineral data, the Songwe geothermal system interpretation was updated, including new reservoir fluid temperature, fluid flow pathway and water–rock interaction models. The classical geothermometers of K-Mg and Na-K-Ca _{(Mg correction)} were used to predict the reservoir fluid temperature and show that fluid emerging in the Songwe area reaches temperatures between 125 and 148 °C. Reservoir fluid characteristics are reconstructed based on the geothermometer calculation and a PHREEQC model in which the deep fluid reacts with certain lithologies. Minerals precipitating at the surface and reservoir depth were used to calibrate the models. The models run at surface temperature were calibrated with minerals precipitating around the springs and suggest that Songwe thermal fluids interact with Red sandstone only, while Ikumbi spring water is the only spring that interacts with all lithologies (simplified referred to as: metamorphic rocks, Karoo and Red sandstone). The model run at reservoir temperature indicates that rising water is also in contact with Karoo sandstones and Ikumbi spring water composition is again influenced by the contact with all lithologies in the graben. Our conceptual model summarizes all data showing the meteoric origin of the fluids, the travel through the basement, rising along the Mbeya fault and the main reaction with sandstones through a lateral travel towards the hot springs. The proposed models reinforce the idea that carbonate dissolution from the sandstone layers is the most common water–rock interaction. Our model is supported by carbonate deposition observed in all springs, dominated by HCO₃ and Na.

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