Lithospheric Structure and Thermal Characterization of Southwest Tanzania

Assessment of geothermal resource potential requires an understanding of the subsurface structure, including basement depth and sediment thickness. For accurate geothermal resource assessment, detailed temperature data, typically obtained by drilling wells, are essential. However, due to the high cost of drilling, most wells are limited to depths less than 5 km. As a result, detailed information on the subsurface temperature distribution is lacking. 3D geological models incorporating thermal properties offer a solution to this problem by providing a reliable prediction of subsurface temperature distribution. In this thesis, I aimed to conduct a quantitative assessment of the thermal and structural framework of the top 10 km of the earth’s crust and used the framework to assess geothermal potential in Southwest Tanzania. To achieve this objective, I constructed 3D geological models for both the upper and deep crust. Forward temperature calculations were performed using a 3D lithospheric model, and a temperature model was developed. The resource assessment utilised these models to estimate geothermal potential. Chapter 2 presents a detailed 3D geological model (upper crust). The model revealed significant variations in sedimentary thickness. The largest thickness (about 11 km) was observed in the Rukwa Basin, and a minimum thickness (of 2.2 km) in the Songwe Basin. Forward and inverse gravity modelling improved the quality of the model in the Songwe basin and Rungwe Volcanic Province (RVP), revealing a range of basement depths from 2 km (a.s.l) to 3.4 km (m.s.l) and significant sediment and volcanic thickness variations. I employed empirical methods and 3D litho-constrained gravity inversion to determine the depths to the Conrad, Moho, and lithosphere-asthenosphere boundary (LAB), as well as the thicknesses of the crust and lithosphere (Chapter 3). The results showed thin crust and lithosphere beneath the rift basins. The thinning corresponds to the regions with high heat flow and geothermal activity, suggesting that uplifted mantle beneath this thinned lithosphere is a significant source of anomalous heat in southwest Tanzania. Chapter 4 introduces a new lithospheric-scale temperature model for southwest Tanzania. This model identifies anomalous temperatures exceeding 150°C at depths of 3-5 km (m.s.l) in the South Tanganyika, Rukwa, RVP, and North Nyasa Basins, indicating potential for electricity production. These thermal anomalies are attributed to the thinned lithosphere and tectonic uplift. Chapter 5 reviews the geothermal potential of southwestern Tanzania. Employing an advanced quantitative techno-economic assessment method from the LEAP_RE Geothermal Atlas 4 Africa, the potential for direct heating, cooling, and power production was analysed. The findings indicate direct heating capacities up to 45 MW, and more than 9 MW for cooling. Electricity generation in most basins is under 2 MW, except for the RVP, which reaches up to 8 MW. The levelized cost of energy (LCOE) for direct heating and chilling is typically lower, under 5 $ct/kWh in the RVP. LCOE for electricity generation is lower in Songwe Basin and modest in RVP. These findings indicate that direct heating and chilling are cost-effective in most RVP and Songwe Basin areas, with electricity generation being the best option in the RVP.