(Special session) Big-data in resource exploration and drilling data processing

[3K0101-06-05] Conceptual modelling of geothermal fluid evolution in southwest Tanzania through water geochemistry and fluid-mineral equilibria

○Nyora Donald Kobare^{1[Student presentation: Doctoral course]}, Koki Kashiwaya¹, Katsuaki Koike¹, Albano Mahecha² (1. Environmental Geosphere Engineering Laboratory, Department of Urban Management, Graduate School of Engineering, Kyoto University, 2. Tanzania Geothermal Development Company)

Chairperson: 木﨑　彰久（秋田大学）

Keywords:East African Rift, Geothermal conceptual model, Reservoir, Geothermometry

The Songwe and Kiejo-Mbaka Geothermal Prospects (SGP and KGP) are intriguing areas for geothermal research within the East African Rift System (EARS). Their strategic location drives the focus on these sites at the intersection of the Usangu, Karonga, and Rukwa basins in southwestern Tanzania. This study in the SGP and KGP involves using water geochemistry and fluid-mineral equilibria to model the evolution of geothermal fluids in understanding the subsurface processes and the potential for power generation. The hydrochemical classification of the geothermal water is identified as alkaline Na-HCO₃, resulting from the interaction of meteoric waters with basement rocks, primarily gneisses, facilitated by the conversion of CO₂ to HCO₃. The abundance of Na⁺ and K⁺ in the samples indicates significant advances in chemical reactions, primarily comprising ion exchange and calcite dissolution within the geothermal system. Analysis through mineral equilibrium indices modelling reveals two distinctive characteristics of the geothermal fluids saturated with carbonate minerals, predominantly aragonite, dolomite, and calcite, evidenced by the extensive development of travertine near discharge zones. Reservoir temperatures are estimated by multicomponent geothermometry are 90–135 °C (Songwe) and 100–145 °C (Kiejo-Mbaka), determined using chalcedony and quartz saturation indices combined with multicomponent geothermometry. The geochemical analysis of the water indicates the existence of four reservoirs: Songwe-Rambo and Kaguri in the SGP, as well as Ilwalilo and Kilambo in the KGP, which serve as the upflow zones for the geothermal fluids. Based on flow rates and temperature differentials, the geothermal power generation capacities for the two prospects are estimated at a maximum of 4 MWe (SGP) and 5 MWe (KGP).