Improving Reservoir Temperature Estimation of Medium-Temperature Geothermal System by Incorporating Mineral Equilibria Conditions into Geothermometric Approaches: A Case Study in the Kiejo Mbaka Field, Southwest Tanzania

Abstract The geothermometric characterization of medium to low-temperature geothermal fields plays a crucial role in geothermal resource assessment by providing estimates of subsurface temperatures based on the chemical compositions of thermal fluids. However, these methods are frequently challenged by mineral equilibria and secondary processes that occur at low temperatures as geothermal reservoir fluids rise to the surface. This study explores the intricacies of saturation indices as valuable indicators of mineral equilibrium, aiming to achieve a comprehensive understanding for accurately estimating geothermal reservoir reactions in a Na-K-HCO 3 -type water system in the Kiejo-Mbaka prospect. A total of 11 samples from thermal springs were collected for analytical analysis of the prospect, displaying temperatures ranging from 30 °C (low) to 64 °C (high) and exhibiting partial to immature equilibration. The reservoir temperature is estimated to range from 19.5 °C to >300 °C using classical geothermometry, suggesting the non-equilibrium state of the water samples. Simulation using PHREEQC in the temperature range of 65 °C to 175 °C indicates that saturation indices models quantify the degree of mineral equilibration with chalcedony, quartz, and chrysotile. Under these conditions, the reservoir temperature needs to be estimated through a chalcedony or quartz geothermometry. Thermodynamic uncertainties reveal oversaturation of calcite, aragonite, and dolomite minerals. Finally, a reservoir temperature of 135 ±20 °C was obtained from the multi-mineral saturation indices models. This estimate remains consistent until the final exploratory temperature gradient drilling test, ensuring the coherence of the findings for practical applications.