Geothermometry and hydrothermal alteration at the Balçova geothermal field, Turkey

•Hydrogeochemical characteristics of the Balçova geothermal field.•Alteration minerals in deep wells in the BGF.•Alteration zones of BGF.•Hydroisothermal cross-section of the BGF based on four wells. The Balçova field is located 9km west of İzmir City, Turkey. The Balçova geothermal field has been studied by many researchers and their studies have also guided this work. However, this paper addresses the structural, hydrogeological and geochemical properties of the field and their relationship to its geothermal potential. The compositions of water samples collected from the deep wells and cold groundwater resources have been classified by using the computer program “Aquachem 3.70”. Two different groups of waters as hot and cold ground water samples were analyzed and 10 different types of hot water and 3 different types of cold water were determined. Cutting samples and cores from selected depths have been studied by petrography and three zones of hydrothermal alteration have been determined. These are: a montmorillonite+kaolinite zone (100–200°C), a transition zone (150–200°C) and a chlorite+illite zone (200–250°C). The calculated reservoir temperatures by using chemical geothermometry applications range from 104° to 260°C. Hydrothermal alteration mineral assemblage of the geothermal field may reflect an acid–alkaline equilibrium of the geothermal system, besides indicating the likely extent of the geothermal system.