Hydrogeochemical similarities and differences between high temperature geothermal systems with similar geologic settings in the Buyuk Menderes and Gediz Grabens of Turkey

Abstract

The Buyuk Menderes Graben (BMG) and the Gediz Graben (GG) are two major graben systems and they have important medium-high temperature geothermal systems, with suitable reservoir temperatures (e.g. 170-276 degrees C) for electricity production such as Kizildere (Denizli city), Germencik, Salavath, Pamukoren (Aydin city) in the BMG and Alsehir (Manisa city) in the GG. These high temperature geothermal systems are located in a highly fractured zone because of the Aegean Extensional Zone effect. Although the main reservoir rocks retain similar compositions for each graben system, the geothermal fluids chemistry characterization can vary along the BMG and GG as a result of differing variables such as a paleo sea effect, the concentration of non-condensable gases (NCG), as well as mineral effects. The water chemistry results and delta O-18-delta D isotope values show remarkable differences within the Germencik and Kizildere systems along the BMG, while the main reservoir rocks are of a similar composition. Chemical, isotope compositions and pH values of thermal waters in Alasehir differ from those in the BMG geothermal systems. Results for delta C-13 isotopes indicate that the origin of the large amounts of CO2 is the marble in metamorphic rocks in the reservoirs in both graben systems. delta S-34, delta S-34- SO4 and delta O-18-SO4 values indicate that the origin of SO4 indicates gypsum and also bacteria effect in the eastern part of the BMG, while different mechanisms such as; oxidation of sulfur, pH, temperature and possible buffer mineral effects on H2S dissolution to the west of the BMG and GG display a greater impact.