Geothermal reservoirs have become very popular in the last decades due to their renewable energy contents. Turkey has a high geothermal energy potential / especially Western Anatolia is a promising region in terms of its highest energy. B&uuml / y&uuml / k Menderes Graben system is a tectonically active extensional region and is undergoing a N&ndash / S extension leading to form geothermal fields in the graben. In the last decade, geothermal exploration, investigations and investments have been increased rapidly through the law related to geothermal energy assigned. The aim of this study is to analyze the geothermal fields located in the graben system in order to investigate their geological, hydrogeological and geochemical features and reservoir characteristics. A data set is compiled from the accessible archives, published papers and documents and several variables have been searched at every field in the graben. The variables include the fluid temperature, the distance to sea, the depth to the reservoir, the reservoir lithology, the chemical constituents, etc. In view of these variables the fields are compared and constrasted and their common characterisrics have been noted. Interpretation of the data set reveals that the fields have some common features and characters however some fields have got notable differences.

This study aims the real-time monitoring of gases (CO2, N2, O2, H2, H2S, CH4, He, Ar) discharging from natural pools in the Tekke Hamam geothermal field (Denizli) in addition to the geochemical characterization of the field along with the Kizildere geothermal field, both located at the eastern segment of the B&uuml / y&uuml / k Menderes Graben. The continuous gas monitoring experiment (November 2007-October 2008) conducted in the Tekke Hamam geothermal field has revealed temporal variations in the gas compositions, gas flow rate and pool temperature. Different variation components, such as daily variation profiles and peak/Multi-day signals, are detected in the monitored data, which are mainly correlated with shallow and deep processes involving mainly meteorological factors and seismicity induced variations, respectively. Particularly, the coupled variations in the gas compositions and flow rate seem to correlate with seismicity induced permeability modifications within the subsurface during the absence of significant meteorological factors, such as high rainfall and varying atmospheric pressure. The noble gas characterization of the fields have revealed both high 3He/4He and 4He/20Ne isotopic ratios, suggesting a mantle contribution of about 18% for Kizildere and 34% for Tekke Hamam, whereas the other noble gases (Ar, Ne, Kr, Xe) are of atmospheric nature. The different mantle contributions observed in both fields can suggest a different mantle-He flux variably contaminated by crustal helium. The chemical (cation-trace element-anion) and stable isotopic (&amp / #948 / 18O-&amp / #948 / D) contents of the thermal waters reveal high temperature water-rock interaction accompanied by the effects of deep origined gases (mainly CO2 and H2S) discharging from the fields.