Geochemical constraints for the origin of thermal waters from western Turkey

The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water-rock interaction. Four types of thermal water are present: (1) a Na-Cl of marine origin; (2) a Na-HCO3 type with high CO2 content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na-SO4 type that is also associated with metamorphic rocks of the Menderes Massif with H2S addition; and (4) a Ca-Mg-HCO3-SO4 type that results from interactions with carbonate rocks at shallow depths. The Na-Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumali Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Canakkale-Tuzla (Br/Cl < sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water-rock interactions; (2) intensification of reactions due to high dissolved CO2 and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (delta(11)B=2.3parts per thousand to 18.7parts per thousand; n=6) can indicate either leaching of B from the rocks, or B(OH)(3) degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. Sr-87/Sr-86 ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (Sr-87/Sr-86 ratio as high as 0.719479) and carbonate rocks (low Sr-87/Sr-86 ratio of 0.707864). (C) 2002 Elsevier Science Ltd. All rights reserved

graben system, Denizli - Saraykoy, western Turkey

The KB-5 well is located at the intersection of the geothermally active Menderes and Gediz graben systems in western Turkey. Significant volumes of "petroleum-like material" (PLM) with its associated thermal water (120 degrees C) erupted onto the surface during drilling from a depth of 120-132 m (i.e., from the claystone and marl-rich Early to Middle Pliocene Kolonkoya formation). The purpose of this paper is: (1) to characterize this PLM, (2) to assess the source characteristics from which the PLM was likely generated, and (3) to recognize the generation mechanism considering the geothermal-gradual versus the hydrothermal-rapid processes. Analytical organic geochemistry using thin layer chromatographic separation followed by gas chromatography-mass spectrometry (GC-MS) was carried out. The PLM is composed primarily of polar NSO compounds + asphaltenes (77%). This feature is similar to hydrothermal petroleums (e.g., Calcite Springs, Yellowstone National Park, Wyoming). The PLM shows n-alkanes ranging from n-C-19 to n-C-38 (i.e., with a maximum at n-C-30) but n-alkanes are depleted in %C-27 alpha alpha alpha R] suggest that the PLM is a petroleum and was most likely generated from a Tertiary source rock with a carbonate-clay-rich lithology. The source organic matter was predominantly terrestrial and bacterial detritus deposited in relatively saline - anoxic environmental conditions. Maturity sensitive biomarker parameters (i.e., T-m > T-s; C-29 sterane 20S/(20S + 20R)=0.35; C-32 hopane 22S/(22S+22R)=0.57; low diasterane/regular sterane ratio) correspond to a maturity level of approximately 0.50-0.60% R-o. This level of maturation could not be reached by such a young (Upper Miocene-Lower Pliocene) and shallow sedimentary column (max. 600 m) in the absence of "instantaneous" hydrothermal activity. PAHs are not detectable, but series of alkylbenzenes, alkyltoluenes, alk-1-enes and triaromatic steranes are present. The lack of organic richness and maturity in the drilled section of the KB-5 well indicates that the petroleum was generated in the deeper sections of the formation. The temperature profile of the well and composition of the hydrocarbons in the petroleum suggest that the temperature of the hydrothermal activity during hydrocarbon generation was < 180 degrees C. (c) 2007 Elsevier Ltd. All rights reserved