Origin of petroliferous bitumen from the Büyük Menderes–Gediz geothermal graben system, Denizli – Sarayköy, 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 °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-C19 to n-C38 (i.e., with a maximum at n-C30) but n-alkanes are depleted in %C27αααR] 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., Tm > Ts; C29 sterane 20S/(20S + 20R) = 0.35; C32 hopane 22S/(22S + 22R) = 0.57; low diasterane/regular sterane ratio) correspond to a maturity level of approximately 0.50–0.60% Ro. 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 °C.