Occurrences and origin of oils and asphaltites from South East Anatolia (Turkey): Implications from organic geochemistry

SE Anatolia hosts the most important oilfields and asphaltite deposits in Turkey. Here, we report on the organic geochemical characteristics of crude oil samples from four wells (Yolacan-1,.Camurlu-28, B. Kozluca-22 and D. Sinirtepe-1) located to the west of Mardin city, one oil from a well (Ozluce-1) to the north of Batman city, and asphaltite samples from the Avgamasya. Herbol and Seridahle localities. Reservoir rocks for the oils from the Ozluce-1 Camurlu-28, B. Kozluca-22 and D. Smirtepe-1 wells are Lower Cretaceous limestones and dolomites of the Alt Sinan Formation. At well Yolacan-1, the oil is reservoired in the carbonates of the Lower Cretaceous Mardin Group. At Avgamasya and Seridahle, asphaltites occur in joint systems in the Upper Cretaceous-Paleocene Germav Formation; the Herbol asphaltites are located in the Eocene Gercus Formation. Except for the Seridahle asphaltite sample, delta C-13 values of all other oils and asphaltites are very similar. Gas chromatograms indicate that while the Ozluce-1 oil sample predominantly contains n-alkanes with medium carbon number (C-13-C-20) the other oil samples predominantly contain n-alkanes with low carbon number (C-4-C-11). Except for the Ozluce-1 oil sample, all the other oil and asphaltite samples show similar C-27. C-29 and C-29 sterane distributions, with C-29 being the dominant sterane. Based on the C-28/C-29 sterane ratio, the Ozluce-1 oil derives from a younger source when compared to the other oil and asphaltites. The diasterane/sterane, Ts/(Ts+Tm). C-29/C-30 hopane ratios and C-24 tetracylic terpane abundance indicate that all the oil and asphaltite samples are of similar character except for the Ozluce-1 oil sample, which is characterized by higher diasterane/sterane and Ts/(Ts+Tm) ratios, a lower C-29/C-30 hopane value and lower C-24 tetracylic terpane content. The C-31-R/C-30 ratio indicates a marine source for all oil and asphaltite samples. Normal alkane, isoprenoid, biomarker and isotope data indicate that the all the crude oils (with the exception of that from Ozluce-1) and all the asphaltites are geochemically similar and were generated from marine carbonate source rocks. The Ozluce-1 appears to have been derived from a clay-rich source rock. Based on geology of the basin and their geochemical characteristics, it is suggested that all oils (except Ozluce-1) and asphaltites were generated from Triassic-Jurassic carbonates of the Cudi Group. The source rock of Ozluce-1 oil may be Cretaceous marl or limestone of the Karababa Formation or clay-rich limestone of the Beloka Formation. (C) 2012 Elsevier B.V. All rights reserved.