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    Comprehensive geochemical correlation between surface and subsurface hydrocarbon occurrences in the Batman-Mardin-Sirnak area (SE Turkey)
    (Elsevier Sci Ltd, 2018) Hosseini, S. Hossein; Horsfield, Brian; Wilkes, Heinz; Vieth-Hillebrand, Andrea; Yalcin, M. Namik; Kavak, Orhan
    Southeast Turkey is the main oil-producing region of the country, located at the northwestern part of the Zagros Basin. In this study, we investigated the geochemical characteristics of both surface and subsurface hydrocarbon occurrences in the Batman-Mardin-Sirnak area in southeast Turkey to determine whether a genetic relationship exists between the two. For this, thirty solid bitumens from twenty-two different veins along with ten heavy oils and thirteen oil seepage samples from nearby oilfields and seeps were studied using diverse geochemical tools. Firstly each of the different hydrocarbon types was considered individually with respect to source organofacies, level of thermal maturity and degree of biodegradation. The results obtained from the source-related parameters demonstrate a marine algal source for all investigated samples. Also, lithology determination based on various diasterane, tricyclic terpane, hopane and homohopane parameters, suggests a carbonate source for all samples, except for the Dadas and Iskar seeps (clastic). Thereafter, considering the influences caused by thermal and biological alteration processes, all heavy oils, less mature solid bitumens as well as the less biodegraded seep oils were selected to be correlated using selected additional parameters. Homohopane distributions reveal that solid bitumens, compared to heavy oils and seeps, are highly enriched in C-31. and depleted in higher homologues possibly due to cracking of high molecular weight homologues to lower ones during and/or after solidification processes. Furthermore, the similarities observed for the relative abundances of six series of aromatic hydrocarbons and their alkylated counterparts substantiate strong genetic affinities within the sample set. Finally, compound-specific stable carbon isotopes of individual components show that solid bitumens are more enriched in C-13 than the heavy oils which is mainly attributed to the preferential removal of light isotopes (C-12) during solidification. Our results strongly confirm that Turkish solid bitumens are genetically related to the nearby heavy oils, thereby providing new information on the petroleum system in this part of the southeast Anatolia.
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    Role of Maturity in Controlling the Composition of Solid Bitumens in Veins and Vugs from SE Turkey as Revealed by Conventional and Advanced Geochemical Tools
    (Amer Chemical Soc, 2017) Hosseini, S. Hossein; Horsfield, Brian; Poetz, Stefanie; Wilkes, Heinz; Yalcin, M. Namik; Kavak, Orhan
    Solid bitumens found in veins and cracks may have formed from once liquid petroleum by thermal chemical alteration such as maturation or by oxidative processes associated with thermochemical sulfate reduction (TSR), biodegradation, or weathering. In this study, we investigated 22 solid bitumen samples from 17 different veins in the Sirnak Province in southeast Turkey using conventional and advanced geochemical tools with respect to their formation. Southeast Turkey is located in an active collision zone from the Alpine orogeny, and compressional pressure occurred with different intensities across the region, generating an overall increase in maturation from west to east. Special focus was paid to the characterization of polar compounds containing nitrogen, sulfur, and oxygen (NSO) of high molecular weight using Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization in negative ion mode (ESI-FT-ICR-MS). The results indicate that, among possible secondary processes, thermal stress plays the major role in transforming solid bitumen compositions across the region after their initial emplacement in subsurface veins and vugs. Hydrogen Index values and amounts of extractable organic matter correlate well with T-max. A progressive shortening of n-alkyl chains and increasing proportions of aromatic hydrocarbons are observed by pyrolysis-GC. While common maturity ratios based on saturated biomarkers are at equilibrium or saturated biomarkers are below the detection limit for samples with T-max > 450 degrees C, those based on aromatic hydrocarbons, such as phenanthrenes or alkyldibenzothiophenes, are useful to describe the whole range of thermal chemical alteration. ESI-FT-ICR-MS results reveal a dominance of nitrogen and sulfur compounds (NySz) with 1-2 pyrrolic nitrogen atoms and 0-2 thiophenic sulfur atoms and nitrogen only (N-y) compounds with 1-2 pyrrolic nitrogen atoms. Condensation, aromatization, and side-chain cracking reactions of NSO compounds take place with ongoing maturation, as indicated by double bond equivalent and carbon number distributions. At highest maturation levels, side-chain cracking has proceeded so far that further condensation and aromatization processes are not possible, which can be interpreted as an indication that solid bitumens have formed from migrated, reservoired petroleum and not from source rock kerogen. An increase in oxygen containing compounds indicates oxidation processes which most likely occurred after emplacement.