Comprehensive geochemical correlation between surface and subsurface hydrocarbon occurrences in the Batman-Mardin-Şırnak area (SE Turkey)

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-Şırnak 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 Dadaş and İşkar 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 C31- 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 13C than the heavy oils which is mainly attributed to the preferential removal of light isotopes (12C) 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.