ORIGINAL PAPER Vitrinite reflectance as a tool for determining level of thermal maturity for the Upper Jurassic Naokelekan and Barsarin Formations in Sargelu location, Kurdistan Region, NE Iraq Dler H. Baban & Shadan M. Ahmed Received: 3 February 2013 / Accepted: 15 March 2013 # Saudi Society for Geosciences 2013 Abstract Ten samples from the Upper Jurassic Naokelekan and Barsarin Formations in an outcrop section near Sargelu Village were studied to evaluate their thermal maturity using vitrinite reflectance method. The studied beds appeared to be mature and within the oil generation window (0.79–1.25 % vitrinite reflectance in immersion oil (Ro)). Some of the reflectance histograms showed bimodal distribution indicat- ing existence of nonindigenous populations which were all ignored during the measurements of the mean value of Ro. Comparison between the results obtained from vitrinite reflec- tance method and some other methods like pyrolysis and gas chromatography showed variations which finally more reality believed to be for the vitrinite reflectance method. Keywords Vitrinite . Naokelekan . Barsarin . Maturation Introduction The use of vitrinite reflectance as a technique for determin- ing maturation of organic matter in sedimentary rocks was first described by Marlies TeichmÜller in her study of the Wealden Basin in 1958 ( Hunt 1996). Vitrinite is composed of clusters of condensed aromatic rings linked with chains and stacked on top of one another. With increasing maturity, the clusters fuse into larger, con- densed aromatic ring structures. Eventually, they form sheets of condensed rings that assume an orderly structure. Both the increase in the size of these sheets and their preferred orientation cause greater reflectivity (Hunt 1996). Vitrinite is one of the petrographic components of coals used for the determination of paleotemperatures based on the thermogenic changes in optical properties of the organic mat- ter. The reflectance of vitrinite is determined by microscopic examination of polished sections, measuring and comparing the electric currents generated in a photomultiplier under the influence of light reflected from the polished surface of the rock sample and from the standard with a known reflection. The reflectance of vitrinite is determined in immersion oil (Ro) and also in air (Chilingar et al. 2005). A histogram profile of vitrinite reflectance used to estimate the maturity level of a sample must have an adequate number of measurements. A rule of thumb is 40–50 vitrinite readings per sample (Law 1999). The lowest value associated with the known generation of conventional oil is about 0.5 %, and 0.6 % is generally recognized as the beginning of commercial oil accumula- tion. The peak of oil generation is at Ro level around 0.8–1. 0 %. At higher Ro levels, the gas/oil ratio increase rapidly with reflectance. The end of oil generation is around Ro=1. 3 %, condensate around Ro =2 %, and dry gas around Ro=3. 5 % (Dow 1977) and it goes with 15 % with graphite (Hunt 1996). The above reflectance intervals and what suggested by other authors for the gas and oil zones all are approximate; the fact is that different source rocks with the same thermal history should have reached the same level of vitrinite reflectance, but not necessarily the same stage of evolution, in terms of oil or gas generation zones (Tissot and Welte 1984). The scope of this study is assessing the thermal maturity state of Naokelekan and Barsarin Formations using vitrinite reflectance technique and evaluating the effectiveness of this tool by comparing the obtained results with works done using other tools about the same formations in the nearest locations to the studied section. D. H. Baban (*) : S. M. Ahmed Department of Geology, Faculty of Science and Science Education, School of Science, University of Sulaimani, Sulaimani, Kurdistan Region-Iraq, Iraq e-mail: dbaban1962@yahoo.com S. M. Ahmed e-mail: shadan.ahmed@univsul.net Arab J Geosci DOI 10.1007/s12517-013-0938-x