Cayirhan oil shales situated 100 km NW of Ankara were formed in the Hirka formation of the middle Miocene age. This formation is a volcanosedimentary sequence and represented by a high content of organic material (TOC 2.08-23.29%, average 9.53%). Paleoecological and paleoclimate conditions during the formation of these shales were outlined by geochemical proxies. Results of the organic geochemical studies determined high Total Organic Carbon (TOC) contents and S-2 values and kerogen is of Type-1 and only a few samples are found to have Type-II kerogen. Based on Tmax values (average 433 degrees C), organic material is immature and early mature stage. Carbon isotope values (delta C-13) are in the range of -33.52 parts per thousand to -24.6 parts per thousand (average -29.23 parts per thousand). Comparison of these data with delta C-13 values from various geologic environments reveals that the carbon isotope range of samples is conformable with that of C3 plants and sedimentary deposits as well as metamorphic and magmatic rocks. The delta C-13 isotopes are fractionated with respect to changes in temperature. Carbon isotope content of cold water planktons is enriched, but as the temperature decreases, dissolved CO2 content is reduced thus lowering the delta C-13 value. Considering the source and living conditions of C3 plants, Cayirhan Oil Shales (COS) of the Hirka formation in the Beypazari basin are assumed to have a lacustrine origin and are formed in warm-water conditions. (C) 2014 Elsevier Ltd. All rights reserved.