SULFUR ISOTOPE CHARACTERISTICS OF MIDDLE MIOCENE ÇAYIRHAN OIL SHALES (BEYPAZARI, ANKARA / TURKEY)

Çayırhan oil shales (ÇOS) were formed in the Hırka formation of the middle Miocene age. The sulfur isotopic compositions of ÇOS, and source rock components were determined from 40 samples. This shales were a high content of organic material (TOC 2.08%-23.29%, average 9.53%). δ³⁴S isotope values for the basin are between -6,37 ‰ and + 26,55 ‰ with and average of +13,86 ‰. The sulphur isotope ratio for organic matter would seem, therefore, to give information concerning the isotopic content of the source sulphur at the time of oil shale formation. When it was based on the change intervals of samples, having an extremely wide range of results can be interpreted from two or more sources of sulfur. The lower values possibly represent plant assimilation of fresh water sulphate. Higher values, particularly those from ÇOS suggest closed basin conditions (or fast sedimentation rate) whereas negative δ³⁴S values from ÇOS suggest open basen (or low sedimentation rate) conditions. Moreover, many studies suggested that the δ³⁴S values could be effective on pH and ƒO2 of the environment. In some studies, a slight decrease in pH values δ³⁴S values due to positive values, while negative values ƒO2 (oxygen fugacity) suggested that without increasing. Therefore wide range of δ³⁴S values of ÇOS in the study area could be affected by ƒO2 and pH.