Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of -1.5 parts per thousand and -2.8 parts per thousand, suggested abiogenic origin from limestone. delta S-34 values of H2S ranged from +11.9 to +13.4 parts per thousand. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120-145 degrees C. Basin modeling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulfate reducing bacteria were found in oil-water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR.