Abstract: There are abundant oil and gas resources in carbonate rocks in Zagros Basin, but the genesis of H₂S is complex. The petrology, homogenization temperature of fluid inclusions and isotopes are analyzed to identify the genesis and distribution law of H₂S. The study results show that gypsum, anhydrite and barite are well developed in the Cretaceous and Cenozoic, H₂S contents (<5%) are low and sulfur isotopic fractionation is large between H₂S and anhydrite, which indicate a high degree of bacterial sulfate reduction (BSR). In addition, thermal degradation of organic kerogen occurred in part of the Cretaceous reservoir, leading to the increase of H₂S content to a certain extent. The sulfate minerals are well developed in the Jurassic-Permian reservoirs with a great burial depth, and hydrocarbon inclusions are observed in calcite cements. H₂S contents are high (up to 40%) and the sulfur isotopic fractionation is large between H₂S and sulfate. Affected by the mixture of organic matter such as hydrocarbons, the carbon isotope of calcite shows a distinctly negative drift (-10‰). These suggest that thermochemical sulfate reduction was dominant. Multiple genetic types led to differences in H₂S contents in various blocks.