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.