Production of hydrogen sulphide in various oil and gas reservoirs has been reported around the world. Hydrogen sulphide is one of the major challenges in the operation and refining process of oil and gas. Therefore, determining the origin and mechanism of hydrogen sulphide production in gas and oil reservoirs is of great importance. The aim of this study was to determine source of H2S generation and production in a gas filed that initially produced very low concentration and eventually some wells faced with increasing level after about ten years which highly affected production and downstream facilities. In this research, detailed geological and comprehensive laboratory studies (petrography and isotopic analysis of gas, rock and water) were performed in different parts of the field to determine the source of hydrogen sulphide. For the first time, isotopic analysis of sulphur in rock, water and gas of Iranian onshore gas fields is presented. Anhydrite δ34S changed from +8.3 to +34.9‰ from the Permian to the Triassic respectively, confirming the Phanerozoic sulphur curve and showing that diagenesis has generally not destroyed the initial sulphur isotope stratigraphy of anhydride. The H2S δ34S varied from − 7.3 to – 11.8‰ in all reservoir unites. According to petrographic studies, the conversion of anhydride to calcite was observed at the upper depths of the upper Dalan Formation. Geochemical evidence from water, rock and gas along with geological, engineering and operational data allows claiming that Thermochemical Sulphate Reduction (TSR) process is the main mechanism of hydrogen sulphide generation in this field. However, negative values of the sulphur isotope in gas samples as well as the burial history and presence of dark condensates in the studied field indicate the possible role of bacterial sulphate reduction and thermal cracking in organic matter with less impact for H2S production.

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