Hydrogen sulfide (H2S) is an undesirable compound in oil and gas reservoirs, which reduces the economic value of gas reservoirs and has toxic and corrosive effects on reservoir operation equipment. Albeit initial generation mechanisms of hydrogen sulfide in gas reservoirs, increasing its concentration during the production phase may affect project economics. Thus, the determination of causes of increasing hydrogen sulfide concentration is of great importance. This study reports a systematic approach to investigate the possible mechanisms behind increasing H2S concentration in a carbonate gas reservoir after ten years of sweet gas production from an onshore field located in SW of Iran. Initial data during the development phase suggested a sweet gas reservoir fluid and all production and processing facilities were designed based on sweet gas production. Some wells showed increasing H2S concentration in two local sections of the field after ten years. Sampling from gas, water, and rock have been done, and isotope analysis was performed for both contaminated and non-contaminated wells. Geochemical and field data analysis were performed to determine the source of H2S increasing in this field. Results showed that the amount of sulfur isotope in the gas phase ranged from - 7.3 to − 11.8‰. A general trend between the concentration of H2S and sulfur isotope was obtained. Sulfur isotope values of rock samples (core, drill cuttings, and outcrop samples), varied between + 8.3% and + 34.9% depending on formation age. Integration of geochemical, geological, and operational data indicated the occurrence and migration of H2S from an anhydrite rich formation at the bottom of the reservoir. All other possible mechanisms was rejected based on isotopic and geological evidence.

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