Several gas fields in Coastal Fars region, southwestern part of Iran have encountered an increase in hydrogen sulfide (H2S) concentration in recent years. Some authors offered thermochemical sulfate reduction (TSR) as the primary mechanism for this phenomenon in Paleozoic gas systems for the given region; however, a relatively low concentration of H2S; ~ 15 ppm for Triassic gas, and ~ 30 ppm for Permian gas, does not correspond with TSR, as the mentioned mechanism is generally responsible for the hydrogen sulfide production in more extensive contents. This study uses sulfur isotopic compositions and formation water chemistry to delineate whether TSR is occurring in the studied area and find an explanation for the low contents of hydrogen sulfide. An excessive decrease in sulfate ion concentrations compared with a level measured in seawater behind an evaporation process reveals that a sulfate reduction mechanism has taken place. Bacterial sulfate reduction is unluckily to proceed, as reservoir temperatures (~90°C) are high for any bacterial activity survival, and δ34S values are not low enough to confirm it. Water dilution with the increase of hydrogen sulfide contents enhances the probability of TSR occurrence; however, due to relatively low reservoir temperature and low contents of Mg2+ ions, its rate is limited, which as a result, has led to low H2S concentrations and sulfur isotope fractionation. In the Permian formation, due to its exposure to a higher temperature and Mg/SO4 ratios relative to the Triassic formation, intensifier TSR is happening, and therefore more hydrogen sulfide is being produced.