Investigating the mechanisms for the gradual and anomalous pattern of increasing H2S content in the Dalan gas field, Coastal Fars of Southern Iran

The Coastal Fars is the most critical natural gas production region in southern Iran. In recent years, an increasing trend in H2S content has been distinguished in some fields in this region. The anomalous increasing trends have been observed in the eastern and western parts of the Dalan gas field. A comprehensive study of gas geochemistry, petrography of rock samples, and production data was performed to investigate the H2S generation mechanisms and disclose the reasons for the anomalous H2S increasing pattern in this field to find the best solution strategy. The geochemical analysis of produced gases and petrography studies revealed that H2S was generated due to the thermochemical sulfate reduction (TSR) process in the Dalan field. The sulfate ions in this process were sourced from anhydrite in the Lower Dalan and Nar members. Production data revealed the anomalous pattern of increasing H2S content in this field. In response to the pressure drop, the rising gas-water contact (GWC) caused the shortest distance between the aquifer and production interval in the Dalan-3 well from the west of the field. The degassing of the dissolved H2S from the water caused increasing H2S in this well. The relationship between the H2S content, produced water, and salinity confirms this conclusion. In contrast, at the Dalan-6 well from the east of the field, there was no direct correlation between salinity and H2S concentration. Despite the rise of the GWC level due to pressure drop, the water zone could not affect the production intervals in the Dalan-6 well. Production logs revealed channeling behind the casing, likely due to poor cement quality, facilitated the connection of the production intervals in the Upper Dalan with the Nar member and caused the H2S concentration increase in the Dalan-6 well. This paper revealed the increasing pattern of H2S content in a field can be irregular in different wells.