Investigating the souring mechanism in two giant carbonate oil reservoirs, southwestern Iran

Abstract In general, a hydrocarbon reservoir with a produced gas stream having more than three parts per million by volume (ppmv) of hydrogen sulfide is considered a sour reservoir. This study aims to present novel ideas on the main souring mechanisms in Bangestan Group, a well-known Mid-Cretaceous hydrocarbon hosting carbonate in Iran, including Ilam, Sarvak, and Kazhdumi formations. Especially, several scenarios are discussed on potential source of H2S in Sarvak Formation and controversial sweetness of the Upper Ilam Reservoir. At first, both of the bacterial or microbial sulfate Reduction (BSR or MSR) and thermochemical sulfate reduction (TSR) mechanisms were rejected based on the reported bottom-hole temperature and lack of anhydrite in rock matrix, respectively. Moreover, the possibility of souring due to migration was disregarded mainly due to the reported constant concentration of H2S over time and among the target oilfield. At the end, thermal decomposition of organic sulfur compounds within the kerogen or crude oil was introduced as the only potential souring mechanism that was further proven through sulfur isotopic analysis. Furthermore, the possibility of H2S absorption in lower horizons of Ilam Formation, and so sweetness of this formation, due to the presence of ferrous minerals was evidenced through detailed petrographic analysis.