ИСТИНА

Coastal Fars is the main gas-bearing region in Iran, where the reservoir units are dedicated to the Paleozoic hydrocarbon system. Geochemical explorations in this region indicate that natural gas is commonly associated with elevated non-hydrocarbon components, especially hydrogen sulfide and carbon dioxide. Previously it was shown that the Thermochemical Sulfate Reduction (TSR) is the most probable mechanism, accounting for H2S occurrence in Permo-Triassic reservoirs in this region; however, its effect on the accumulated gas’s chemistry transformation has not been studied thoroughly. In this study, the molecular and isotopic composition of 12 gas samples in addition to the previously-published results of integrated analyses on rock and condensate, were utilized to trace potential alterations caused by the given phenomenon exhaustively. A slight increase in aromatic to saturate hydrocarbon components ratio with the extent of TSR and the presence of reported sulfur-rich pyrobitumen (Saberi, et al., 2014) indicate liquid hydrocarbon involvement in the primary stage of the sulfate reduction process. Further, the differential increase of δ13C of gas components, decrease of δ13C (methane) - δ13C (ethane) (from -4.3 to -13.1‰), and increase in carbon-dioxide concentration with the increase of hydrogen sulfide along with gas dryness (from 91 to 96%) show the dominancy of C2-C4 hydrocarbon gas components in the second stage of TSR. Comparative low reservoir temperature (~90°C) does not correspond to the contribution of methane into the given phenomenon; however, a noticeable increase in δ13C (methane) (from -41.7 to -34.7‰) with the increase of hydrogen sulfide was seen. Thermochemical Sulfate Reduction impact on studied parameters is analogous to thermal maturation, but the process from heavy-hydrocarbon-dominated TSR to methane-dominated TSR presents different trends of δ13C (methane) - δ13C (ethane), ln(C1/C2) from those of maturation (Hao, et al., 2008). Simultaneous carbon-dioxide content increase and decrease in its isotopic composition with the extent of TSR indicate its presence relevancy to the given phenomenon; however, carbon isotope values of CO2 (from -5.92 to -13.93‰) are too heavy to verify this idea. According to Dai et.al (Dai, et al., 1996) it can be concluded that carbonate's dissolution has contributed to carbon-dioxide gas. Hence, a series of TSR corresponding to condensate, wet gas, and dry gas stages, respectively, has led to higher aromatic/saturate ratios, heavier hydrocarbon components, lighter carbon dioxide molecules, and relative gas dryness in the studied fields.