The South Pars field, which is the Iranian part of the world largest non-associated gas accumulation (taking into account its Qatari part, North Field, with 1342 tcf proved reserve), is hosted by Upper Permian Dalan and Lower Triassic Kangan carbonates. Carbon isotopic and molecular composition studies were undertaken to investigate the origin of non-associated gases from this field. In general, evaluation of the δ13C values of methane, ethane and propane and gas wetness (C2–C5/C1–C5) indicates a thermogenic origin. On the other hand, results of the isotopic analyses showed no mixing of biogenic gases in all of the analyzed gas samples. A further evaluation using natural gas plot, carbon isotopic composition of methane to pentane vs. 1 / n, showed δ13C1 < δ13C2 < δ13C3 as normal trend. However, the carbon isotopic compositions of butane and pentane indicate a minor reverse trend in the normal progression of carbon isotopic compositions. This reversal in carbon isotope ratio among butane and pentane is interpreted as a consequence of mixing from multiple sources with different maturities.
δ13CCO2 in South Pars field is similar with those of thermogenic origin. The diagram of δ13CCO2 vs. δ13CCH4 indicates that South Pars gases plot into the isotopic compositional field for methane derived from thermogenic origin. The H2S concentration is much lower than the Khuff Formation and it seems it has originated from different processes. Apparently, the Lower Silurian organic-rich shales, which are the main source for hydrocarbons in the Arabian Plate and southern Iran, including South Pars field, generally contain kerogen type II with some contribution of type III in some areas.
Burial history modeling indicates that at the end of the Cretaceous time pre-Permian sediments remained immature in the Qatar Arch, and their present day maturity is lower than values obtained based on isotopic data. Therefore, lateral migration of gas from the nearby Silurian source rock kitchens towards the north and south of the Qatar Arch is the most probable origin for the significant natural gas accumulations in South Pars field.
