The Shanul onshore gas field in Coastal Fars (Zagros, SW Iran) produced essentially sweet gas for about ten years before hydrogen sulfide (H2S) concentrations began rising in some wells. Integrated petrographic and gas–water–rock sulfur-isotope studies identify thermochemical sulfate reduction (TSR) as the dominant H2S-generating mechanism, even though the reservoir is unusually cool (about 100 °C) for TSR, with possible minor contributions from bacterial sulfate reduction (BSR) and thermal cracking of sulfur-bearing organic matter. What remains unresolved, as acknowledged by the source study, is the quantitative and predictive account: (i) the fraction of the produced H2S attributable to TSR, BSR, thermal cracking, and migration/mixing, across wells, reservoir intervals, and production time; (ii) whether the negative gas sulfur-isotope signature (δ34S of −7.3 to −11.8‰) is typical of Middle East Permo-Triassic (Khuff-equivalent) carbonates regionally or specific to this field; and (iii) the kinetics of incipient low-temperature TSR needed to forecast future H2S escalation under production and to design mitigation. Later work has qualitatively confirmed the TSR origin (petrographic replacement textures, one-dimensional PHREEQC reaction-transport sensitivity modeling, analogous souring increases at the Dalan field and Gavbandi-High areas), but no study yet delivers a partitioned H2S budget, a calibrated production-souring forecast, or a regional δ34S synthesis for these reservoirs. Here PHREEQC denotes a widely used computer code for equilibrium and kinetic geochemical reaction modeling.

Background

Hydrogen sulfide (H2S, 'sour gas') is toxic and corrosive, and an unexpected rise of H2S in a field developed and equipped for sweet gas damages project economics, facilities integrity, and safety. H2S in carbonate gas reservoirs can originate from several processes. Thermochemical sulfate reduction (TSR) is the abiogenic reduction of sulfate (typically from anhydrite or gypsum) by hydrocarbons at elevated temperature, producing H2S, CO2, and carbonate minerals; it is generally regarded as effective mainly above roughly 120–140 °C, so its occurrence at only ~100 °C is anomalous and 'incipient'. Bacterial sulfate reduction (BSR) is the microbial analog, operative at lower temperatures and usually over geological time. Thermal cracking of sulfur-rich organic matter (e.g., dark condensates, sour kerogen or oil) releases H2S during deep burial. In addition, H2S measured at a well can reflect migration from sulfate-rich units, dilution and mixing during production, or degassing of dissolved H2S as aquifers rise — effects that can mimic or mask in-situ generation. The sulfur isotopic composition δ34S (the per-mil deviation of the 34S/32S ratio from a standard) of H2S, anhydrite, and formation water is the principal discriminant among these sources, because BSR strongly fractionates sulfur isotopes (producing isotopically light H2S), TSR produces smaller fractionations, and cracking inherits the organic sulfur signature. The Shanul field lies in Coastal Fars on the Zagros foldbelt of SW Iran, producing from the Permo-Triassic Dehram Group: the Triassic Kangan Formation and the Permian Dalan Formation, the latter subdivided into the Upper Dalan, the anhydrite-bearing Nar Member, and the Lower Dalan — the Iranian equivalents of the Khuff Formation that hosts the giant sour-gas reservoirs of the Arabian platform. After roughly a decade of sweet production, some Shanul wells showed rising H2S. A comprehensive 2021 study (the source of this problem) presented the first Iranian onshore gas–water–rock sulfur-isotope dataset: anhydrite δ34S of +8.3 to +34.9‰ (Permian to Triassic, consistent with the Phanerozoic seawater curve and showing that diagenesis did not destroy the primary stratigraphy), H2S δ34S of −7.3 to −11.8‰ across all reservoir units, and petrographic anhydrite-to-calcite conversion in the upper Dalan. Integrating these with geological, engineering, and operational data, the authors concluded that TSR is the main H2S mechanism in this field, with the negative H2S δ34S values, burial history, and dark condensates indicating a possible lesser role of BSR and thermal cracking. They explicitly left open: the quantitative share of each process in the cumulative H2S (no interval-matched cores or cuttings existed for the Nar and Lower Dalan in the Shanul wells, so those units were represented by analog samples from nearby fields); whether the negative gas δ34S trend is widespread in Middle East Permo-Triassic carbonates or field-specific; and the time-dependence and kinetics of ongoing souring at the incipient ~100 °C stage, for which integrated reservoir-flow and geochemical reaction-transport modeling plus time-series isotope monitoring were called for. Subsequent regional work documents the same incipient-TSR pattern: at the Dalan field (Coastal Fars) H2S increases are also TSR-derived, with sulfate from anhydrite in the Lower Dalan and Nar members, and well-level anomalies driven by gas–water-contact rise, degassing of dissolved H2S, and behind-casing channeling; in the Gavbandi-High area, TSR is demonstrably rate-limited at ~90 °C (producing only ppm-level H2S), with BSR excluded on temperature and isotopic grounds; and a one-dimensional PHREEQC model of Shanul itself shows H2S evolution to be sensitive to Fe-bearing minerals, pressure drawdown, and pH. None of this yet quantifies the source partitioning or forecasts the souring trajectory.

Problem Statement

The problem, as posed by the source study on the Shanul gas field and left open by the subsequent literature, is to convert the qualitative TSR interpretation of Shanul-type souring into a quantitative, predictive account — a single scientific objective with three mutually supporting requirements: (1) Determine, with stated uncertainty, how the H2S budget of the Shanul reservoir (and the analogous Coastal Fars Permo-Triassic reservoirs) is partitioned among thermochemical sulfate reduction, bacterial sulfate reduction, thermal cracking of sulfur-bearing organic matter, and migration/mixing (including production-induced effects such as aquifer rise, degassing of dissolved H2S, and dilution), as a function of well, reservoir interval, and production time. Reducing the residual uncertainty in this partition requires interval-matched gas–water–rock sampling and isotopic analysis across more wells and intervals, including direct downhole core or cutting samples from the Nar and Lower Dalan intervals, which remain unsampled in the Shanul wells. (2) Establish whether the negative H2S δ34S\delta^{34}\mathrm{S} signature (−7.3 to −11.8‰) characterizes Middle East Permo-Triassic (Khuff-equivalent) carbonates regionally or is specific to individual fields, by a multi-field isotopic comparison. (3) Quantify the kinetics of TSR at the reservoir's low temperature (~100 °C) sufficiently to forecast the time evolution of H2S content in producing wells — including whether and how fast souring will escalate under continued production — so that mitigation measures can be designed on a predictive rather than reactive basis. The partitioning, its regional calibration, and its temporal projection are jointly needed for a predictive account of reservoir souring.

References

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