Thermochemical sulfate reduction (TSR) — the abiotic high-temperature reaction between sulfate and hydrocarbons that generates H2S in deeply buried carbonate reservoirs — is the accepted origin of the highly sour Permian Changxing and Triassic Feixianguan gas fields of the Sichuan Basin, yet the 2010 synthesis by Liu Wenhui and co-workers, which proposed a four-factor enrichment model as a basis for predicting the distribution of highly sour gas fields, left the H2S formation mechanism only partly resolved. Linked issues remained open: the mechanistic role of Mg2+ (catalysis, ion-pair reactivity, or a solubility artifact) relative to CaSO4 systems; whether formation-water chemistry — especially dissolved MgSO4 concentration — controls TSR and the massive generation of deep H2S; the provenance and temporal availability of soluble sulfate in reservoir formation waters; the extent to which H2S destruction by metal-bearing lithologies during migration shapes final gas compositions and distributions; and the causal (not merely correlational) relation between dolomitization or hydrothermal fluid events and H2S generation. Published work through 2026 shows major progress on all five fronts — MgSO4 contact-ion-pair evidence from the Tazhong field, Raman speciation of Mg2+–SO4 pairing, DFT and gold-tube activation energies for methane TSR, three-stage in-situ Raman kinetics with H2S autocatalysis via S0 and trisulfur radical, basin-history kinetic models validated on the Puguang and Hetianhe fields, and isotope-constrained sulfate sources — but the mechanistic Mg2+ question is unsettled, kinetic parameters still vary widely, sulfate provenance partitioning remains inferential, and H2S sink rates are unquantified. A mechanism-grounded, pre-drill capability for predicting H2S distributions in these reservoirs therefore remains an open, coherent problem in petroleum geochemistry.

Background

Deep carbonate gas reservoirs in northeastern Sichuan (China), hosted in the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation (P2ch–T1f) reef and oolitic-shoal facies (carbonate reefs and shoals built of sand-size rounded grains), contain some of the world's most sour (H2S-rich) natural gases, with H2S contents reaching about 17% in fields such as Puguang. The accepted source of this H2S is thermochemical sulfate reduction (TSR): the abiotic, non-microbial reduction of sulfate coupled to the oxidation of hydrocarbons at reservoir temperatures generally above roughly 120 °C. Understanding TSR matters because H2S is toxic and corrosive, so drilling into an unpredicted sour reservoir has fatal and economic consequences, as recognized already by Cai Chunfang and co-workers in 2004 (Marine and Petroleum Geology 21:1265–1279), who modeled methane-dominated TSR in the Feixianguan Formation 'towards prediction of fatal H2S concentrations'.

Two mechanistic traditions frame the open questions. In one, experimentally observed TSR is strongly autocatalytic in H2S: Zhang Tongwei, Alon Amrani, Geoffrey S. Ellis, Qisheng Ma and Yongchun Tang (2008, Geochimica et Cosmochimica Acta) showed H2S initiation accelerates sulfate reduction, and Qisheng Ma, Geoffrey S. Ellis, Alon Amrani, Tongwei Zhang and Yongchun Tang (2008, Geochimica et Cosmochimica Acta 72:4565–4576) used density functional theory (DFT, a quantum-chemical method) to identify reduction of sulfate S(VI) to sulfite S(IV) as the rate-limiting step, with activation barriers strongly dependent on the sulfate species present. In the other, cation-specific chemistry matters: gold-tube hydrous pyrolysis (laboratory pyrolysis in sealed gold capsules at high temperature and pressure) by Lu Hong and co-workers (2010, Organic Geochemistry) and by the source paper itself showed that systems supplied with magnesium sulfate (MgSO4, highly soluble) yield far more H2S than systems supplied with anhydrite (CaSO4, poorly soluble).

The source paper for this problem — Liu Wenhui, Tenger, Gao Bo, Zhang Zhongning, Zhang Jianyong, Zhang Dianwei, Fan Ming, Fu Xiaodong, Zheng Lunju and Liu Quanyou (2010), 'H2S formation and enrichment mechanisms in medium to large scale natural gas fields (reservoirs) in the Sichuan Basin', Petroleum Exploration and Development 37(5):513–522 — synthesized field distributions (36 wells with H2S from 0% to 17.10%), petrography, Raman gas-inclusion data, and gold-tube experiments into a four-factor model for volumetric H2S enrichment: (1) source (hydrocarbons plus soluble sulfate, possibly in magnesic fluids), (2) energy (burial heating to at least about 120 °C, into the wet-gas/condensate cracking window, the temperature range in which oil and condensate thermally crack to gas), (3) space (early dolomitized reservoir volume and paleo-oil pools contemporaneous with charge), and (4) preservation (reducing, closed conditions without H2S-destroying lithologies). Dolomitization — replacement of limestone by dolomite, here taken to signal Mg2+-rich fluids — spatially coincides with the sour wells. Crucially, the authors themselves acknowledged that 'it is not clear how [Mg2+] is involved in the reaction mechanism' while writing it above the reaction arrow as a promoter, and their MgSO4-versus-CaSO4 comparison did not control dissolved sulfate concentration, so a solubility artifact could not be excluded. On this basis they proposed that formation-water chemistry, especially the MgSO4 concentration, may be one of the main factors controlling TSR and the massive generation of deep H2S, and inferred — from dolomite-genesis studies rather than from measurements — that the sulfate-rich (magnesic) fluids of these reservoirs may have multiple sources, with upward-migrating deep fluids as one possible supply route. They treated the consumption of H2S by metal-bearing lithologies during migration qualitatively, and stated that the complex causal relationship among H2S abundance, TSR, Mg2+ and reservoir dolomitization requires further in-depth study: these unresolved issues are the problem stated here. Later work supplies the vocabulary for one of these issues: in concentrated Mg-bearing brines, Mg2+ and SO4 form contact ion pairs (directly associated MgSO4 species, CIPs) whose concentration is temperature- and salinity-dependent (Xiaolin Wang and co-workers, 2016, Chemical Geology 435:10–21), and trisulfur radical anion S3− is a reactive intermediate-valence sulfur species that forms from H2S and sulfate at elevated temperature.

Problem Statement

For the sour-gas reservoirs of the Sichuan Basin (paradigmatically the Changxing and Feixianguan formations), establish a quantitative, mechanism-grounded basis for predicting H2S generation and enrichment — elucidating an H2S formation mechanism able to predict H2S contents and their spatial distribution in these reservoirs, testable against the observed H2S contents of the basin's wells, the goal the 2010 synthesis identified. This jointly requires: (1) establishing the molecular-scale role of Mg2+ in TSR — whether magnesium genuinely catalyzes sulfate reduction, enhances reactivity through MgSO4 contact ion pairs, or acts through sulfate solubility and speciation effects — and how that role is reconciled with the H2S/S3−/S0 autocatalytic pathway, including the product distributions of MgSO4- versus CaSO4-sourced systems; (2) establishing whether formation-water chemistry — especially the concentration of dissolved MgSO4 — is one of the main factors controlling TSR and the massive generation of deep H2S, the control hypothesis the synthesis proposed, and how dissolved SO4^2- and Mg2+ speciation, availability and quantitative concentration thresholds at basin temperature–pressure–salinity conditions govern the kinetics of TSR and H2S generation; (3) quantifying the provenance, transport, and temporal availability of soluble sulfate in the reservoir formation waters (relative contributions of connate evaporated seawater — brine trapped in the sediment at deposition — diagenetic or evaporite dissolution, and upward-migrating deep fluids), which the synthesis could only infer; (4) characterizing the preservation factor quantitatively — how much H2S is consumed during migration by reaction with metal-bearing lithologies and other oxidants, and how that consumption shapes final gas compositions and H2S distributions, beyond the qualitative treatment in the synthesis; and (5) resolving the causal sequence and feedbacks linking dolomitization and hydrothermal fluid events to H2S generation, as distinct from a common Mg2+-rich fluid history. These five elements are jointly required — a complete resolution must address all five, and no single element standing alone constitutes a solution — and they jointly support one objective: converting the qualitative four-factor (source, energy, space, preservation) enrichment model into a mechanism-grounded capability for predicting H2S contents and distributions in these reservoirs.

References

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