@article{MA2026, 
author = {Anlai MA and Yan BAO and Zhong WANG},
title = {Experimental simulation of thermochemical sulfate reduction in different sulfur-bearing systems using crude oil from well Fengshen-1, Dongying Sag, Bohai Bay Basin},
year = {2026},
journal = {Oil & Gas Geology},
volume = {47},
number = {3},
pages = {1018-1032},
keywords = {n-alkane, diamondoid, thiadiamondoid, dibenzothiophene, thermochemical sulfate reduction, molecular geochemistry, Dongying Sag, Bohai Bay Basin},
url = {https://www.sciopen.com/article/10.11743/ogg20260320},
doi = {10.11743/ogg20260320},
abstract = {Thermochemical sulfate reduction (TSR) is a common secondary alteration occurring in deep carbonate reservoirs. Earlier TSR simulation experiments have focused on reaction kinetics, gaseous product compositions, carbon isotopes of individual gas components, and sulfur isotope fractionation in H2S, but the formation and evolutionary patterns of TSR-sensitive molecular markers remain poorly understood. To address this gap, a condensate sample from well Fengshen-1 in the Minfeng subsag of the Dongying Sag, Bohai Bay Basin, was selected as the study sample. Closed-system simulation experiments were conducted for 80 h under three sulfur-bearing systems: CaSO4, MgSO4, and elemental sulfur. Quantitative analyses of gas composition, compound-specific carbon isotopes of gas components, and liquid-phase products were then used to systematically characterize the alteration effects of TSR on liquid hydrocarbon molecules. As indicated by the experimental results, all three systems generated both hydrocarbon and non-hydrocarbon gases, and H2S yields increased in the order of CaSO4 system < MgSO4 system < elemental sulfur system. Methane carbon-isotope fractionation followed the sequence CaSO4 system < elemental sulfur system < MgSO4 system, whereas CO2 carbon isotopes showed the opposite trend, following the order of CaSO4 system > elemental sulfur system > MgSO4 system. Across all three systems, n-alkane concentrations in the liquid products decreased by 42% to 57%, with Pr/nC17 and Ph/nC18 ratios also decreasing markedly, indicating that isoprenoid alkanes degraded faster than n-alkanes. At the same time, aromatic hydrocarbon concentrations, alkyl dibenzothiophene (DBT) series concentrations, and dibenzothiophene/phenanthrene ratios all increased markedly, suggesting sulfur radical-mediated aromatization reactions. Diamondoid concentrations and adamantane maturity indices also increased substantially, and a complete C0-C3 thiadiamondoid series was first detected at Easy%Ro of 1.21%, confirming the progressive enrichment of thiadiamondoids via sulfur-radical addition. On this basis, this study establishes an evolutionary model for TSR-sensitive molecular markers, demonstrating that the stability of the pristane/phytane (Pr/Ph) ratio, coupled with the contrast in degradation rates between n-alkanes and isoprenoid alkanes, serve as two indicators for evaluating TSR alteration intensity. It also shows that the staged generation of thiadiamondoids can serve as a new proxy for evaluating TSR intensity, while isotopic fractionation differences among the three sulfur-bearing systems provide critical geochemical evidence for tracing TSR reaction pathways in hydrocarbon reservoirs. These findings have significant implications for identifying TSR-induced secondary alteration processes in deep and ultra-deep carbonate reservoirs at the molecular geochemical level.}
}