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%R_o 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.

Thiadiamondoids (TDs) have recently attracted increasing attention as molecular proxies for thermochemical sulfate reduction (TSR) reactions in reservoirs. However, their formation mechanisms, as well as the generation and evolution processes, remain poorly understood. In this study, simulation experiments with a duration of 160 h were conducted on the model compound 1,3-dimethyladamantane (1,3-DMA) using the CaSO₄, MgSO₄, and elemental S systems, with measurements at the 10th, 20th, 40th, 80th and 160th hours during the simulation process being presented. The results indicate that at the end of simulation, the MgSO₄ system exhibited the lowest residual amounts of 1,3-DMA, suggesting the highest degree of TSR. Four types of non-hydrocarbon compounds with adamantane structures were detected in the liquid products in the three experiment systems: adamantanones, adamantanols, adamantanethiols (ATs), and thiaadamantanes (TAs). Among these, adamantanones exhibited the highest concentrations in the three simulation systems. In addition, TAs were dominated by C₃-TAs in the CaSO₄ and MgSO₄ systems and by C₂-TAs in the elemental S system. The simulation experiments revealed a strong correlation between the concentrations of TAs and adamantanones, suggesting that adamantanones might be the intermediates for TAs. Combined with the synthesis mechanism of TAs from thiaadamamantane-4,8-dione, TDs might have two different genetic mechanisms: (a) low temperature cationic carbon ion rearrangement from diagenesis to early catagenesis stage, and (b) a free sulfur radical mechanism in high-temperature TSR process during middle-late catagenesis. TAs exhibited different generation and evolution processes across different experiment systems. Notably, the MgSO₄ system revealed that TAs undergo generation, accumulation, and destruction process, corresponding to Easy%R_o values of 0.89%–0.98%, 0.98%–1.21%, and >1.21%, respectively. Among these three simulation systems, dibenzothiophenes (DBTs) concentrations consistently trended upwards, indicating TAs have lower thermal stability than DBTs.

Source rocks in the Upper Ordovician Qrebake Formation in the Shunbei area of the Tarim Basin have been newly discovered in recent exploration. Investigating their geochemical characteristics holds great significance for future hydrocarbon exploration. Using comprehensive geochemical experiments, analyses, and tests on source rock samples from six wells in the Shunbei area, we explore their organic matter abundance, types, and thermal maturity in the Upper Ordovician Qrebake Formation. The results indicate that the selected source rocks consist primarily of grayish-black mudstones, dark gray calcareous mudstones, and dark gray argillaceous limestones. The grayish-black mudstones, among others, exhibit total organic carbon (TOC) content ranging from 0.20 % to 2.81 % (average: 1.59 %) and Rock-Eval pyrolysis-based hydrocarbon generative potential (S₁+S₂) from 0.43 to 12.05 mg/g (average: 5.59 mg/g). The dark gray calcareous mudstones show TOC content varying from 0.46 % to 2.24 % (average: 1.47 %) and S₁+S₂ from 0.18 to 1.97 mg/g (average: 0.91 mg/g). In contrast, the dark gray argillaceous limestones manifest TOC content ranging from 0.22 % to 3.41 % (average: 1.72 %) and S₁+S₂ from 1.37 to 18.56 mg/g (average: 9.04 mg/g). Notably, wells SB5, SB7, and SB71X present source rocks of the highest organic matter abundance. Source rocks in the Qrebake Formation are formed in a weakly reducing, brackish marine sedimentary environment, with parent materials originating primarily from low aquatic algae. Most samples exhibit hydrogen index (HI) values ranging between 289 and 512 mg/g (average: 416 mg/g), suggesting humic-sapropelic organic matter (Type Ⅱ₁) with high hydrocarbon generative potential. Source rocks from well SB7 present vitrinite equivalent reflectance (R_o,eq) ranging from 0.55 % to 0.88 % (average: 0.72 %), indicating low-to-moderate maturity. In well SB71X, the R_o,eq ranges from 0.53 % to 0.70 % (average: 0.62 %), suggesting low-maturity source rocks. Meanwhile, source rocks from well SB5 display R_o,eq between 0.85 % and 0.95 % (average: 0.90 %) of moderate maturity. Source rocks in the formation measure from 20 to 25 m in thickness, distributed primarily in the intra-platform depressions in the Nos. 5 and 7 fault zones in the Shunbei area.