Formation and evolution of thiadiamondoids in petroleum: Evidence from thermochemical sulfate reduction simulation experiments with 1,3-dimethyladamantane

Abstract

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.