oa Formation and Evolution of Thiadiamondoids: Evidence from Thermochemical Sulfate Reduction Simulation Experiments with 1,3-Dimethyladamantane

Thiadiamondoids 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 that lasted from 10 to 160 h were conducted on the model compound 1,3-dimethyladamantane (1,3-DMA) using the CaSO4, MgSO4, and elemental S systems. Four types of non-hydrocarbon compounds with adamantane structures were detected in the liquid products in the three experiment systems: adamantanones, adamantanols, adamantanethiols, and thiaadamantanes. Among these, adamantanones exhibited the highest concentration in the three simulation systems. In addition, the thiaadamantanes were dominated by C3-thiaadamantanes in the CaSO4 and MgSO4 systems and were dominated by C2- thiaadamantanes in the elemental S system. The simulation experiments revealed a strong correlation between the concentrations of thiaadamantanes and adamantanones, suggesting that adamantanones might be the intermediates for thiaadamantanes. The thiaadamantanes exhibited different generation and evolution processes across different experiment systems. Notably, the MgSO4 system revealed that thiaadamantanes undergo generation, enrichment, and destruction processes, corresponding to Easy%Ro values of 0.89%, 0.98%–1.22%, and >1.34%, respectively.