Thiadiamondoids as a Molecular Fingerprint of Thermochemical Sulfate Reduction of Hydrocarbons in Deep Petroleum Reservoirs

Zhibin Wei, Stanford University, Department of Geological and Environmental Sciences, Stanford, California, [email protected]

Thermochemical sulfate reduction (TSR) takes place in some hot petroleum reservoirs, resulting in hydrocarbon destruction, accumulation of H₂S and CO₂, and generation of organic sulfur compounds and S⁰, which is accompanied by dissolution of sulfate minerals and partial replacement by calcite. Thus, TSR reactions play an important role in oil destruction and oil deadline predictions in an oil-bearing sedimentary basin. Despite the availability of some clues of TSR occurrence provided by existing tools such as petrographic, fluid inclusion and isotopic data, none of them alone provides the ability to identify the existence of TSR. This study is thus to diagnose the TSR process in deep petroleum reservoirs using thiadiamondoids as a molecular fingerprint and to quantify the TSR oxidation of petroleum hydrocarbons by thiadiamondoid analysis. Thiadiamondoids were detected in a series of oils and condensates with a broad maturity range from Smackover Formation, Gulf of Mexico, using SIR-GCMS and MRM-GCMS. Our results show that the concentrations of thiadiamondoids coincidently increase with the extent of the TSR process, suggesting that thiadiamondoids can be used as a unique indicator for TSR occurrence. Therefore, the investigation of thiadiamondoids regarding their formation mechanisms and their applications to predicting the occurrence and extent of TSR may allow for the better understanding of secondary oil-cracking as well as making reasonable decisions in petroleum exploration and development. Case studies of thiadiamondoids in the Eastern Saudi Arabia and Western Canada Sedimentary Basin have demonstrated their potential as a more specific proxy for TSR occurring in deep petroleum reservoirs.