Heteroatomic Cage Compounds as Molecular Probes For Thermochemical Sulfate Reduction (TSR) In Hot Petroleum Reservoirs

Wei, Zhibin¹, J. Michael Moldowan¹, Fred Fago¹, Jeremy Dahl¹, Kenneth E. Peters², Peter D. Jenden³, Martin Fowler⁴ (1) Stanford University, Stanford, CA (2) U.S. Geological Survey, Menlo Park, CA (3) Saudi Arabian Oil Company, Dhahran, Saudi Arabia (4) Geological Survey of Canada, Calgary, AB

Thermochemical sulfate reduction (TSR) is an abiogenic process in which hydrocarbons are oxidized by inorganic sulfate in deep, hot petroleum reservoirs. TSR destroys hydrocarbons, but also generates large quantities of organic sulfur compounds, hydrogen sulfide (H2S), carbon dioxide, and elemental sulfur. It can severely diminish the amounts of producible hydrocarbons and increase production and processing costs. It is therefore important to develop improved methods to detect the occurrence of TSR and predict the abundance and distribution of produced H2S. Our recent work on heteroatomic diamondoids (thiadiamondoids and diamondoidthiols) suggests that these thia- and thiol-cage compounds represent unique molecular probes to recognize and quantify the extent of TSR in oil or condensate samples. We studied suites of oil and condensate samples generated from the Oxfordian Smackover Formation source rock in the Gulf of Mexico having a broad range of thermal maturity and degree of TSR alteration. Case studies were also carried out using these heteroatomic cage compounds to geochemically characterize TSR in Eastern Saudi Arabia and the Western Canada Sedimentary Basin. The present study confirms that heteroatomic cage compounds originate by sulfurization of polycyclic hydrocarbons during TSR. The data show that these cage compounds can be used to improve understanding of secondary oil-cracking and thus reduce risk in petroleum exploration and development. Accurate prediction of the extent of TSR helps optimize drilling decisions in regions where TSR is important.