The molecular and sulfur isotope distribution of volatile compounds in natural gases and condensates from Alberta, Canada

Compound-specific S isotope analysis (CSSIA) of volatile organic sulfur compounds (VOSC) in petroleum gases was applied for the first time to two natural gas samples from the Triassic section of the Alberta Basin in Canada. For comparison, CSSIA was conducted for five condensate samples from adjacent wells. The analysis of VOSC in the gas samples revealed similarity in the sulfur isotope composition for thiols, sulfides, and H₂S suggesting an isotopic equilibrium during the reaction between H₂S and hydrocarbons in the reservoir. In contrast, one of the gas samples contained alkyl thiophenes with distinct δ³⁴S values that suggest a different source, probably from oil or bitumen. The molecular and isotopic analysis of condensates revealed several phases of thiol interaction with H₂S bearing similar δ³⁴S value as the H₂S in Gas A and B. Thermally stable OSC such as alkyl-, benzo- and dibenzo- thiophenes of all condensate samples kept their previous δ³⁴S values and not the ones expected in isotopic equilibrium, indicating a recent migration of H₂S to the reservoirs. The δ³⁴S value of the H₂S reacting with the gas and condensate samples is isotopically similar to H₂S which could have generated during complete microbial or thermochemical sulfate reduction of the Charlie Lake anhydrite in the Triassic section, therefore making this process the likely source of the H₂S. This study demonstrates the potential of sulfur isotope analysis of VOSC to interpret origin, migration pathways and provide a relative time frame for in-reservoir processes impacting present day natural gas properties.