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TitleThe molecular and sulfur isotope distribution of volatile compounds in natural gases and condensates from Alberta, Canada
AuthorKutuzov, I; Said-Ahmad, W; Turich, C; Jiang, CORCID logo; Luu, N; Jacksier, T; Amrani, A
SourceOrganic Geochemistry vol. 151, 104129, 2020 p. 1-14,
Alt SeriesNatural Resources Canada, Contribution Series 20200443
Mediapaper; on-line; digital
File formatpdf; html
Lat/Long WENS-120.0000 -118.5000 55.6667 54.8333
Subjectsfossil fuels; Science and Technology; gas
Illustrationslocation maps; tables; diagrams; cross-sections; graphs
ProgramGeoscience for New Energy Supply (GNES) Canadian Energy Geoscience Innovation Cluster (CEGIC)
Released2020 09 29
AbstractRoutine chemical analysis of natural gas generally focuses on composition of gaseous hydrocarbons using molecular ratios of alkanes and their isotopic properties such as stable carbon isotopes (delta-13C) and hydrogen isotopes (delta-2H) to determine origin and correlation of gases. Analysis of volatile organic sulfur compounds (VOSC) in natural gas may potentially provide additional information. A newly developed method for compound-specific S isotope analysis (CSSIA) of VOSC in petroleum gases was applied for the first time to two natural gas samples (Gases A and B) from Triassic section of the Alberta Basin in Canada. For comparison, five condensate samples from adjacent wells were also analyzed for their molecular and sulfur isotopic compositions of VOSC and polyaromatic compounds such as benzo- and dibenzo- thiophenes (BTs and DBTs). The gases are of thermogenic origin, with CO2/H2S ratios of 0.03 to 0.08 and H2S with delta-34S values of +15.6 to +16.2 permille. The similarity in the sulfur isotope values for the thiols (~ +19.5 permille), sulfides (~ +21 permille), and H2S, suggest an isotopic equilibrium during the reaction between H2S and alkanes in the reservoir. In contrast, one of the gas samples contained alkyl thiophenes with distinct delta-34S values (+26.3 to +27.5 permille) that suggests interactions with residual source petroleum (oil) which was previously affected by TSR. The distinct delta-34S values of these thiophenes from the other, less thermally stable VOSC (i.e. thiols) suggest non-equilibrium and a relatively recent migration of H2S to the reservoir. The delta-34S values of the alkyl thiophenes were similar to the anhydrites of the Doig formation therefore making it the likely source of sulfate for the TSR. The molecular and isotopic analysis of condensates revealed large variety of VOSC and heavier OSC up to methyl-DBT (MDBT) which covered a range of delta-34S values from +10.6 to +23.1 permille. The isotopic values of VOSC in the condensates revealed conflicting delta-34S trends which allowed identification of three distinct condensate groups (I, II and III) undergoing several phases of thiol interaction with H2S. Due to their low thermal stability thiols were quick to react with H2S in the reservoirs: thiols of group I are in isotopic equilibrium with H2S bearing the same delta-34S as the H2S in Gas A and B. Thiols of group III are close to isotopic equilibrium with isotopically identical H2S while thiols of group II have only began reacting with isotopically identical H2S and are far from isotopic equilibrium. Thermally stable OSC such as alkyl thiophenes, BTs and DBTs of all samples kept their previous delta-34S values and not the ones expected in isotopic equilibrium, indicating that the migration of H2S to the condensate reservoirs is recent. The delta-34S of the H2S reacting with the gas and condensates samples is isotopically similar to H2S which was generated during sulfate reduction of the Charlie Lake anhydrite in the Triassic section, therefore making this process the likely source of the H2S. The occurrence of isotopically similar H2S over large area of the studied basin suggests the sulfate reduction of the Charlie Lake anhydrite is a regional phenomenon which could be traced throughout the basin to identify gas migration and in-reservoir processes. The results of this study demonstrate 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.
Summary(Plain Language Summary, not published)
Compound specific sulfur isotope analysis (CSSIA) was for the first time applied to the volatile sulfur-containing organic compounds in natural gas and condensate samples. The results were used to establish the genetic relationship among the gases and condensates, the relationship of the organosulfur compounds to hydrogen sulfide in the reservoir, and the timing of fluid migration and accumulation into the reservoir.

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