Geochemical characterization of selected Western Canada oils by C5–C8 Compound Specific Isotope Correlation (CSIC)

Forty-two oils/condensates from the Western Canada Sedimentary Basin (WCSB) ranging in reservoir age from Devonian to Cretaceous are compared using the stable carbon isotopic ratio of individual hydrocarbons in the gasoline-range fraction (C5–C8) of these oils. Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS), using a modified solid dilution, purge and trap inletting system can provide reproducible isotopic ratios for the ca. 26 dominant n-, iso-, and cyclo-alkanes and aromatics in the gasoline range. The resulting isotopic signatures, or gasoline “isotopograms”, for oils/condensates provide a diagnostic geochemical technique for oil–oil and oil–source correlations. In many instances, the non-straight-chained hydrocarbons are more diagnostic than n-alkanes. Hierarchical cluster analysis provides a non-subjective approach to the treatment of individual isotopic information. This study confirms that oils from similar or the same sources produce isotopic signatures that are highly correlated. Clear carbon isotopic relationships between individual hydrocarbon compounds in the oils suggest a steady-state kinetic relationship for the generation of oil and gas, as opposed to a thermodynamic equilibrium relationship. This mechanism explains the consistent shift in isotopic ratios between similar compounds, such as the methylalkanes and dimethylalkanes.