RHENIUM-OSMIUM SYSTEMATICS OF THE DUVERNAY PETROLEUM SYSTEM, ALBERTA, CANADA: IMPLICATIONS FOR RHENIUM-OSMIUM SYSTEMATICS OF PETROLEUM

The organic-rich intervals within the late Devonian Duvernay Fm are amongst the most prolific petroleum source rocks and are proven as one of the main sources of conventional, and more increasingly unconventional hydrocarbons produced in Western Canada. The Duvernay strata are considered to have undergone main hydrocarbon generation during the Late Cretaceous / Early Eocene Laramide Orogeny. The generated crude oils have biomarker signatures typical for hydrocarbons derived from Type II marine organic matter, including distribution of normal alkanes showing decrease in amounts with increasing carbon number, pristane/phytane ratios greater than 1.0 (commonly around 1.5), smooth homohopane profile, C29>C27>C28 regular sterane distribution and a predominance of diasteranes over regular steranes. The oils display a wide range of thermal maturities and have been classified into several oil sub-families based on maturity and compositional characteristics indicating some localized differences in source organic facies.

Herein we apply the rhenium-osmium (Re-Os) isotope system to establish the absolute generation age of Duvernay Fm sourced oils, as well as to investigate the relationship of the source rock ¹⁸⁷Os/¹⁸⁸Os composition at the time of oil generation (Os_g) with that of the initial ¹⁸⁷Os/¹⁸⁸Os composition of the oil (Os_i).

All oil samples analysed during the present study are from two Devonian reservoirs, Leduc and Nisku. The asphaltene contents of these Duvernay-generated oils are very low (≤1%). The abundances of Re and Os in the asphaltene fractions are low (Re 0.1-1ppb; Os ~10ppt, respectively). Although the Os isotope composition measurements are challenging, the Re-Os isotope data nominally yield an Early Palaeocene (Danian) age. Including uncertainty the Re-Os age spans the Late Cretaceous (Campanian) to Early Eocene (Ypresian), which overlaps with the duration of the Laramide Orogeny, and thus the proposed timing of oil generation of burial models. The Os_i of oils ranges from 0.68 to 0.85 with an average of 0.74. These values are similar to the Os_g of Duvernay Fm. The highly similar Os_i and Os_g of the oil and source rock support the inference of previous studies that the ¹⁸⁷Os/¹⁸⁸Os of an oil is inherited from its source rock, and that the Re-Os systematics of the oil yield the age of oil generation.