--> Integration of Elemental and Stable Isotope Chemostratigraphy to Characterize Paleoenvironmental Diachroneity in the Duvernay Formation (Mid-Frasnian), a Mixed Carbonate-Siliciclastic Unconventional Play in the Western Canada Basin

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Integration of Elemental and Stable Isotope Chemostratigraphy to Characterize Paleoenvironmental Diachroneity in the Duvernay Formation (Mid-Frasnian), a Mixed Carbonate-Siliciclastic Unconventional Play in the Western Canada Basin

Abstract

Elemental geochemistry is widely used to characterize the depositional history of unconventional plays. Although this method can inform chronostratigraphic interpretations, there are risks involved in using the same dataset for paleoenvironmental reconstruction as well. Diachronous facies cannot be fully recognized with elemental geochemistry alone; it must integrated with other methodologies, such as stable carbon isotope chemostratigraphy. Diagnostic signals recorded by δ13C curves in marine sedimentary rocks are well documented to be isochronous, facies independent, and highly resistant to late diagenetic alteration, making this a powerful tool for chronostratigraphic correlation. Elemental and stable isotope chemostratigraphy are integrated herein to investigate paleoenvironmental diachroneity in the Duvernay Formation, a mid-Frasnian carbonate-rich shale play in the Western Canada Basin. Twelve wells are included in the study: eight from the Kaybob area and four from Willesden Green. During the mid-Frasnian, these were partially isolated sub-basins, further compartmentalized by local reef build-ups. Changes in paleoredox have been modelled between sub-basins using a multi-trace element approach, providing a framework for predicting the spatial and temporal distribution of preserved organic matter across the play. The timing of anoxia can be calibrated by a series of diagnostic δ13Corg excursions, which are correlated with a high degree of confidence between wells. These fluctuations, which are interpreted as the mid-Frasnian “punctata Excursion” constrain the Duvernay Formation in the study wells to the transitans, punctata, and hassi conodont Biozones. Furthermore, they demonstrate that development of anoxia is not synchronous between the Kaybob and Willesden Green, or even between closely spaced wells. This fine-scale diachroneity occurs below the resolution of traditional biostratigraphic methods, underscoring to the applicability of stable carbon isotopes for chronostratigraphic correlation in unconventional plays.