--> The Application of δ13Corg Stable Isotope Chronostratigraphy to Correlate Upper Palaeozoic Black Shales of the Northwestern Frontier, Yukon Territory, Canada

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The Application of δ13Corg Stable Isotope Chronostratigraphy to Correlate Upper Palaeozoic Black Shales of the Northwestern Frontier, Yukon Territory, Canada

Abstract

Upper Devonian to Mississippian black shales of the Western Canada Sedimentary basin form an important and prolific unconventional hydrocarbon target. Recent resource assessments of Horn River Group shales in the Horn River basin (78 Tcf), Cordova Embayment (67 Tcf), Liard basin (due early 2016) and Mackenzie Plain (191 Bbl) highlight the potential for extending this play into northwest Canada. In Yukon, Upper Palaeozoic organic-rich shales are ubiquitous northeast of the Tintina trench where they represent deposition in a hydrographically restricted series of sub-basins offshore of ancestral North America. Dating these Yukon shales using biostratigraphic techniques is difficult due to high thermal maturities well within the dry gas window that have degraded sparse microfossils beyond recognition. Although conodont biozones used for correlation in Northwest Territories and Alberta suggest Canol-Muskwa-Duvernay equivalency, this relationship remains to be tested in Yukon. The inability to differentiate stacked, siliceous black shale units in outcrop also hampers their utility as field analogues for the subsurface. A solution to this problem was to use organic carbon isotope (δ13Corg) analysis of outcrop samples and well cuttings to establish a chronostratigraphic framework via a conodont biozonation scheme. Positive δ13Corg excursions characteristic of the punctata and hassi conodont zones in the Canol Formation of north Yukon suggest a Givetian-Frasnian age and correlation in-part to Alberta's Duvernay Formation. In Liard basin in southeast Yukon, the base of the punctata excursion occurs within the middle of the Evie Formation (Horn River Group), whereas the Muskwa Formation correlates to only the uppermost Canol Formation of north Yukon. The data also highlight that outcrops mapped as Canol Formation in north Yukon contain a major (+4 to 5%) positive shift interpreted as the latest Devonian ‘Hangenberg’ Excursion, and are assignable to the Ford Lake Formation in the region. This younger δ13Corg signature was used to correlate the Ford Lake shales with the lower Exshaw Formation in Liard basin. Finally, conodont biozones were used to correlate existing geochemical data from Yukon shales to Upper Palaeozoic eustatic sequences in an attempt to identify allogenic controls on organic-rich shale deposition. In a frontier region where well coverage is sparse and of vintage, this depositional framework will be critical for guiding future shale exploration in Yukon.