Impact of the Prairie Evaporite Dissolution Collapse on McMurray Stratigraphy and Depositional Patterns, Shell Albian Sands Lease 13, Northeast Alberta

Abstract The Cretaceous McMurray Formation in NE Alberta contains nearly a trillion barrels of bitumen, a significant portion of which is being developed via surface mining and in-situ thermal methods. The focus of this report is the structure and stratigraphy of the Cretaceous McMurray Formation and its relationship to the configuration of the underlying Devonian section in the area of Shell Canada’s Albian Sands Lease (Township 95, ranges 9 and R10W4M), a joint oil sands mining venture between Shell Canada (60%), Chevron Canada Limited (20%) and Marathon Oil Canada Corporation (20%). The structural and stratigraphic relationships between the two intervals has been the source of several recent investigations due to industry related incidents that demonstrated the integrity of the underlying Devonian succession can be compromised by vertical pathways associated with faults, sinkholes, or other features. Key findings of this work include the following: The present morphology of the Pre-Cretaceous unconformity is primarily due to structural deformation (differential subsidence) related to dissolution and collapse of the underlying Prairie Evaporite Formation and overlying Devonian units of the Beaverhill Lake Group rather than erosion relief. Two types of collapse structures are recognized: a) large scale sag folds that are 1-to-10 kilometres in extent; and b) small scale breccia pipes that are 10- to-100 metres in diameter. The sag folds are interpreted to have formed in response to the dissolution of halite. The breccia pipes, which postdate the sag folds, represent sinkhole features that formed in response to the dissolution of gypsum. Rather than onlapping the unconformity, Lower McMurray strata thin and converge across structural highs and thicken and diverge across structural lows, demonstrating much of the dissolution driven subsidence was contemporaneous with the deposition. The subsidence produced up to 80 metres of accommodation and involved the coherent sagging and faulting of large intact segments of the underlying Devonian section. The arrangement of major stratigraphic packages indicates subsidence features shifted locations through time rather than persisting through the entire Lower McMurray. Changes in sedimentation style between sand-rich fluvial packages to mud-rich lacustrine dominated packages within the lower McMurray reflect changes in relative rates of subsidence. Sand-rich fluvial units are localized in settings with low-to-moderate subsidence rates, while mud-rich fluvial-lacustrine units are localized in settings with moderate-to-high subsidence rates. The Middle-to-Upper McMurray section is composed of four high relief unconformity bound units that display flat/horizontal stratal relationships with the underlying structure of the Pre Cretaceous unconformity. In contrast to the Lower McMurray, stratigraphic relationships indicate it was largely impacted by falls and rises in relative sea-level (cycles of negative and positive accommodation) rather than dissolution driven subsidence that, while still active, had begun to wane.