--> Evolution of a Large Tidally Influenced Meandering River System in the Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta, Canada

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Evolution of a Large Tidally Influenced Meandering River System in the Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta, Canada

Abstract

The 3-D reconstruction of meanderbelt deposits from ancient strata can provide significant insight into the long-term (i.e., centuries to millennia) evolution of fluvial systems. A significant challenge to such analyses is limited exposures in outcrop belts and widely spaced or low-resolution perspectives in subsurface datasets. A particularly dense and high quality dataset consisting of 130 km2 of high-quality 3D seismic data and over 350 well penetrations from the Cretaceous McMurray Formation of northeastern Alberta, Canada provides a unique perspective of an immense ancient river system. The high-resolution dataset features large-scale meandering channel elements, associated with paleochannels that were 400–600 m wide and up to 50 m deep. The data reveals evidence for intra-point bar rotation, neck cut-offs, point bar accretion and other morphodynamic processes. The stratigraphic expression of these processes is recognized through creation of a 3-D geocellular model that includes core-calibrated lithologic properties and seismic-constrained surface projections. A specific objective is to characterize the ancient expression of morphodynamic processes that are commonly observed in modern systems, yet rarely described from the rock record. The analysis also provides insight into sandstone and, inherently, reservoir distribution within the ancient meanderbelt. Reconstructed paleochannel migration patterns reveal the evolutionary history of eight individual meanderbelt elements, including point bars, counter point bars, abandoned channel fills, side bars and mid-channel bars, which have been mapped in core, FMI and seismic data and incorporated into the geocellular model. Results of the study show intra-point bar erosion surfaces bound lateral accretion packages characterized by unique accretion directions, internal stratigraphic architecture and lithologic properties. Individual lateral accretion packages fine as they evolve, as do entire point bars. Seismic and FMI characterization also reveals a multi-phase channel abandonment history that includes vertical aggradation, sidebar development and mid-channel bar deposition, which is not typically recognized in ancient meandering river deposits.