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Tidal-Fluvial Sedimentology and Stratigraphy of the McMurray Formation in the Surmont Area of the Athabasca Oil Sands


Channels of the Aptian-Albian McMurray Formation carried the discharge of a continental-scale drainage system flowing northwards toward a tidally-influenced embayment of the Boreal Sea. These channels incised valleys greater than 40 km wide and filled them with tidal-fluvial sediments over 80 m thick. Deposits resulting from meandering channel migration show a complex architectural arrangement of abandoned channels and large-scale stacked point bars. Preserved channels show average depths of 34-51 m, widths of 550-1050 m and width to depth ratios of between 12 and 30:1. Seismic geomorphology slices near the top of the McMurray Formation reveal sinuous meandering channels that have undergone translation, expansion, rotation and oscillation. Depositional style in the later channel belts includes point bars that migrate laterally and downstream, counter point bars that dip away from the concave bank, side bars that result from reverse channel migration, and in-channel bars. Internal point bar geometries show extensive lateral accretion surfaces up to 35 m high with dip angles of 5 to 15 degrees in the direction of migration. Complex channel migration patterns result in partial preservation of earlier bar deposits of the same channel belt, and variable styles of facies connectivity ensue from the path of the channel bend migration, depth of channel scour, and temporal change in discharge history. An understanding of the complex organization of subsurface properties and architectural elements in the McMurray Formation is essential to successfully managing hydrocarbon recovery by steam-assisted gravity drainage (SAGD). Hybrid geocellular models provide a technique for capturing a detailed 3D characterization of reservoir architecture by subdividing the McMurray stratigraphy with a series of correlated surfaces that result from channel base migration. The resulting framework yields a stratigraphically-constrained grid whose cells are populated stochastically with geological properties and calibrated to an extensive dataset from delineation boreholes. Production data and time-lapse seismic volumes help calibrate successful numerical simulations of SAGD from these reservoir models.