Stratigraphic Characteristics and Organization of Tidal-Fluvial Channels in the Athabasca Oil Sands

The Athabasca Oil Sands in NE Alberta hold around 170 billion barrels of recoverable resources, most of which is contained in clastic marginal marine deposits of the McMurray Formation. These marginal marine deposits accumulated in a wide variety of depositional settings from fluvial channels to shallow marine shorefaces and deltas, but the thickest and most productive deposits are frequently found in tidal-fluvial channel systems. Major channels formed during high discharges resulted in average depths of around 30-40 m, widths of 400-700 m and steep width to depth ratios of between 10 and 20:1. Sedimentological, ichnological and biostratigraphic data suggest the channels were formed in a brackish-salinity, tidally-influenced setting. Tidal influence is suggested by paleocurrent data indicating multidirectional flow both upstream and downstream, with syneresis cracks, mud drapes and inclined heterolithic stratification common. A low diversity trace fossil suite and palynological data are suggestive of brackish water conditions. 3D and 4D seismic data suggest a complex architectural arrangement of abandoned channels and large scale (up to 10 km long) stacked point bars. Seismic geomorphology analysis also suggests the presence of tidal bars separating flood and ebb flows, both normal and counter point bar geometries, both upstream and downstream point bar migration, and extensive lateral accretion surfaces up to 35 m high with dip angles of between 5-15 degrees in the direction of migration. McMurray deposits contain extensive mud clast breccias, often occurring at significant boundaries between lateral accretion surfaces or channel bases. Barriers to flow may result from abandoned channel deposits, IHS mudstone beds, channel base muds and mud clast breccias. The McMurray Formation in the eastern Athabasca region accumulated in a low accommodation environment over a number of depositional cycles. This, combined with the large scale channel development in each cycle resulted in pervasive incision and reworking, removing virtually all regional and local correlation surfaces. An understanding of the complex arrangement of lithofacies and architectural elements in the McMurray Formation, as well as the generation of a sequence stratigraphic framework for interpretation and correlation, is essential to successfully producing the large bitumen reservoirs of the McMurray, and minimizing the steam to oil ratio required for production.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California