Earth Sciences Report 2001-06

Author(s) Date 2001-08-31

Most of the bitumen resources in the Athabasca oil sand area are contained in fluvial and estuarine channel deposits of the Lower Cretaceous McMurray Formation. The subsurface of the Clarke Creek area is characterized by data from about 60 wells and 4 high-quality seismic lines. Integration of subsurface data shows that seismic surveys give an accurate three-dimensional picture of the various structures in the subsurface. Identification of bounding surfaces of specific strata and lithofacies associations has identified channel complexes. A channel complex is a lithofacies package defined by major bounding surfaces, with genetically related facies and consistent paleoflow patterns.

Two channel complexes can be recognized in the Upper McMurray Formation at Clarke Creek, both interpreted as estuarine meandering-channel–point-bar complexes. The older channel complex was oriented to the northwest, with prominent low-angle cross-bedding (seen in core and interpreted on dipmeter logs and seismic sections) striking parallel to the main channel axis and dipping variably at 90° to the channel-axis trend. This cross-bedding is interpreted as being due to the lateral migration of point bars within the estuarine complex. A younger channel complex removed much of the older estuarine meandering-channel–point-bar complex. The younger channel complex trends to the north, and has similarly associated lateral-accretion surfaces, due to point bars bordering along the axial trend of the main channel.

The lower part of each channel complex is characterized by medium- to large-scale trough-crossbedded sand units, with high porosity and permeability, representing main channel deposits and containing the highest bitumen grades. Tops of channel complexes are outlined by dipping surfaces of lateral accretion cross-bedding (sandy or muddy, inclined, heterolithic stratification) of estuarine point-bar successions, with reduced permeabilities and containing lower bitumen grades. These channel complexes may represent hydraulically equivalent zones of similar porosity and permeability.

A large percentage of the oil sands are contained in the central, younger channel complex. Results from the present study indicate that channel complexes can be realistically imaged by seismic methods. Recognition of channel complexes could help define thick bitumen deposits in other oil sand areas.

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Langenberg, C.W., Hein, F.J. and Berhane, H. (2003): Three-dimensional geometry of fluvial-estuarine oil sand deposits of the Clarke Creek area (NTS 74D), northeastern Alberta; Alberta Energy and Utilities Board, EUB/AGS Earth Sciences Report 2001-06, 51 p.