Cyclic Sedimentary Record in Point Bar Deposits, Cretaceous McMurray Formation, Alberta Basin, Canada
University of Calgary, Department of Geoscience Calgary, Alberta, Canada
Evidence for cyclic sedimentation is common in ancient deposits, however, record of climate induced cyclicity is less expected in fluvial deposits where erosion eradicates significant portions of the stratigraphic record. In the McMurray Formation of Alberta, Canada, cyclical sedimentation is recorded by alternating mudstone and sandstone beds attributed to fluvial sediments deposited under tidal influence.
The McMurray Formation is the most volumetrically important bitumen-bearing unit of the Athabasca Oil Sands, and point bar deposits dominated by interbedded sandstone and siltstone account for a significant proportion of the reservoir. The dimensions of the channels examined are well constrained by 3-D seismic data, cores, and wireline logs. Statistical analyses of wireline log curves using Fourier transforms demonstrate variable cyclicities with wavelengths of 1-4 m in the spectra. Cycles of < 2 m are attributed to seasonal variations in sedimentation, while thicker cycles are interpreted to record depositional fluctuations related to inter-annual climatic variations or celestial sunspot cycles with periods of 7 years or less.
Depositional cyclicity is most pronounced in the muddy portions of point bars where current energy and erosion are interpreted to have been lowest: (1) distal, downstream portions of point bars, including counter point bars, and (2) the youngest scrolls of point bars, which were commonly associated with the early phase of channel abandonment. Sedimentological heterogeneity within the McMurray Formation significantly impacts heavy oil development from the point bar deposits. Thus, validating the strength and location of depositional cyclicity provides a measure of predictability in a complex reservoir system.
AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid