CYCLIC SEDIMENTARY RECORD IN POINT BAR DEPOSITS, CRETACEOUS McMurray FORMATION, ALBERTA BASIN, CANADA

Evidence for cyclic sedimentation is common in ancient deposits attributed to numerous depositional settings. However, record of seasonal or climate induced cyclicity is less expected in fluvial deposits where erosion commonly eradicates significant portions of the stratigraphic record. In the subsurface Cretaceous 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 account for a significant proportion of the reservoir. The dimensions of the channels examined, approximately 30 m deep and 500 m wide, are well constrained by 3-D seismic data, and cores and wireline logs from wells spaced 400 m apart on average (data from > 100 wells studied). Individual point bars cover >10 km², and are dominated by sandstone or interbedded sandstone and siltstone. Statistical analyses of wireline log curves, using Fourier transforms and correlograms, demonstrates variable cyclicities with wavelengths of 1-4 m in the spectra. Cycles of < 2 m are attributed to seasonal variations in point bar sedimentation, while thicker cycles are interpreted to record inter-annual climatic variations (e.g., El-Niño Southern Oscillations) with periods of 7 years or less, or depositional fluctuation related to celestial sunspot cycles.

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.