Sedimentology and stratigraphic architecture of a point bar deposit, Lower Cretaceous McMurray Formation, Alberta, Canada

Abstract

Abstract A single channel and point bar deposit are examined in the subsurface of northeastern Alberta from the Lower Cretaceous McMurray Formation. High-quality 3-D seismic, core and wireline log data were used in order to constrain the stratigraphic architecture and the facies distribution. Mudstone-dominated abandoned channel fill provides a mold of the 32–36 m deep paleo-channel; seismic time slices reveal that these channels were 500–584 m wide. The associated point bar deposit ranges in thickness from 30–40 m. Seven facies are identified in the point bar strata studied, including massive sandstone with siltstone rip-up clasts at the base of the point bar sequence, cross-stratified sandstone that is common in upstream locations on the point bar, various interbedded sandstone and siltstone units that comprise IHS packages, and siltstone. Reservoir quality in the McMurray Formation is directly related to numerous factors, including the overall thickness of bar deposits, and the distribution of siltstone beds that can act as barriers or baffles to fluid flow. The overall thickness of the point bar deposit studied is greatest where increased scour around the apex of the paleo-channel bend resulted in increased accommodation; amalgamated sandstone units are up to 23–30 m thick in these areas. Stratigraphic cross-sections and mapping of lithological trends across the point bar deposit demonstrate that siltstone beds increase in abundance upward through the section, and in downstream reaches of the point bar. The average cumulative thickness of siltstone beds increases from 18 cm to 67 cm in the downstream direction, which corresponds to a decrease in siltstone bed frequency. Small-scale upward-fining packages that range from 5–12 m thick are locally observed within the point bar deposit, and are interpreted to represent erosion and reactivation following storm events or episodes of meander-bend rotation.