Stratigraphic Architecture of a Large-scale Point-bar Complex in the McMurray Formation<subtitle>Syncrude’s Mildred Lake Mine, Alberta, Canada</subtitle>

Abstract

Canada's largest bitumen resource is contained within the McMurray Formation, a complex deepening-upward fluvial-estuarine succession typified by abrupt facies changes, inclined stratal geometries, and high-relief unconformities. Within this succession, fluvial-estuarine point-bar reservoirs represent a significant fraction of the resource that can be developed through surface mining and in-situ thermal recovery processes such as steam-assisted gravity drainage (SAGD). Point bars up to 60 m (197 ft) thick and 40 km2 (15 mi2) in area have been imaged in published high-resolution three-dimensional (3-D) seismic time slices at several SAGD developments. The dimensions of associated abandoned channels and channel belts are consistent with discharge through a drainage area the size of the Early Cretaceous Western Interior. At Syncrude Canada Ltd.'s Mildred Lake mine, closely spaced core-hole data are tied to high-wall exposures of a point-bar succession that is 55 m (180 ft) thick and occupies an area of at least 15 km2 (6 mi2). Data are integrated using two 3-D visualization tools: light detection and ranging (LIDAR), a laser technology that produces high-resolution digital terrain models of the outcrop, and LogVu3D, an application that displays large sets of geophysical logs in a 3-D volume. The point-bar model developed here describes sand body dimensions, stratal stacking patterns, lithofacies distributions, and mudstone heterogeneity at a variety of scales. First-order depositional trends reveal a central thick sand-rich ore body, deposited primarily by lateral accretion, and flanking mud-rich bars with fining-upward trends and thinner basal sands. The change from mud-prone to sand-prone deposition is interpreted to be a function of variable discharge and sediment load. Depositional trends in the strike direction are related to the dynamics of stream flow around the meander bend. Erosion of inclined heterolithic strata (IHS) at the high-energy upstream head of the point bar and along the lower reaches of lateral accretion surfaces disrupts mudstone continuity and provides a source of clast deposition that is volumetrically more important than clasts derived from cutbank erosion. Although IHS commonly transition tangentially into basal clast-rich channel sands, significant IHS erosion associated with channel forms oriented subparallel to dip also occurs along the entire length of lateral accretion bedding. In low-energy areas on the downstream end of the point bar and along the upper reaches of lateral accretion surfaces, mudstone beds tend to be continuous, closely spaced, and highly bioturbated. In the point-bar complex, stratal discordances are ubiquitous and indicate that the process of lateral accretion was discontinuous and accompanied by abrupt reorganizations of point-bar growth. The stratigraphy is organized into a hierarchy that emphasizes these discontinuities and subdivides the succession into beds, bedsets, stories, lateral accretion sets, and bars. The geomorphology of modern and exhumed ancient point bars and 3-D seismic time slices of the McMurray Formation point bars provide analogs from which lateral accretion sets and bars defined in high-wall exposures may be expressed in to plan views. Lithofacies are grouped into six genetically related associations that represent members of a continuum interpreted to be a function of changing flow energy, sediment transport, and location on the point bar. These associations, organized within the stratigraphic hierarchy, form the foundation for quantification of reservoir heterogeneity. Industry reservoir performance data indicate that in-situ steam chamber growth, steam-oil ratios, production rates, and recovery efficiencies are particularly sensitive to reservoir heterogeneity in the form of IHS. The IHS mudstone bed thickness, frequency, and length distributions were measured and, after correcting for outcrop bias, are shown to vary systematically. In moderately to highly bioturbated mudstones, beds are typically longer than horizontal well spacing and steam chamber widths and are considered likely seals to steam chamber growth. In contrast, mudstone beds in clast-associated IHS facies are an order of magnitude shorter. Erosion of mudstones on lateral accretion surfaces provides pathways for steam access and bitumen drainage. Clast-associated mudstones constitute baffles to steam rise, and the rate and shape of steam chamber development will be strongly influenced by the distribution of this facies. A conceptual model of steam chamber growth in a heterogeneous point bar is presented that has implications for steam chamber definition, resource assessment, reservoir modeling, and development well planning.