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High Resolution Reservoir Characterisation of Marginal-Marine Heavy Oil Deposits: Example of the Cretaceous Upper Mannville Formation of East-Central Alberta, Canada


A detailed understanding of reservoir architecture is a critical component of enhanced recovery techniques. In East-Central Alberta, the Cretaceous oil sands contain one of the greatest accumulations of heavy oil and bitumen in the world. The 80-100 m thick Grand Rapids Formation is of particular interest as it is easily accessible (300-400 m below the surface) and hosts significant volume of crude bitumen trapped in a succession of thin deltaic to marine coarsening-up shoreface parasequences incised by marine-influenced channels, typically 100 to 2,000 m wide and 3 to 30 m deep. The stratigraphic relationship between the “background” regional parasequences and the channel fills is critical to establish as the sandstones of each type have different reservoir properties, the latter being the best reservoir in terms of permeability and oil saturation. Intense drilling activity over the last 60 yrs has provided an unparalleled data base that can be used to better understand the 3D distribution of these deposits. Although the general sandbody distribution is relatively well-established, the sandstone bodies have numerous internal heterogeneities which result in increased reservoir complexity. This study is based on the detailed analysis of ∼1100 wells and 55 cores, with a cumulated length of ∼1700 m, collected from a ∼400 km2 area located in the Cold Lake region. The extremely dense dataset offers a high-resolution image of the subsurface Geology and allows for the construction of a very high resolution sequence stratigraphic framework. The range of sedimentary facies, recognised from sedimentological and ichnological analysis of cores, are part of two depositional environments: marine-influenced fluvial and marginal-marine. In both environment, the unconsolidated nature of reservoir sandstones leads to an average porosity of 34-38% throughout the study area with permeability values ranges from 500 to 4500 mD depending on the facies. 3D mapping of sedimentary facies demonstrates that large lateral variations in reservoir properties are present within the principal reservoirs. These variations and associated heterogeneities are likely overlooked in classical interpretations, but are consistent with existing production data. These depositional reconstructions and reservoir characterizations provide important insights into meso-scale reservoir complexities and can be used to improve reservoir models of marginal marine reservoir successions.