Sequence Stratigraphic and Depositional Facies Framework of the Lower Cretaceous McMurray Formation, Kearl Oil Sands Project, Alberta

Thomas Nardin^1,*, Joan Carter², Bob Falls², John Irish², R. Stancliffe², Bogdan Varban², Nancy Wilson², Lidia Zabcic², and Lynn Pratt^3,*
1T.R. Nardin Geoscience Consulting, Calgary, AB
²Imperial Oil Resources Oil Sands Development and Research, Calgary, AB
³Lateral Development Corporation, Calgary, AB
*Under Contract with Imperial Oil Resources Oil Sands Development and Research

The Lower Cretaceous McMurray Formation in the Athabasca area of northern Alberta represents Canada's most important oil sands resource with an estimated 800 billion bbl in place. Approximately 15% of this resource can be exploited by surface mining. Imperial Oil and ExxonMobil have obtained regulatory approval to develop 4.4 billion bbl of bitumen at 300,000 bpd over a 50 year period at their Kearl Mine.

By integrating well, core and seismic data, a sequence stratigraphic framework was constructed for the McMurray Formation at Kearl to establish the basis for reservoir models that are essential for resource assessment, ore grade prediction, mine planning and pit design. The Kearl study was based on 840 wells (75% cored) with an average spacing of 600 m and 43 km of high-quality 2D seismic. These data were integrated with previous stratigraphy and facies studies at Syncrude’s North Mine and from outcrop to identify regionally significant sequence boundaries, establish facies recognition criteria, interpret depositional environments and determine reservoir dimensions and distributions.

Regionally, the McMurray Formation is a deepening-upward fluvial-estuarine complex deposited within an Early Cretaceous paleo-valley system which is controlled by the structure on the Sub-Cretaceous unconformity. The main valley trended north-northwest toward the Boreal Sea. Tributary systems are evident in isopachs along the flanks of the Athabasca Anticline to the west and the Pre-Cambrian shield to the east. Kearl is optimally located near the confluence of this eastern tributary with the main valley resulting in high-quality reservoirs comprised of thick deposits of quartz-rich, coarse sands.

In the northern Athabasca area, the McMurray Formation is characterized by abrupt lithofacies changes, inclined stratal geometries and high-relief unconformities. Facies are commonly preserved as erosional / depositional remnants. This stratigraphic complexity can be organized within a sequence framework in which four major sequences are recognized (Figure 1). The oldest sequence (I) is dominated by thick amalgamated braided stream deposits overlain by a shoaling-upward succession of lacustrine muds, paleosols and coals. Sequence II is a thick, heterolithic assemblage of fluvial, floodplain, and estuarine deposits. Sequence III is distinguished by large-scale fluvial-estuarine point bars that are up to 70 m thick and represent the primary reservoirs at Kearl. Bayhead delta muds contained within deeply incised valleys typify Sequence IV. Pleistocene tills, sands and gravels directly overlie the McMurray over most of the area but where the Wabiskaw Member (“D” facies) of the Lower Cretaceous, Clearwater Formation is preserved it unconformably overlies the McMurray Formation.

The results of this study have provided an improved understanding of McMurray depositional systems that is utilized for reservoir modeling, resource characterization and assessment, mine pit and tailings pond design, drilling programs, ore processing and substrate stability evaluations. It has also provided a conceptual model and analog that can be used to guide interpretation where subsurface control is sparse.

Figure 1. The diagrammatic cross section illustrates the sequence stratigraphic architecture for the McMurray Formation in the northern Athabasca and provides a conceptual model that can be used to guide interpretation, particularly where subsurface control is sparse.

AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada