Facies Analysis and Architectural Elements Within a Fluvio-Estuarine Sedimentary System: The Lower Cretaceous Bituminous Sandstones of the MacMurray Formation, Alberta (Canada)

摘要

In north-eastern Alberta (Canada), the fluvioestuarine McMurray Formation constitutes the main bitumen accumulation of the Athabasca Heavy Oil Province. Deposited within the fluvial-to-marine transition zone, the McMurray clastic succession is characterized by the lateral and vertical juxtaposition of a wide variety of depositional processes. This led to complex lithofacies variations responsible for uncertainties in predicting reservoir heterogeneities. In this context, a realistic paleogeographic reconstitution is necessary to localize sandy reservoirs and optimize the exploitation of bituminous sandstones. The sedimentological study of the McMurray reservoir, based on outcrops and subsurface data (cores, well logs, seismic profiles), has been undertaken along a 150 km north (distal) - south (proximal) transect. Facies analysis permits to constrain depositional environments within the McMurray Formation. We document in detail facies associations corresponding to continental (fluvial/palustral/paleosols facies), estuarine (interaction between tide and fluvial processes) and marine environments (wave-tide to wave dominated processes). A particular attention has been paid to the nature, size, geometry and bounding surfaces of architectural elements (I.e. fluvial or estuarine channels, HIS/point-bars, tidal bars, incised valley, emerged surfaces). Stratigraphic correlations, combined with cartographic analysis, allow to evaluate how these sedimentary bodies are related in time and space. The McMurray Formation has been previously sub-divided into three units: the fluvial dominated lower McMurray, the fluvioestuarine middle McMurray and the marine dominated upper McMurray. We notably demonstrate, on the base of size and amplitude of HIS, that the middle McMurray may be subdivided into stratigraphic sub-units.We also point out the significance of oxidized surfaces overlain by abrupt grain size changes. Such surfaces are important clue for sequential analysis and may have a regional extent. They should be taken into account as a potential accurate correlative tool for the McMurray Formation.