Stratigraphic architecture of fluvial deposits of the Cretaceous McMurray Formation, Athabasca oil sands, Alberta, Canada

Abstract

In-situ thermal recovery of bitumen requires a detailed understanding of the subsurface stratal architecture of oil-sands deposits. In the present study, 455 well-logs and 10 drilled cores acquired at Christina Lake (Township 76–77, Range 6W4), northeastern Alberta, Canada are examined to reveal the stratigraphic architecture of the oil-sands deposits of the Early Cretaceous McMurray Formation, focusing on the lower fluvial succession overlying unconformably Paleozoic limestone. The lower McMurray succession comprises 3 major lithologic units of thick sand, interbedded sand/mud, and thick mud, which represent main fluvial channels, proximal floodplains with crevasse splays close to the main channels, and distal floodplains, respectively. Spatial correlation of well-log data shows the stratigraphic architecture of the succession and the paleotopography of the underlying unconformity surface. Thick channel sand bodies forming good bitumen reservoirs occur mainly along the axis of a paleovalley with minor intercalation of fine-grained floodplain deposits, which is ascribed to frequent interconnection of channel bodies resulting from slow increase of accommodation space during lowstand and the limitation of channel locations to the narrow axial part of the paleovalley during the early stage of valley-filling. The channel bodies tend to decrease in thickness and proportion toward the valley margin and upsection at the expanse of increase in proportion of fine-grained deposits. This overall stratigraphic architecture is in agreement with the sequence stratigraphic model. The succession, however, locally displays significantly different architectures from the model, controlled by the paleotopography. The recessed areas between topographic highs on the valley margins are dominated by fine-grained floodplain deposits, because those areas were not suitable for being occupied by channels.