Year of Publication: 2007
Abstract:
The understanding of the bedrock topography, buried bedrock valleys and channels, drift thickness and glacial aquifers in the surface-mineable and in situ-recoverable oil sands north of Fort McMurray has been updated by the acquisition and interpretation of more than 35 000 new borehole logs from the oil sands industry. Interpretations of these new data enable the construction of a three-dimensional model of the bedrock topography and subcrop, as well as the major buried aquifers contained within buried valleys and channels. From this model, a series of maps and cross-sections has been generated depicting the subsurface distribution of previously known and newly discovered buried aquifers that underlie the oil sands operations in the region.
Numerous buried fluvial erosional features have been mapped on the bedrock surface, either as bedrock valleys formed prior to the last glaciation, or as bedrock channels formed by glacial meltwater. Names have been assigned to the major valleys and channels to facilitate common understanding and discussion between industry, government and research institutions. Many of the buried channels exhibit features indicative of erosion by subglacial meltwater under a significant hydraulic head. These channels, referred to as tunnel channels, are commonly narrow, deeply entrenched, discontinuous to anastomosing and unconstrained by the topography of the pre-glacial landscape. Subsequent deposition of glacial sediment has effectively masked any surface expression of the buried valleys and channels on the modern landscape. As a consequence, and given their narrow form and discontinuous nature, many channels fall between regional oil sand resource-evaluation boreholes and remain undetected following initial exploration drilling. Mapping of the bedrock topography and buried drift aquifers also has been complicated by glaciotectonism, which has disrupted the normal stratigraphic setting in some areas by the processes of glacial thrusting, displacement and superposition of pre-existing strata on younger units.
Most of the buried bedrock valleys and channels contain a thick infill, as much as 90 m thick, of water-saturated coarse fluvial sediment ranging from fine sand to metre-sized boulders. These constitute buried aquifers that may be targets for the supply of potable water for municipal and industrial use. In places, the tops of the aquifers lie within 5 m of the surface. Elsewhere they are buried at greater depth. Unlike continuous and extensive aquifers found in large, preglacial bedrock valleys south of the study area, buried glacial aquifers in the Fort McMurray area are confined to isolated channels and valleys. Although they do not form a continuous, well-connected network throughout the oil sands region, buried valleys and channels can function as natural pathways for the subsurface movement of water or other fluids at the scale of an oil sand operator's lease.
