THE GLACIO-STRATIGRAPHIC ORGANIZATION OF QUATERNARY SEDIMENTS IN THE OILSANDS (IN SITU) AREA, ALBERTA: IMPLICATIONS FOR HYDROGEOLOGICAL MODELLING

Large volumes of high-quality water are needed in the steam-enhanced thermal recovery methods used to extract bitumen from the oilsands (in situ) areas of Alberta. Preglacial and glacial aquifers that occur within the more than 300 m thick glacial sedimentary succession in parts of the area are capable of providing this water supply. Predictive mapping of these aquifers is complicated by: the tendency of glacial genetic units to violate the law of original horizontality; the existence of unpredictable, vertically cross-cutting glacial sluiceways filled with stratified sediment or diamict; the existence of glacially thrusted terranes; and, the recognition that a layer-cake depositional architecture of these sediments is over simplistic, being more of a three-dimensional labyrinth type.

A glacio-stratigraphic architecture is presented to interpret the hydrostratigraphic sequences resulting from multiple glaciations. It demonstrates an increasing confidence of interpretation as new methods of till differentiation are applied, from recognition of cyclic till sedimentary sequences in petrophysical logs, to characterization by mineralogy and geochemistry, and lastly to mapping of nonglacial events from weathered profiles on buried till surfaces. The resulting three-dimensional geological model provides predictive capabilities for assessing impacts on the hydrogeological system. Key elements of the model are the recognition that: nonstratified diamict (till) units and stratified fluvial complexes are associated with advance and retreat phases of each glaciation; till sheets form the foundation of the framework, with major aquifers nested between; distinct mineralogy of tills can assist in regional correlations and determine relative stratigraphic age; there is a cyclicity in the vertical facies associations; and, bedrock topography is a first-order control on the distribution of fluvial sediments. The implications of this complex architecture include: there is an increased probability of encountering regional aquifers at interfaces of regional tills; and, glaciofluvial downcutting and superposition of sediments produces a stair-stepped hydraulic connection resulting in an increased likelihood of hydraulic communication between surface water bodies and deep aquifers.