Hypogenic karst beneath the Athabasca Oil Sands: Implications for oil sands mining operations

Abstract

The Athabasca Oil Sands, located in northeast Alberta, largely comprise a stacked succession of Early Cretaceous fluvial and marine sediments that were deposited directly above Middle and Late Devonian limestones, dolostones, calcareous shales and evaporites. Dissolution of halite and anhydrite from the Prairie Evaporite Formation by hypogenic karstification has resulted in the diachronous subsidence of overlying stratigraphic units and severe brecciation of important aquitards. These aquitards are required to protect oil sands mining operations from in-pit influxes of saline water sourced from Devonian aquifers. Sequence stratigraphic, palynologic and groundwater isotopic evidence suggests that karstification began prior to the Early Cretaceous, was active during deposition of the McMurray Formation and is still ongoing today in the Athabasca Oil Sands mining area. Groundwater flow associated with hypogenic karstification has important implications for the development of the Athabasca Oil Sands. For in-situ projects, Devonian aquifers are considered both as a resource for the moderately saline water required for steam generation and as a disposal zone for waste water. In open-pit oil sands mines, some of the most effective aquitards are removed by mining operations, thereby increasing the possibility that saline water from the Devonian aquifers could enter the mine pits. This poses safety, environmental and economic risks to mining operations that require a thorough understanding of the geologic, hydraulic and geomechanical controls related to hypogenic karstification. This paper, presents our current understanding of the hypogenic karst system in the vicinity of the Kearl mine.