IMPACT OF PALEOZOIC KARST UPON GROUNDWATER FLOW IN THE VICINITY OF THE ATHABASCA OIL SANDS

Extracting oil from the Athabasca Oil Sands requires abundant water supply, much of it from groundwater, and geological layers capable of receiving large amounts of injected waste water. Hence good understanding of geological structures and knowledge of the dynamics of existing regional groundwater flow systems and their response to the imposed stresses of water production and waste water injection is of paramount importance to successfully determine and manage any changes to the pattern of groundwater flow.

The presentation outlines the existing groundwater flow systems affected by the ubiquitous karst in Paleozoic layers. The Paleozoic karst causes the low flow potential drain found by Hitchon (1969). This drain collects groundwater flow from the Cretaceous layers above and from deeper Paleozoic layers to discharge into the Athabasca River.

The Devonian Grosmont Formation underlies the southwestern and northwestern part of the Cretaceous Athabasca Oil Sands. Permeabilities as high as 250 Darcy (2.5*10^-3 m/sec) were measured in boreholes within this formation, which acts as a regional collector and transmitter of groundwater in the area of its occurrence. Heavy oil is already being extracted from the Cretaceous layers above the Paleozoic, creating a noticeable effect on the pattern of groundwater flow in the Grosmont Formation by water extraction and injection. In the future additional extraction operations will target the Grosmont Formation itself. Pronounced effects on the regional and local groundwater flow systems are anticipated for the area of the Wabasca Oil Sands, located between the Wabasca and Athabasca Rivers.

To the east of the occurrence of the Grosmont Formation, karst in lower Paleozoic layers also collects and transmits groundwater flow from the Cretaceous and Paleozoic layers into the Athabasca and the Clearwater Rivers. Due to the geometrical configuration of these two main rivers there exist three additional major regional groundwater flow systems, one to the south of Ft. McMurray in the area of the Stony Mountain Uplands and two more to the north of Fort McMurray east and west of this stretch of the Athabasca River. All of these systems have already been subjected to substantial water extraction and waste water injection.