The Hydrogeology of the Athabasca Oil Sands Area, Alberta

The area of study includes most of the Athabasca Oil Sands deposit and extends from Tp77 to Tp100, and R1 to R25 west of the fourth meridian: a total of about 67,260 sq. km (25,970 sq. mi). Holocene and Cretaceous clastic rocks unconformably overlie Devonian nonclastic rocks resting on Precambrian basement. Regional dip is toward the southwest. A major fault passing north-northwestward under Fort McMurray is hypothesized, with a downthrow west of about 273 ft (83 m); it appears to be of post-Devonian age. Partial solution of Prairie Evaporite Formation salts extends about 16 km (10 mi) west of the fault; to the east solution is almost complete. Collapse features are particularly notable at the Cretaceous-Devonian contact.

West of the fault, three hydrostratigraphic units are defined: 1.The K-O hydrostratigraphic unit, consisting of Holocene and Cretaceous sediments, is characterized by alternating vertical and horizontal groundwater flow controlled by zones of low and high hydraulic conductivity, respectively. Unsaturated zones extend up to 10 km(6 mi) from outcrop into the units of high hydraulic conductivity. Total dissolved solids contents in groundwaters are usually less than 10,000 mg/L and commonly less than 5000 mg/L 2.The D-2 hydrostratigraphic unit, consisting essentially of Upper Devonian strata, has dominantly horizontal groundwater flow towards a zone of hydraulic heads located roughly coincident with the fault at elevations generally equal to those of the Athabasca River. Total dissolved solids concentrations are usually less than 40,000 mg/L, but are higher to the southwest and in the north-center of the area. 3.The D-1 hydrostratigraphic unit, consisting of the Methy, McLean River, and La Loche Formations that underlie the Elk Point evaporites, has horizontal groundwater flow toward the fault zone. Freshwater hydraulic heads west of the fault are approximately at land surface, but decline eastward to the elevations of major rivers. Total dissolved solids concentrations exceed 200,000 mg/L west of the fault.

The D-1 and D-2 hydrostratigraphic units merge east of the fault. Groundwater flows west towards the fault, with total dissolved solids concentrations seldom exceeding 50,000 mg/L.

On a regional basis, the oil sands have a finite hydraulic conductivity commonly of 10-6 to 10-4 cm/sec.

Surface mining ventures will generally require that the oil sands and underlying aquifers be depressurized. Hydrogeological conditions will affect depressurization:1.pumping volumes will vary widely; 2.in produced water, total dissolved solids may range up to 300,000 mg/L; 3.hydrogen sulphide gas contents will vary greatly;4.induced infiltration from the Beaverhill Lake Formation or the Athabasca River may occur;5.collapse features and faults caused by evaporite solution may have affected the oil sands sufficiently that loss of pressures and fluids could occur in in situ operations.

Structural conditions are suitable for injection of waste liquids into the D-2 hydrostratigraphic unit only west of the limits of evaporite solution. Waste injection into the Beaverhill Lake Formation may be possible where the overlying Woodbend Group is sufficiently thick.