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The Lower Cretaceous Grand Rapids Formation of north-central Alberta consists of three massive sandstone members: upper A, middle B, and lower C. Maximum thickness of the formation is 90 m. In the subsurface, the A and B sands are bitumen impregnated and form the Wabasca oil sand deposit, with reserves estimated at 10.5 × 109 cu m (66 × 109 bbl).
The Grand Rapids Formation was deposited in a broadly regressive setting, interrupted by two transgressive pulses that resulted in shales forming between the three sandstone members. The C sand and the lower part of the B sand were deposited under offshore marine conditions which gradually shallowed to distributary channels, nearshore bars, and coastal swamps at the close of B sand deposition. The shales between the A and B sands were initially formed in brackish environments, which changed to an offshore marine environment as the result of transgression. The A sand consists of nearshore marine deposits overlain by distributary and tidal-channel sands. Coal seams that cap the A and B sands in the outcrop area indicate a coastal-swamp to interdistributary-bay setting.
The clay mineral assemblage in the sandstones consists of kaolinite, illite, montmorillonite, and minor chlorite. In general, the bitumen-saturated sandstones have lower clay content with a larger proportion of kaolinite and less montmorillonite than the nonsaturated, water-bearing sandstones. Examination of the sandstones in thin section and by scanning electron microscopy shows that most clays are either authigenic or have authigenic overgrowths giving the appearance of authigenic minerals.
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