Diagenetic Features of Grand Rapids Formation, North-Central Alberta, Canada: ABSTRACT

The Lower Cretaceous Grand Rapids Formation of north-central Alberta consists of three massive sandstone units separated by shales and siltstones. Average thickness of the formation is approximately 90 m. In the subsurface these sandstones are impregnated with bitumen and form the Wabasca oil sands deposit, with resources estimated at 10.5 × 109 cu m (66 × 109 bbl). The Grand Rapids sandstones are arkosic to lithic sandstones with a fairly heterogeneous mineralogy in which the predominant constituents are quartz, feldspar, chert, rock fragments, mica, glauconite, and locally siderite. They are poorly consolidated, the main cementing agents being authigenic clays. Kaolinite tends to be the dominant clay mineral in the oil-saturated sandstones while montmorillonite and Fe-rich chlorite predominate in the underlying water-bearing sandstones. Scanning electron microscope studies show that kaolinite occurs as booklets, irregularly scattered throughout the sandstone in a pore-filling habit. Montmorillonite and chlorite have both pore-lining and pore-bridging habits, while illite is mostly a pore-bridging clay. Montmorillonite-chlorite mixed layer clays are found in some samples. The clays in the sandstones all appear to be authigenic in origin. Calcite is the only other common cementing agent. The calcareous cement formed early, apparently replacing most of the original matrix material and locally forming large concretions up to 5 m in diameter in the lower sandstone unit. Other authigenic minerals which may be local cementing agents include siderite, K-feldspar, pyrite, and zeolites(?). End_of_Article - Last_Page 735------------