Extensive deposits of heavy oil occur in updip pinch-out of the Bluesky and Gething Formations (Lower Cretaceous) of northwestern Alberta. In-situ extraction technologies require a detailed knowledge of porosity, permeability, and mineralogy within the reservoir and the effect of diagenesis on these properties.

Marine sands in the upper part of the Gething Formation are composed predominantly of quartz and chert with lesser amounts of clastic carbonate, rock fragments, and feldspar. Emplacement of heavy oil forming the Peace River oil sands effectively stopped or slowed diagenesis. Thin-section petrography and scanning electron microscopy provide the means of establishing a diagenetic sequence and of timing of oil migration. Three wells with abundant core have been chosen to illustrate the relations among diagenesis, hydrocarbon migration, and reservoir quality.

Authigenic minerals, in their probable order of emplacement, include pyrite, quartz overgrowths, feldspar overgrowths, kaolinite, and illite. Kaolinite and illite are most abundant in the water sands. An unusual secondary carbonate mineral, dawsonite. NaAlCO₃(OH)₂, occurs in only the richest oil sands but the timing of its deposition is in question. Secondary porosity was formed after feldspar overgrowths but before deposition of kaolinite. Oil migration took place after part of the kaolinite formed.

Diagenesis is an ongoing process and the various stages probably continued until migration of oil into the reservoir. Porosity is better in the good oil sands than in the water sands. Permeability is reduced by the heavy oil.

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