Historically, the Muskeg Formation has been a secondary target in the prolific Rainbow Field, where most of the oil is found in the Middle Devonian Keg River carbonate buildups. With the increasing production maturity of the Keg River pools, the Muskeg Formation has emerged as an attractive target for enhanced recovery. Reservoir description, which preceded waterflood design in the Muskeg C pool, has revealed a more complex reservoir than expected.
The Muskeg C pool appears to be characterized by simple geology. It consists of interbedded carbonates and evaporites deposited in a peritidal setting. The trap is formed by drape over an underlying Keg River buildup. Each bed can be correlated across the C pool, and as a result of a large thickness and porosity development, is related to structural elevation. Correlation between porosity and permeability is good.
Twenty years of production data, however, suggest a more complex reservoir. Reservoir pressure behaviour, water cut and water saturation are incompatible with a simple geological model. Geological, geophysical and engineering evidence suggests the presence of solution collapse breccia zones, which provide vertical communication between the Muskeg Formation and the underlying Keg River buildup. These zones explain not only the reservoir behaviour of the C pool, but also the distribution of oil in the Muskeg Formation overlying the neighbouring Keg River buildups.