Exploration in the Rainbow Lake area of Alberta has disclosed the presence of several biohermal reefs of Middle Devonian age. The reefs, designated the Rainbow Member of the Keg River Formation, are characterized by either a pinnacle or an atoll form and develop to a maximum height of 800 ft. The carbonate rocks of the Upper Keg River Member are considered to be the offreef time equivalent of the Rainbow Member. The evaporites and carbonates of the Muskeg Formation are believed to have been deposited subsequent to reef growth.

Crinoidal beds are present in the uppermost Lower Keg River and lowermost Upper Keg River. Structural control in the form of low-relief warping and normal faulting is believed to have exerted considerable influence on the initiation, distribution, and geometry of these crinoidal beds and reef growth. Fourteen broad but distinct lithofacies, representing six depositional environments, are recognized in Rainbow Member reefs. The stromatoporoid organic-reef facies developed to within 30 ft of the top of the Rainbow Member. Deposition of skeletal rudite and lutite, predominant in the upper reef complex, was controlled by the configuration of the wave-resistant organic reef.

Five distinct pore systems are recognized in Rainbow Member reefs and are related to lithofacies and diagenesis. The various pore systems are classified into reservoir facies by reference to laboratory derived capillary-pressure and relative-permeability criteria. Because an empirical relation can be derived between lithofacies and pore systems and because pore systems can be referenced to relative permeability data, it is possible on the basis of geologic considerations to construct a reservoir model that provides an accurate method of determining fluid flow.

In the Rainbow Member reservoirs both original pore systems and those of diagenetic origin are present. Pore systems of diagenetic origin are related to the lithofacies and to the type, degree, and age of diagenesis. Dolomitization and solution are the most important diagenetic processes which have influenced reservoir potential.

Similarities in patterns of facies distribution exist in all Rainbow Member reefs. Each reef, however, bears the imprint of a variable diagenetic history which has resulted in significant variations in reservoir properties. There appears to be a direct relation between areal size of the bioherms and complexity of diagenetic history.