ABSTRACT

Exploration in the Rainbow Lake area of Alberta has disclosed a number of 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 carbonates of the Upper Keg River Member are considered to be the off-reef 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 occur in the uppermost Lower Keg River and lowermost Upper Keg River Members. 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 about 30 ft of the top of the Rainbow Member. The deposition of the skeletal rudite and lutite facies, dominant in the upper part of the reef complex, was controlled by the configuration of the wave-resistant organic reef.

Five distinct pore systems are recognized in the Rainbow Member reefs and are shown to be 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. As an empirical relationship can be derived between lithofacies and pore systems, and as pore systems can be referenced to relative-permeability data, it is possible to construct a reservoir model based on geologic considerations 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 shown to be related to the lithofacies and to the type, degree and age of diagenesis. Dolomitization and solution are the most important diagenetic processes that have influenced the reservoir potential of these reefs.

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