Data obtained from comparatively recent discovery and exploitation of hydrocarbon-bearing Rainbow Member reefs in northern Alberta provide an excellent opportunity to examine the regional sedimentary and structural setting of the Black Creek basin, and specifically, the development of varied reef forms within the Rainbow sub-basin.

An intracratonic basin first developed during pre-Middle Devonian (lower Elk Point) time. Basin "growth" continued through this period and into Middle Devonian (upper Elk Point) time, when it reached its maximum expansion during deposition of the lower Keg River carbonates. Minor thickness and faunal changes take place in this unit; in some places, the areas where these changes take place are believed to have been loci for later Rainbow Member reef growth. In other places, such as the Hay River bank, the coincidence of reef growth and major structural elements indicates tectonic control.

The Rainbow Member reefs are characterized by high relief (up to 800 ft) and two basic geometrical forms: pinnacle (large and small) and atoll. A third form may be in the Shekilie barrier and Hay River bank reefs, but little information is available.

The increasing salinity of the seas, caused by development of the Shekilie barrier and lowering of the sea level, gradually terminated Rainbow Member reef growth. Muskeg evaporites (the Black Creek Member salt at the base, overlain by the Muskeg anhydrite) filled the Black Creek basin. The origin of the Black Creek salt beds and the time relations of reef and offreef beds have been interpreted differently by different authors.

Detailed lithologic studies reveal the presence of 14 facies representing six depositional environments: basin, forereef, organic reef, backreef, lagoon, and shoal. Superimposed on the original facies is a variable diagenetic history: an early stage of penecontemporaneous dolomitization and a later stage of white dolomite infilling. Both are common to the atoll reefs. Geological rock types are classified in terms of five reservoir facies. The link established between rock and reservoir properties leads to a better understanding of fluid-flow behavior and recoveries in the pools. At present, there are in excess of 1.2 billion bbl of in-place reserves in the Rainbow field, whereas in the Rainbow South field, in-place reserves are in excess of 200 million bbl.

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