Abstract The Rainbow Lake Field is located some 400 miles northwest of Edmonton, Alberta. The main production comes from Middle Devonian reefs which are encased in evaporites. These reefs vary in size from a quarter section to nine sections and range in height to over 800 feet. Additional production comes from overlying units which are structurally draped over the reefs. These reefs have been interpreted to be ecologic reefs that owe their reservoir qualities to primary fabric and mesodiagenetic processes. To the contrary, we hope to show that these build-ups are “cementation reefs”, i.e. they are primarily products of pervasive cementation of mechanically deposited skeletal debris and that their reservoir characteristics are mainly the result of eodiagenetic processes.

Abstract Data obtained from exploration for hydrocarbon-bearing Middle Devonian Rainbow Member reefs in northern Alberta, Canada, provides an excellent opportunity to examine the regional geological history of the Black Creek basin and the evolution of varied reef forms within the Rainbow portion of this basin. During pre-Middle Devonian and Middle Devonian time, a cyclical sequence of redbeds (clastics and evapo- rites) of the Lower Elk Point subgroup were deposited in a shallow epicontinental sea. The incipient development of the Black Creek basin occurred at this time. Subsequently, negative epeirogenic movement resulted in widespread deposition of fine-grained, dark carbonate rocks of the Lower Keg River Member. Local faunal changes, as well as local changes in thickness (interpreted to be caused by “lime-mud” mounds), occur in the upper part of this unit. During the time of Upper Keg River deposition crinoidal beds were deposited in the form of a bank about 50 ft thick. The crinoidal bank extended throughout and beyond the Rainbow part of the Black Creek basin. In this bank, reef-constructing organisms flourished in several localities. The loci for concentration of reef organisms which led to rapid Rainbow Member reef growth, are believed to be the mud-mound topographic highs that were present in the underlying Lower Keg River Member. Structural control of reef growth, if it did exist, was subtle. The reefs that grew in the Rainbow subbasin are characterized by pinnacle and atoll forms having vertical relief of up to 820 ft. Relatively rapid basin subsidence, combined with directional aspect of climate, paleography of the sea floor, and local bathymetry, controlled the external geometry, and to some extent, the internal facies of the reefs. Detailed lithologic studies reveal 14 facies representing six depositional environments—basin, bank, forereef, organic reef, backreef, and lagoon. Superimposed on the original facies is a variable diagenetic history. The growth of Shekilie barrier-reef complex across northwestern Alberta and adjacent areas of British Columbia and the Northwest Territories, and regional tectonic movements, altered depositional patterns late in Elk Point sedimentation. The barrier formed by the reef complex and structure prevented the free flow of normal marine waters southeast into the Black Creek basin. In this basin salinity of the water increased and the Black Creek Member salt was deposited, followed by the Muskeg anhydrites. The Muskeg evaporites completely infilled the Black Creek basin and covered the Rainbow Member reefs except for those present in the Shekilie barrier complex. The evaporite cover provided an effective seal for hydrocarbon entrapment. Reserves from the Rainbow Member pools in the Rainbow field and the Rainbow South field are estimated to be in excess of 1.2 billion and 165 million bbl of oil-in-place, respectively.