Middle Devonian Reef Production, Rainbow Area, Alberta, Canada

Abstract

The discovery of oil in a reef of Middle Devonian age in the Rainbow area of northwestern Alberta by Banff Oil Ltd. and Aquitaine Company of Canada Ltd. in March 1965 sparked a period of intensive exploratory effort which resulted in the finding of an estimated 2.2 billion bbl of oil and 1.5 trillion cu ft of gas. The productive Rainbow-Zama trend extends 110 mi in a general north-south direction and is 20 mi wide at its widest part. Additional discoveries have been made in the Slave Point Formation, Sulphur Point Formation, and Zama Member, all of which overlie the main producing formation--the Rainbow Member reef of the Keg River Formation. Several unique events have occurred in the relatively short exploration and production history of the Rainbow part of the productive trend. The adaptation of Common Depth Point seismic technique to the problem of finding carbonate reefs within an evaporite sequence represented a substantial advance in utilizing the seismic approach in exploration. Furthermore, for the first time, seismic and drilling operations in the muskeg terrane of northwestern Alberta were undertaken on a year-round basis. In the field of reservoir engineering, detailed stratigraphic studies of reef cores were utilized extensively in providing the base for evaluation of secondary recovery schemes, using one-, two-, and three-dimensional mathematical models, which were constructed to simulate the producing formation and movement of contained fluids. Recovery factors of up to 88 percent have been accepted by the local regulatory body, the Alberta Oil and Gas Conservation Board. An approval program of sequential depletion of several separate pools also provides the most economic method of production and conservation. The seismic data on which the location of the discovery well at Rainbow was based were obtained during 1953-1955, before the use of Common Depth Point techniques in northern Alberta. Seismic interpretation was made extremely difficult by the presence of a severe multiple problem. The selection of the location involved very close coordination between geologist and geophysicist. The availability of high risk capital for costly exploration in a remote area considered to have essentially sour gas prospects made it possible to drill the well. Reefs ranging in age from Ordovician to Pennsylvanian have been mapped in outcrop in Canada's Northwest Territories. Prospects for hydrocarbon production from reefs in this part of Western Canada's sedimentary basin are considered excellent.