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The reservoirs showed a rapid pressure decline, indicating poor communica-
tion and a low primary recovery of about 16% of the oil in place (which sec-
ondary recovery is expected to double).
The youngest reefs, and the first to be discovered, occur in the area be-
tween Calgary and Edmonton. These are of Late Devonian (Frasnian) age, in
three major trends running more or less parallel to each other (and to those
in the Swan Hills area) in a NNE-SSW direction. The stratigraphic relation-
ships are shown in Fig. 12-9, the production coming almost exclusively from
the reef limestones (mostly dolomitized) of the Leduc Formation. Minor,
but still important production comes from dolomites of the D-2 reservoir of
the Nisku Formation, overlying the Woodbend Group, in anticlines formed
by differential compaction and draping over the reefs. Dolomitization has
destroyed the original diagnostic features of the reefs, both texture and or-
ganic content.
These reefs grew in shallow seas that transgressed towards the south-east
over a contemporaneous biostromal carbonate platform (Cooking Lake For-
mation) that has not been dolomitized. Prevailing winds were from the pre-
sent north-east. The reefs themselves are generally flat-topped (“table” reefs
of Andrichuk, 1958) ranging in size up to about 30 km in greatest horizontal
dimension. Due to the transgression and to subsidence, the reef complexes
grew to thicknesses between 180 and 300 m. Their growth was terminated

Fig. 12-8. Structure contour map on top of Beaverhill Lake Group, Swan Hills area, Alberta.
Contours in feet below sea level. Horizontal ornament, oil; crossed, gas. (Simplified from
Hemphill et al., 1970, p. 82, fig. 21.)