Page 290 - Petroleum Geology

Throughout the late Middle Devonian and Late Devonian, the basin accu-
mulated a dominantly transgressive sequence of sediments, the transgression
progressing generally from north to south. North of the Peace River arch, in
middle Devonian times, grew the reefs of the Rainbow-Zama area. These
reefs grew to the south of the great Presqu'ile barrier reef along which, to the
north-east, occur the lead-zinc deposits of Pine Point on the southern shore
of the Great Slave Lake. South of the Peace River arch, in latest Middle and
early Late Devonian times, grew the reefs of the Swan Hills area. Further to
the south, and a little later, grew the reefs of the Leduc area. In all three areas,
important petroleum reserves exist in the reefs themselves, with some in
structures formed by differential compaction over them.
This whole area has suffered very little deformation. Dips are generally less
than 1"; and faulting is relatively rare to the east of the foothills, with normal
faults usually throwing less than 30 m. Decades of meticulous research have
failed to reveal any structural control on the reef trends.
The Presqu'ile barrier reef was a strong influence on the environment, sep-
arating an area of terrigenous sediment with some reefs to the north and
west (the Mackenzie basin) from the Rainbow-Zama area of carbonates and
evaporites with many reefs to the south and east.
To the north and west of the Presqu'ile barrier reef there are isolated reefs
with diameters up to about 12 km, associated with marine shales. These have
been disappointing from the petroleum point of view (Hriskevich, 1967).
To the south and east of the barrier reef, detailed facies analysis enabled
Langton and Chin (1968) to recognize two distinct types of these Middle
Devonian (Givetian) reefs: they are either pinnacles (without lagoonal or
back-reef facies) or atolls. They grew on a platform of carbonate and argilla-
ceous carbonate of the Keg River Formation, crinoids apparently forming
the nucleus to reef growth. Stromatoporoids appear to have been the domi-
nant frame builder, with corals in a subsidiary role. The reef structures are
up to about 250 m high, and the rim development suggests that the prevailing
wind was from the present north-east. Some original porosity is present, but
most is attributable to diagenesis, local differences of which have led to varia-
tions in reservoir characteristics. As with the Silurian reefs of the Great Lakes
area, the reefs of the Rainbow-Zama area were terminated by environmental
changes, probably brought about by the growth of the reefs themselves, that
resulted in the deposition and accumulation of evaporites and carbonates
that acted as caprocks (Fig. 12-6).
Normal faults with throws up to 25 m are common in the Rainbow area,
and the faulting took place during or soon after the reef growth because the
faults appear to be confined to the reefs - and could have been caused by
their own weight (Barss et al., 1970, pp. 42-44).
In the Rainbow area, the pattern of accumulations has no obvious indica-
tion of differential entrapment: there are reefs with a little oil or a little gas,
reefs that have oil with associated gas caps, reefs with oil and no gas, reefs