Page 288 - Petroleum Geology

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accumulation of evaporites. This environmental change seems to be due larger-
ly, if not entirely, to the growth of the encircling reefs.
The north-western belt of pinnacle reefs, studied by Gill (1979) and referred
to in Chapter 9 in the context of differential entrapment, grew on a carbonate
platform - the Lockport Formation - and euxinic conditions developed in
the deeper water in which the Cain Formation accumulated. The Cain For-
mation is only 7-8 m thick, and consists of a basal dark grey to black argilla-
ceous mudstone with a mean total organic carbon content of 0.3%, followed
by cyclic calcite and anhydrite with halite. .The carbonate components are
rich in bituminous matter.
The reefs were dolomitized and secondary porosity development made
them good potential reservoir rocks. They were then sealed by evaporites.
The regional dip of the Lockport Formation, on which the pinnacle reefs
grew, is less than 2".
Thus, before the end of the Silurian period all the ingredients of a petro-
leum province existed: source rock, carrier bed, cap rocks and stratigraphic
traps. The size of the traps, had it not been for subsidence, would have been
very small because the average basal area of a pinnacle reef here is only 0.3
km2 (Gill, 1979, p. 612); but subsidence gave them vertical dimensions of 90
to 180 m at depths from 900 to 2100 m. The fluid capacity of the reefs,
even then, might have been quite small had it not been for the diagenetic
development of porosities averaging about 6%.
The source of the petroleum is clearly not in the reefs themselves because
the very strong evidence of differential entrapment and the existence of gas-
bearing, oil-bearing and water-bearing reefs of the same type indicate the main
source to be down-dip of the petroleum-bearing reefs, and that it generated
both oil and gas. The evidence of migration and differential entrapment re-
quires a common carrier bed and a common source that had more energy
than the carrier bed and the deepest reservoir. There may have been other
contributary sources in some areas, with these energy relationships to partic-
ular parts of a carrier bed and reservoir, but the main source must have been
in hydraulic continuity with the Lockport Formation. Sharma (1966, p. 347)
suggested the underlying Clinton Formation as source rock for the Peters
reef in St. Clair county, Michigan; but Gill (1979, p. 618) pointed out that
it does not appear to be in a favourable facies in positions that satisfy the re-
quirements for the main source, while the Cain Formation overlying the
Guelph and Lockport Formations does.

DEVONIAN REEFS OF WESTERN CANADA

The prolific and widespread reefs of the Silurian Period were followed in
the Devonian Period by perhaps even more prolific and widespread reefs
around the world (although they were, perhaps, more concentrated during