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organic composition to the undolomitized, but it seems likely that they were,
and that while reefs seem to have been prone to dolomitization, it is not
related to particular reef environments.
Not all the southern reefs contain petroleum; and the distribution of gas,
oil and gas, and oil fields has a pattern that suggests differential accumula-
tion. This was one of the areas that Gussow (1954, 1968) used to illustrate
his hypothesis, and was probably the one that suggested it to him. Both the
observation of variety and the argument for differential entrapment lead to
the conclusion that the source of the petroleum lies outside the reef trends
themselves, down-dip, probably in the contemporaneous fine-grained sedi-
ments that accumulated in quieter, deeper water (a conclusion supported by
Deroo et al., 1977, from geochemical evidence).
The overlying Nisku Formation of the Winterburr, Group also contains im-
portant reserves of oil (with some associated gas) in anticlines formed by
differential compaction over the deeper reefs. Recently these have been added
to by the discovery of oil in a reef facies of the Nisku, called the Zeta Lake
Member, in the Pembina area to the south-west of Edmonton (Chevron Stan-
dard Limited, 1979). These reefs are small in area, flat-topped, and up to
110 m thick. Stromatoporoids appear to be subsidiary to corals and algal en-
crustations. The pools are small, with recoverable reserves varying from about
160,000 m3 (lo6 bbl) to about 6.4 X lo6 m3 (40 X lo6 bbl). Diagenesis has
had a marked influence on reservoir properties. Limestone reservoirs average
3.5% porosity and 300 md permeability, but dolomitized reservoirs average
14.5% porosity and 2.3 darcies permeability. Most pools will be produced
from a single well, with another well for pressure maintenance. The source
of the oil is considered to be local, either in the Ireton or in the off-reef Nisku
strata.
The presence of Nisku reef oil suggests that the other Nisku accumulations
have their own source, rather than resulting from spillage from the Leduc For-
mation reservoirs underlying them.
The Devonian reefs of the Western Canada basin have been important for
petroleum geology both in providing a natural laboratory for study and in
stimulating studies that have been, and will be applied elsewhere.

CRETACEOUS REEFS OF MEXICO

Mexico was one of the early countries to produce important quantities of
crude oil, and her petroleum history is particularly interesting. Their first
well, drilled near oil seeps in 1869, ten years after Drake’s well in Pennsylvania,
found some oil at 28 m. Serious production began in 1904, also as a result of
drilling near seeps, and by 1908 it could be said that La Faja de Oro - the
Golden Lane - had been discovered (Guzmh, 1967; Viniegra-0. and Castillo-
Tejero, 1970). In the following year, a number of prolific wells was drilled,