Page 283 - Petroleum Geology

seems to be morphology). The stratigraphic record of reefs is virtually con-
fined to reefs of these types because oceanic reefs on seamounts and around
volcanic islands, by their very nature, are unlikely to be preserved in the
stratigraphic record - or be recognized if they are.
The foundations of fossil reefs show the same general characters as those
of the present day. Devonian reefs in Alberta, Canada, grew on carbonate
banks or shelves; some as fringing reefs around the Peace River arch. Some
Silurian reefs in the Great Lakes area of North America grew on bioclastic
calcarenites and some on limy muds into which the reef mass settled (Shrock,
1939; Lowenstam, 1950). The petroleum potential of a reef seems to be very
largely dependent on the existence of a permeable base on which it grew.
Reefs without a permeable base rarely contain commercial quantities of pet-
roleum.
The termination of a reef is also important because, to become a petroleum
reservoir, it must be covered by a fine-grained, relatively impermeable cap
rock. In many cases this occurred by simple drowning of the reef, when reef
growth (for one reason or another) failed to keep up with sea level. Mudstones
or marls then accumulated in due course, and differential compaction may
have draped them over the reef like an anticline (for example, the Cretaceous
reefs of Mexico). Some reefs grew to modify their own environment to the
point of self-extermination by the creation of hypersaline conditions in which
evaporites were deposited and accumulated.
The final ingredient required is a petroleum source rock in hydraulic con-
tinuity with the reefs. The environment of a living reef, as discussed earlier,
is not favourable for the preservation of organic matter because it is richly
oxygenated and has relatively high water energy. Small quantities of petro-
leum may be generated in situ, as found in Silurian reefs quarried for commer-
cial dolomite near Chicago, but the evidence for a more distant source is com-
pelling. The main source material seems to be situated in the areas of deepen-
ing water between reefs or reef trends, or outside them, where euxinic (anoxic)
conditions may develop during the transgression. Such source rocks may well
be contemporaneous with the reefs, but they will normally accumulate on
top of the lateral equivalent of the permeable base on which the reefs grew,
because this is the nature of the accumulation of sediment in a transgressive
sequence. Primary migration will be downwards to the permeable carrier bed,
and secondary migration will then be up-dip towards the reef (see Fig. 11-1).
The area of interface between source rock in the loose sense and the carrier
bed may be very large and, with the slight relief of the carrier bed, there may
be many migration paths. Not all of these will necessarily reach a reef, and
not all reefs will necessarily lie on a migration path. In view of the slight relief
inferred from the present slight relief, the migration paths may be greatly af-
fected by water flow in the carrier bed; but this will generally be from the
deeper parts receiving the waters of compaction to the shallower parts to-
wards the land of the time, and so be towards the reefs in general.