Page 39 - Petroleum Geology

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quired is the maximum depth at which reef growth can start, for this estab-
lishes the maximum thickness of reef that can develop with constant water
depth. Whatever the true figure may be for past Periods, it is unlikely to
exceed 100 m (300 ft), and may well be half that figure.
This concept places many of the Silurian and Devonian organic reefs of
North America (many of them petroleum reservoirs), the Cretaceous reef
reservoirs of Mexico, some of the Mesozoic and Tertiary reef reservoirs of
the Middle East, and the Paleocene reef reservoirs of Libya, into the trans-
gressive category.
The association of evaporites with carbonates is an important association
for petroleum because evaporites can provide seals to the carbonate petro-
leum reservoirs, as they do in the Middle East, northern U.S.A., some areas
of western Canada, and the Permian basin of West Texas and New Mexico.
In all these areas evaporites tended to cover large areas eventually. In western
Canada, evaporites formed behind the barrier reef that grew in middle and
late Devonian times across Alberta into the Northwest Territories (Fig. 1-5),
apparently as a direct result of the enclosing of the seas by reefs. We note
that evaporites are commonly intercalated with, and eventually terminate
the development of extensive carbonate sequences, and that such sequences
are significantly associated wit; I petroleum occurrences.
As regards the association between regressive sequences and sandslsand-
stones, little need be said here. This is due almost certainly not so much to
lack of carbonate, but its relegation to insignificance by the quantity of ter-
rigenous clastic material.
These two associations, carbonates and transgressions, and regressions and
sands/sandstones, are sufficiently distinct to suggest that the rate of sedi-
ment supply is a material factor that determines whether a sedimentary basin
preserves the record of a transgressive or a regressive phase in the develop-
ment of the physiographic basin. Massive sediment supply, such as is observ-
able now on the U.S. Gulf Coast, the Mackenzie Delta in the Arctic, the
Niger Delta, the large deltas of the Indian subcontinent, South-East Asia and
China, is the principal feature of the present development of these physiog-
raphic basins, and so similar features are inferred for the regressive sequences
recorded in sedimentary basins.

CLASSIFICATION OF SEDIMENTARY BASINS

Sedimentary basins are of great variety, each being individually unique.
Their ages vary, their time spans vary, and the type and distribution of the
sediments they accumulated vary. A complete classification would require
two separate classifications - one of the physiographic basins, the other of
the geometry of sedimentary basins - because, as we have seen, the nature
of the sediments that accumulate in a sedimentary basin depends on the