Page 281 - Petroleum Geology

256

in the living reef; and secondly, drilling is unlikely to yield all the informa-
tion required for a rigorous classification or recognition of features.
The morphological distortions imposed by the dimension of time are of
two broad types. Subsidence, which is one essential ingredient of a transgres-
sive sequence, requires the living reef to grow upwards or sideways to main-
tain its living community at or near sea level (Fig. 12-1). Lateral growth will
leave in the stratigraphic record a rock unit that would probably be called a
biostrome, although at any time during its development it would have been
called a reef. Lateral growth cannot take place-once the sea floor around the
reef is deeper than the depth tolerance of the colonial organisms. Vertical
growth, which is particularly favourable for subsequent petroleum accumula-
tion, distorts the vertical dimension. As noted in Chapter 1, the vertical dimen-
sion of many fossil coral reefs, commonly 150-200 m, far exceeds the pre-
sumed depth tolerance of the reef-building organisms, and so is evidence of a
deepening sea and transgression.
Because petroleum-bearing reefs are commonly prolific, well spacing tends
to be larger than for sandstone reservoirs, with the result that the density of
information may be rather sparse. Moreover, if the water contact is high in
the reef, few wells will penetrate the water-bearing part.
Lowenstam (1950) stipulated that a reef must properly contain organisms
that were frame builders, and that these organisms grew to produce a wave-
resistant structure, with sediment retention and sediment binding playing an
important part. This criterion of biological potential to build a wave-resistant
structure, clearly satisfactory for reefs studied in outcrop, may not be deter-
minable in the subsurface (although it may be implied by the morphology).
If we insist on the information required to satisfy a definition of reefs in bio-
logical, ecological, and morphological terms, many of the so-called reefs are
not true reefs (for example, the Capitan Formation in the Guadelupe Moun-
tains of New Mexico; Achauer, 1969).
The strict definition of coral reef must, of course, require that corals were
important frame-builders. Again, the information obtained from the subsurface
may not be sufficient to establish this. Dolomitization and recrystallization
may destroy the evidence, and corals are rarely the dominant frame builder.
Playford (1969) assessed corals as third in importance as frame builders after
algae and stromatoporoids in the Devonian reefs of Alberta and those of
Western Australia. Girty’s early work on the Guadelupian fauna in New Mexico
did not reveal corals (Girty, 1908).

Fig. 12-1. A subsiding reef maintains its living community near sea level by vertical or
lateral growth.