Page 37 - Petroleum Geology

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either mudstone/shale and sand/sandstone or mudstone/shale and carbonate/
evaporite as the dominant association of lithologies. These associations
reflect the tendency of physiographic basins to generate, transport and
deposit sediments of a similar character over great spans of time. But they do
change, and some important sedimentary basins record such changes in asso-
ciations (for example, the Maracaibo basin in Venezuela, and the East Borneo
basin in Indonesia).
More significantly, two associations are evident in transgressive and regres-
sive sequences. There is an association between transgressive sequences and
carbonates/evaporites, and between regressive sequences and sands/sandstones.
Not all transgressive sequences contain carbonates, but no major regressive
sequence contains significant carbonates*.
The upper Devonian organic reefs of the Western Canada basin are well
known because many of them are important petroleum reservoirs. The re-
sults of much competent research have been published (see, for example,
Barss et al., 1970, Hemphill et al., 1970, for accounts of two areas). There
is no doubt that the reefs grew during a dominantly transgressive phase of
the development of the physiographic basin. In general, the more southerly
their position, the younger their age (Fig. 1-6); and many are overlain by
mudstones, calcareous mudstones and marls of a deeper-water facies than
that implied by the reefs themselves. Are organic reefs always transgressive?
What is a transgressive reef?
The matter of transgressive and regressive reefs is complicated and not to
be related solely to changes of sea level relative to the land. The reason for
the complication lies in the nature of an organic reef. Lowenstam’s widely
accepted definition of a reef requires that it contain organisms that were
frame-builders, the organisms growing to create a wave-resistant structure;
and that the organisms were important for retaining sediment and binding it
(Lowenstam, 1950). There are thus two important parameters: the biological
potential to build and the environmental potential to kill or destroy. These
determine the form of the reef, whether biohermal or biostromal, whether
isolated as patch or pinnacle reefs, or associated with a back-reef facies that
may include evaporites.
A reef is a facies that depends on an environment of a physiographic
basin. So the simplest concept is that a reef that migrates landward is trans-
gressive, and a reef that migrates seaward is regressive. Another simple con-
cept, but more important from a petroleum point of view, is that when the
true thickness of a reef exceeds the presumed depth tolerance of one or

* The thick and extensive carbonate sequences of Iran and Iraq are not clearly transgres-
sive, according to geologists with experience of these areas with whom I have discussed
this point; nor are they clearly regressive. Perhaps this is a case of close balance between
subsidence and carbonate accumulation without terrestrial clastic supply.