Page 280 - Petroleum Geology

PART 2. PETROLEUM GEOLOGY OF TRANSGRESSIVE SEQUENCES

CHAPTER 12

FOSSIL CORAL REEFS

SUMMARY

(1) Dominant transgressive sequences tend not to be folded, so this is the
geological context of stratigraphic traps, of which fossil reefs are probably
the purest form.
(2) Reefs become large petroleum traps when their vertical dimension has
been extended by upward growth with a rising sea level, relatively. It seems
necessary for them to have grown on a permeable platform that acted as a
carrier bed, because the reef organisms themselves were not the source of the
petroleum. Ultimately they must be exterminated by drowning and/or
smothering by muddy sediment.
(3) Diagenesis has a marked influence on reservoir properties. Dolomitized
reefs tend to be more porous and permeable than limestone reef reservoirs,
and they are also more common.
(4) Fossil reefs commonly contain very large quantities of recoverable
petroleum and are capable of producing it at a great rate. Crude oil quality
tends to be rather heavier than that from sandstone reservoirs, with relatively
high sulphur content, but no wax.

INTRODUCTION

The most remarkable feature of dominantly transgressive sequences is that
they tend not to be folded. The corollary for petroleum geology is that this
is the habitat of stratigraphic traps. Fossil coral reefs are arguably the purest
form of stratigraphic trap; they are strongly associated with transgressive se-
quences that have almost invariably not been folded, whatever their age,
with regional dips commonly less than 5", many less than 1". When fossil reefs
become petroleum reservoirs, they can contain very large quantities of oil
and gas, and individual wells are usually very productive.
We must be careful with terminology because the detailed terminology of
present-day reefs is rarely applicable to fossil reefs. There are two separate
influences on comparisons between modern reefs and subsurface reefs. First,
the dimension of time may exaggerate or blur the morphology that existed