Page 311 - Petroleum Geology

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rock accumulation continued into the Paleocene for the nearby Intisar fields
- probably with some deepening of the water.

UNCONFORMITY TRAPS

The beauty of an unconformity trap is that it exists because of a rather
long and sometimes complicated succession of events, each of which is essen-
tial for the entrapment of the petroleum. First, an alternating sequence of
potential reservoir rocks and cap rocks must accumulate. Then they must be
deformed and truncated by an erosion surface. Then the erosion surface must
be covered by relatively fine grained sediment that will become the cap rock,
and may also be the petroleum source rock. This unconformity must then
subside to depths at which petroleum in its source rock (wherever it is) is
generated and can be expelled to migrate to the trap. There is no real time
limit on these events, but they are unlikely to occur in less than 10 m.y., and
may well take 100 m.y. In addition to all these events, the erosion surface
must have a configuration that closes the reservoirs into spaces of minimum
potential beneath the unconformity, or faults must exist, otherwise any pet-
roleum generated would migrate along the subcrop of the permeable carrier
bed and dissipate, or pass through the unconformity to a different type of
trap.
Little thought seems to have been given to unconformities since the days
of Hutton - largely, perhaps, because in outcrop there is relatively little to
see. But subsurface geology provides a clearer picture of both stratigraphy
and structure around them, so the study of unconformity traps is of interest
to both “pure” and applied geologists.
The word “unconformity ’’ suggests to many geologists subaerial erosion
of sedimentary rocks that had suffered orogeny (i.e. uplift) followed by the
accumulation of another sequence of sediments, usually marine. This sort of
unconformity, though real, is rarely a petroleum trap because the beds im-
mediately overlying the unconformity surface are usually sufficiently porous
and permeable for any oil or gas to escape, or accumulate in a different sort
of trap. What is required for a petroleum trap is that the post-unconformity
sediments shall be fine-grained and very extensive. This requires transgression
over a surface of very low relief by a sea with little energy - less than that
required, in general, for erosion and removal of the products of erosion -
yet a supply of mud. These conditions are also conducive to the preservation
of organic matter in the muds. Once these muds have accumulated, further
subsidence results in the completion of the post-unconformity sequence.
These conditions were fulfilled over wide areas of the world during the
Cretaceous, and more locally at other times (particularly during the late
Jurassic and during the early Tertiary), and they are sufficiently peculiar to
have deserved more attention that they have received.