Page 312 - Petroleum Geology

The question of petroleum source rocks for unconformity traps is intrigu-
ing. There are, of course, only three possibilities: either they are in the pre-
unconformity sequence, or in the post-unconformity sequence, or both.
If pre-unconformity source rocks exist, there is one requirement that must
be met: generation and primary migration of oil or gas from them must be
delayed, at least in part, until the sealing post-unconformity bed has accumu-
lated. This is not a great difficulty provided the erosion leading to the uncon-
formity is not so deep that deep burial before uplift and erosion would be
implied. A relatively thin post-unconformity seal will be sufficient to trap
the petroleum.
A post-unconformity source cannot, of course, generate any petroleum
until the trap already exists, and such generation will occur after substantial
subsidence and burial of the unconformity. Primary migration will then take
place downwards through the subcrop of porous and permeable beds, and
secondary migration will be against the subcrop to the trap. When post-un-
conformity beds contain a petroleum source rock, they must be rather thicker
- at least so that the zone of downward migration embraces the petroleum
source rock sensu stricto. The source rock sensu lato may therefore also be
the seal. This is not a strict requirement, because the source rock may exist
anywhere over the subcrop, but it is very likely because of the requirement
of low relief on the unconformity surface. This requirement of low relief
also means that very extensive source rocks may exist that will generate very
large quantities of petroleum.
There are geological arguments that suggest the location of the source
rock relative to the accumulations. First, if there is oil of similar quality in
several reservoirs of differing rock type in one trap, there is good reason to
postulate a post-unconformity source. Similarity of formation water will re-
inforce this conclusion. If, on the other hand, there is a variety of crude oils
and formation waters, whether in similar or dissimilar reservoir rocks, differ-
ent sources are indicated, at least one being a pre-unconformity source rock.
Of course there will be ambiguities. From a post-unconformity source, petro-
leum will only enter those permeable rock units that subcrop against it. This
need not be in the trap itself, but the corollary is also important: a reservoir
that does not subcrop in the trap is not proof of a pre-unconformity source
because it may subcrop anywhere where the secondary migration paths will
lead to the trap.
The surface of the unconformity is also important in relation to the sub-
crop because this, in the absence of faulting, determines the trap. It also
determines the probable directions of secondary migration, but this is less
important. The very size of some unconformity traps suggests a great area of
petroleum genesis, but the migration paths are restricted stratigraphically. If
the surface of unconformity does not form a perfect seal, this will usually
be indicated by concordant oil/water contacts, or gas/water contacts, in the
subcropping reservoirs. In such a case, similarity of crude oils is to be expect-