Page 270 - Petroleum Geology

used to assess areas of petroleum potential, so we may be discarding areas on
account of a false impression of immaturity.
Meanwhile, it is essential to understand clearly that petroleum can and
does occur in sedimentary sequences that have chemical indications of imma-
turity. As Hunt (1979, p. 528) wrote, “Considerable research still needs to
be done to define the immature-mature boundary with different types of
both marine- and land-derived organic matter”.

PETROLEUM MIGRATION AND FAULTS

Geochemical evidence for deep source rocks, stratigraphically removed
from the accumulations, required an explanation of the migration from the
one to the other. During the last decade, it has become almost universally
accepted that faults are the conduit for this migration. This hypothesis is a
consequence of the geochemical, to a great extent, because the consensus of
opinion up to the mid-1960s was that faults generally acted as barriers to
petroleum migration. In the mid-l970s, great interest was shown in the hypo-
thesis proposed by Price (1976; and in verbal presentations before that) that
petroleum was generated at depths of 10 km or more, and migrated in solu-
tion in water up faults. Such ideas are stimulating in themselves, and stimulate
further research into the possible processes. But the physical difficulties of
fluid migration in fault planes has received little attention.
We must be careful to distinguish between lateral migration through faults,
from one block to the other, and in faults, up and down fault planes (Fig.
11-7). As regards lateral migration through faults, there is good field evidence
that many faults do not permit this migration, and that some do. As regards
migration in faults, up or down (but mostly up), there is also good field evi-
dence that many do not permit this migration, and that some do. To simplify
the discussion, we shall take these two dichotomies separately, starting with
lateral migration through a fault.
Any fault trap is evidence that lateral migration is prevented (note the
tense) by the fault, but we must distinguish between fault traps that are con-
tained by juxtaposed porous and permeable rock units that could have been
carrier beds, from those that are contained by fine-grained, relatively imperme-
able juxtaposed rock units that could not be carrier beds. We are not concerned
for the moment with the latter, but with a fault plane that is itself a barrier
to migration due to the fine-grained material or gouge in it.
In general, movement on such faults must have caused a reduction in mean
pore size so that the capillary displacement pressure required for further
migration exceeds that available in the migrating oil. We can therefore visualize
a range of fault-plane textures varying from a clayey fault gouge or smear
with very high capillary displacement pressures to one with no significant
change of texture from one side to the other. We infer that the trapping capa-