Page 102 - Petroleum Geology

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the solution for the release of petroleum to a separate phase in (or on the
way to) the accumulation.
At the other extreme, migration as a separate phase in water has the merit
that this is the state found in the accumulation. The problems relate to the
mechanical difficulty of transporting petroleum as a separate phase when it
is disseminated through the pore spaces with water (and disseminated it must
be at some stage between generation and accumulation, because the source
material itself is disseminated). This difficulty is particularly great if primary
migration through a fine-grained source rock as a separate phase is postulated.
Transport in a colloidal state has the merit that it reduces the difficulties
of the alternatives - but it also has a problem relating to the need for an
agent to emulsify the petroleum and an agent to deemulsify it.
Combinations of these have also been proposed, of which perhaps the
most attractive is transport as a separate phase, with residual petroleum
being dissolved in the formation water.
From the great diversity of processes proposed, the only conclusions that
would receive general support from petroleum geologists would be the fol-
lowing:
(1) Mudstones are the principal source rocks of petroleum, so primary
migration takes place in mudstones along energy gradients generated by com-
paction. In some areas, notably the Middle East, fine-grained carbonate rocks
appear to have been petroleum source rocks.
(2) Petroleum, probably as a separate phase, is expelled from the source
mudstone during compaction, and migrates through permeable carrier beds
under the influence of gravity (and the hydrodynamic field) to the trap - or
the surface.

Entrapment of petroleum

The accumulation of petroleum in a trap is accomplished when the
physical properties and geometry of the rocks prevent further migration.
Since petroleum is less dense than water, the barrier to further migration is,
in general, such that it prevents upward, or an upward component of, migra-
tion.
The main forms of petroleum traps are well known: they consist of the
anticlinal trap, the fault trap, and the stratigraphic trap (with which uncon-
forrnity traps are classified in spite of the fact that this is a structural term).
The entrapment of petroleum in anticlines was one of the first principles of
petroleum geology, and it has dominated petroleum-geological thinking. Its
essential features are (Fig. 4-4): (a) a geometrical closure, that is, the struc-
tural contours on top of the reservoir form closed rings; (b) a reservoir rock
that has permeability (which implies porosity); and (c) a fine-grained, rela-
tively impermeable cap rock that overlies the reservoir and seals it.
One must be specific and state that the closure must be on top of the