Page 222 - Petroleum Geology

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The macroscopic velocity through the carrier bed is approximately qlf, that
is, about 160 pm/s, or 14 m/day. There are no obvious difficulties in ac-
cumulating enough oil for a giant field in a million years - or even a few
hundred thousand years.
The rate of primary migration per unit area of interface between mudstone
and carrier bed is undoubtedly very slow; but this is probably compensated
by the very large areas of such interfaces.
If gas, and gas only, is migrating, the principles are the same except that as
gas moves to positions of different pressures and temperatures (usually lower)
its volume changes significantly. If gas and oil are migrating together in separate
phases, the principles are the same, but the details become very complex be-
cause gas can dissolve in oil and water. If oil has gas in solution . . . there are
many ,variations on the theme. Oil is also slightly soluble in water, so any oil
left behind in a discontinuous phase will probably be removed eventually in
solution in moving water.
Accumulation of petroleum in a trap

When petroleum, trickling along one or more paths of local minimum poten-
tial, arrives in the trap and begins to accumulate, migration ends but a new
set of physical changes begins:
-Water is displaced downwards from the top of the reservoir.
- The oil/water or gaslwater contact is displaced downwards.
- The pressure in the petroleum increases while petroleum accumulates.
-The petroleum in the accumulation continues to move in response to
these changes.
Migrating petroleum has negligible kinetic energy, so the newly-arrived
petroleum is added to the accumulation at the interface, without penetration
into the accumulation (much as cream poured into a jug, rather than milk).
The water contact will be rather lower near the point of entry. Due to the
density difference between oil and water, and gas and water, the potential
energy of the accumulation will be greater where the water contact is lower,
and there will be sympathetic movement within the accumulation in the direc-
tion that tends to restore equilibrium.
Within the accumulation, a new physical environment develops. Within
the continuous oil phase, the pressure decreases with elevation above the oil/
water contact according to the relationship:
(9.10)
This rate of decrease is less than that in a continuous water phase, so that, if
we take their pressures to be equal at the oil/water contact, the oil at any
depth within the reservoir above this is at a higher pressure than the water in
continous phase with it. This greater pressure is applied to the water, which
acquires the pressure gradient Ap,/Az = pog, and so acquires a downward