Page 209 - Petroleum Geology

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is mechanically unstable. As the oil is forced out of the mudstone over a
wide area, the waterloil interface will become wavy, with a tendency for
“diapiric” oil bodies to form; and the amplitudes of those near the dominant
wavelength (see p. 332) will be amplified at the expense of the others (Fig.
9-4). As soon as one oil “diapir” reaches its critical vertical dimension, it will
move and tend to drain others. This oil will move vertically upwards, in static
water, until it reaches the cap rock.
The criterion with any change of lithology encountered during this vertical
migration, and there may be several within real carrier beds, is that the cap-
illary displacement pressure required for further progress must be less than
that existing in the oil. Vertical migration ceases as soon as the pressure in
the oil is insufficient to overcome the capillary resistance. The cap rock is a
fine-grained material of which the capillary displacement pressure exceeds -
and usually exceeds by a wide margin - that existing in the oil. The oil is
then diverted along this lithological interface, in the direction of decreasing
energy, in the updip direction literally and strictly.
When the oil is diverted along the base of the cap rock, the situation is
comparable with that at the termination of primary migration from an over-
lying source rock, but not identical to it. The criterion for up-dip migration
is the same as before: the difference of vertical elevation within the continuous
oil phase must exceed the critical vertical dimension.
For migration downward from an overlying source rock, there is no mechan-
ical instability at the oil/water interface, and the critical vertical dimension
depends on the relief of the cap-rock/carrier-bed interface. For source rocks
sufficiently rich to generate enough oil to form a commercial accumulation,
the critical vertical dimension must be achieved sooner or later, unless the
source rock is directly over the trap.

Fig. 9-4. “Diapiric” water/oil interface at the beginning of secondary migration at the bot-
tom of the carrier bed.