Page 205 - Petroleum Geology

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tends to compact. Because compaction can only take place if the pore fluids
can be compressed or expelled, a potential gradient is generated in the fluids
in the mudstone. When compaction is due to gravity, the direction of this
gradient is essentially vertical - upwards and/or downwards to the nearest
permeable bed in which the pore fluids are at a smaller potential. If the mud-
stone is underlain and overlain by such beds, pore fluids in the mudstone will
tend to move both upwards and downwards during compaction (Fig. 9-2).
There will be a physical surface of maximum potential near the middle of
the mudstone that separates the upward tendency from the downward ten-
dency to flow. This surface is both a hydraulic and a chemical insulator: no
fluid can move across it. If the petroleum source rock is above this insulating
surface, any products of organic diagenesis that can move will move upwards
towards the overlying bed. If the source rock is below this surface, any prod-
ucts that can move will move downwards towards the underlying bed. If the
source rock straddles the surface, movable products of diagenesis will move
towards both overlying and underlying beds. And if, as seems likely, petroleum
generation involves a net increase in volume, it will also increase pore pres-
sures and the potential gradients. Within a mudstone, this will tend to shift
the insulating surface into the zone of generation, and so divide it (at least
temporarily) into upward and downward zones of migration.
The rate at which the products of organic diagenesis move depends on their
state (gas, liquid, or in solution in water), on the potential gradient, and on
the effective permeability of the mudstone to that fluid. Those products of
organic diagenesis that are taken into solution in the pore water will migrate

Fig. 9-2. Pressure-depth diagram of upward and downward migration of pore water from
compacting mudstone to underlying and overlying sandstones at normal hydrostatic pres-
sures.