Page 82 - Petroleum Geology

61

FLUID MIGRATION FROM ABNORMALLY HIGH PORE PRESSURES

Water tends to flow from positions of higher energy or potential to posi-
tions of lower energy, and the directions of flow are normal to the surfaces
of equal potential (called equipotential surfaces) through the water. For
water to flow through a sedimentary rock, there must be a potential gradient
within the pore water and the rock must have permeability to water. The
hydraulic conductivity (K) of a rock, or its coefficient of permeability, has
the dimensions of a velocity, LT-'. The dimension of time means that it is
unlikely that any natural sedimentary rock is totally impermeable. Indeed,
we can and have measured mudstone permeabilities (see, for example, Magara,
1971, p. 241, fig. 9), so they are not normally impermeable to water even on
a short time scale.
In a mudstone compacting under the force of gravity, so that the vertical
component of total stress is a function of the overburden thickness, surfaces
of equal total stress will be horizontal or nearly horizontal. The vertical com-
ponent of effective stress (u) is a function of both total stress and pore-fluid
pressure, and surfaces of equal effective stress will also be horizontal or nearly
horizontal. Lack of mechanical equilibrium between solids and liquids there-
fore induces equipotential surfaces in the liquids that are also horizontal, or
nearly so; and pore-water movement tends to be vertical (upwards or down-
wards) when induced by gravitational compaction. Pore water tends to migrate
from the highenergy or high-potential zones within the mudstone to posi-
tions of lower energy above or below it. Commonly, an extensive permeable

Fig. 3-16. Pressure-depth plot for sandstone interbedded with abnormally pressured mud-
stones. Mudstone pore-fluid migration is downwards to the upper sandstone interface, up-
wards to the lower interface.