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ness or of permeability in the aquifer. The hydraulic gradient of an aquifer
with constant volumetric rate of flow varies with cross-sectional area and
with permeability according to Darcy’s law:
Ah11 = QIK A (9.8)
so that if the aquifer thins in the direction of flow, or if the hydraulic con-
ductivity (K) decreases in the direction of flow, the hydraulic gradient, and
hence the slope of the potentiometric surface, increases. With down-dip flow,
it is possible that this increase in the slope of the potentiometric surface is
sufficient to arrest updip migration of oil, or to combine with capillary bar-
riers to form an accumulation.
Similarly, when an aquifer is faulted across the flow path with a throw
that is less than the thickness of the aquifer, water flow will be impeded and
there will be a potential drop across the fault, the amount depending on the
permeability of the fault plane and the reduction of area normal to flow at
the fault plane (Fig. 9-16). If the dip of the carrier bed/aquifer is greaterthan
the critical hydrodynamic dip, the change in water potential gradient across
the fault could act as a hydrodynamic trap irrespective of the permeability
of the fault to oil.

Fig. 9-14. Constructed map of u for po/p = 0.85
Fig. 9-15. Constructed map of u for po/p = 0.9.