In previous potentiometric studies of variable-density aquifers, particularly studies related to oil exploration, certain gravitational effects apparently have been ignored. These include the effects of troughs formed by permeability barriers within the aquifers, and the effects of structural troughs, saddles, anticlines, and synclines. In intermontane regions these gravitational effects probably are negligible in comparison with observed head differences; in most other regions they can change appreciably the heads, or the potentials, that are available to cause flow.

A gradient in potential is not necessarily associated with flow, even though corrections are made for the average rate of change in density of water. Gravitational effects can cause the interface between water and an oil or gas deposit to be tilted, even if the water under the deposit is static. These effects can reduce the rate of flushing of brine by fresh water, or they can prevent flushing.

Previous potentiometric studies should be reevaluated to ensure that all gravitational effects have been taken into account.

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