Current hydrodynamic theory is valid, in part, only for systems in which ground-water density is constant. Ground-water density is a function of salinity, temperature, and pressure. In the strict sense, constant density systems do not exist in nature on a regional scale.

A new concept of hydrodynamic analysis is presented in which actual flow--inducing pressure gradients are mapped. These gradients are, in reality, the flowing pressure forces that cause ground waters to migrate through rocks. Basic data needed to map these flowing pressure forces are (1) structural configuration of the aquifer, (2) ground-water density distribution in the aquifer, and (3) formation pressures. The flowing pressure gradients are both modified by and used to interpret changes in aquifer transmissibility (permeability-thickness/viscosity) between pressure control points.

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A comparison between actual flow-inducing pressure gradients in an aquifer and other representations of water potentials, such as h_w, indicates possible errors in depiction of flow rate and direction by the latter. By disregarding ground-water density variations, in some methods of analysis, we can seriously misinterpret hydrodynamic phenomena.

When the flowing pressure forces in an aquifer are known, it is a simple matter to derive oil potentials in terms of these new quantities. This is done in essentially the same manner as shown by Hubbert in l953. The mapping of flow-inducing pressure gradients provides us with a simpler and more accurate way of describing dynamic ground-water systems.

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