Investigations of underpressured basins can be improved by identifying factors which impede accurate interpretation of pressure-depth (P-D) plots. The majority of P-D data from drill-stem tests are poor quality and are biased in distribution toward exploration and production; thus, true P-D trends, are obscured. Plotted P-D data may not only reflect flow conditions and underpressuring, but they can also be distorted by the hydrogeologic setting, i.e., surface topography, basin structure, and potentiometric surface. For example, surface topography can cause plotted data to appear more underpressured than it really is. Varying surface topography in conjunction with basin structure can cause P-D gradients to erroneously indicate potential cross-formation flow. If the effects of the hydrogeologic setting are not identified, P-D trends may be misinterpreted.

In the Palo Duro basin, the problems of the hydrogeologic setting are reduced by delineating regions with little variation in surface topography and structure. The P-D plots for these different regions vary considerably. To evaluate the effects of the hydrogeologic setting on these P-D plots, theoretical models of hydrostatic conditions were computed for the hydrogeologic setting of each region. Comparison of regression lines through real P-D data to those through modeled data distinguishes true hydrologic conditions from the misleading effects of the hydrogeologic setting.

P-D plots indicate regional underpressuring in Palo Duro basin. Variations in potential vertical flow within the underpressured basin are indicated by variations in P-D gradients across the basin. Potential upward flow is indicated along the western flanks of the Amarillo uplift, while potential downward flow is indicated in the southwestern and eastern parts of the basin.

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