Hydrodynamic effects on oil accumulations generally can be recognized at an early stage of exploration but become of critical importance with increased drilling and discoveries. At the mature stage, hydrodynamic concepts can be readily applied in exploration and development to reduce risk and to increase success ratios. The south flank of the Williston Basin is an example of a mature area with significant hydrodynamic effects on oil accumulations. Early exploration was aimed at stratigraphic traps in the Mission Canyon Formation, but the development of major fields showed that all are strongly influenced by hydrodynamic flow and some oil accumulations may be largely independent of porosity pinchouts.

Examples of hydrodynamic effects are illustrated by the Billings Nose fields, and the Elkhorn Ranch and Knutson fields. All of these accumulations have hydraulic gradients on the order of 10 ft./mi. (2 m/km) or more; tilted oil-water contacts with gradients of about 25 ft./mi. (5 m/km); displacement of oil downdip to the northeast; and variable formation-water salinities that range from nearly fresh to highly saline. Some producing zones have been described as purely hydrodynamic traps, lacking both structural and stratigraphic closure. Future success will depend on applying hydrodynamic concepts in exploration and development drilling, and prediction methods are illustrated by possible extensions to existing one-well fields. Simple graphic techniques can estimate the limits of production before drilling, but a knowledge of local structure is most important to the interpretation.

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