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ABSTRACT: Exploration for Stratigraphic Traps in a Mature Hydrodynamic Setting, Williston Basin, North Dakota

Robert R. Berg

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 readily can be 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 accumulation. 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 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. These accumulations have hydrodynamic gradients on the order of 20 ft/mi (4 m/km) or more; tilted oil-water contacts with gradients of 30 to 50 ft/mi (6 to 10 m/km); displacement of oil downdip to the northeast; and variable formation-water salinites 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, and prediction methods are illustrated by possible extensions to existing one-well fields. Simple graphic techniques can estimate the limits of production before rilling, but a knowledge of local structure is most important to the interpretation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990