One of the generally accepted inferences with regard to the Williston basin has been that it should be relatively free from hydrodynamic influences. However, a large proportion of the pools in this basin have inclined oil-water contacts and, in some, tilting is an essential feature of the trap. Two excellent examples of such fields are the Poplar pool in northeastern Montana and the North Tioga pool on the north end of the Nesson trend. The reservoir in the Charles formation at Poplar has a readily demonstrable tilt in the oil-water contact of approximately 40 feet per mile north-northeast. At North Tioga a dip of the same order of magnitude, but toward the southeast, is apparent in the water table in the Mission Canyon formation. In both places, log and sample studies sh w that the tilting can not be ascribed to an "apparent condition" arising from stratigraphic changes. The tilt at Poplar is merely an interesting aberration in an essentially structural accumulation. On the other hand, hydrodynamics is a necessary component of the trap at North Tioga.

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