ABSTRACT

Cross-basinal hydrodynamic flow pervasively affects the oil accumulations and fluid migration history of the Bakken-Madison petroleum system of the Williston Basin. The Madison freshwater potentiometric surface is gently tilted across the basin about 10-15 ft/mi (2-3 m/km) to the east and northeast.

Madison oil fields around the basin show tilted oil-water contacts which closely match in magnitude and direction the tilts predicted from the regional hydraulic gradient. In the Billings nose fields, the tilts of the oil-water contacts are 25 ft/mi (5.6 m/km) to the east-northeast. Madison fields of the Nesson anticline are hydrodynamically modified, and hydrodynamic flow appears to be the main trapping mechanism for the oil. Along the Nesson anticline the oil-water contacts are tilted between 25 ft/mi and 60 ft/mi (5 and 12 m/km) to the east and southeast. In Montana, at Poplar field, the oil water contact is tilted 20 ft/mi (4 m/km) to the north-northeast.

Evolution of the Bakken-Madison petroleum system occurred in two phases: a late Cretaceous through Paleocene hydrostatic phase; and an Eocene to Recent hydrodynamic phase. The system was hydrostatic when the Bakken began oil generation in the latest Cretaceous, prior to Rocky Mountain uplift. The system became hydrodynamic when the Rocky Mountains were uplifted in the Laramide, synchronous with peak Bakken oil generation in the Eocene. Hydrodynamic flow profoundly modified both primary and secondary oil migration pathways during the early and Middle Tertiary.

Since the Oligocene, very little Bakken oil has been generated, yet oil in many fields is trapped by cross-basinal flow. Therefore, hydrodynamic flow and Rocky Mountain paleotopography must have persisted since late Eocene time.