Abstract: Effect of Cross-Basinal Hydrodynamic Flow on Oil Migration and Accumulations in Bakken-Madison (Mississippian) Petroleum System; Williston Basin, North America

William D. Demis

Cross-basinal hydrodynamic flow pervasively affects oil accumulations, play types, and oil-migration history of the Bakken (and other source rocks)-Madison petroleum system of the Williston basin.

The Madison aquifer contains fresher water (<20,000 ppm dissolved solids) on the west and southwest sides of the basin. The central basin contains brine (>200,000 ppm dissolved solids). Previously published computer-based simulations of the Madison aquifer shows the potentiometric surface is tilted east across the basin at 10-15 ft/mile (2-3 m/km).

New data on oil column tilts for 12 major Madison fields around the basin show orientations roughly coincident with the simulated potentiometric surface. Within the central brine lens, all of the major oil fields of the Nesson anticline are shifted to the east side of the structure by hydrodynamic flow. Tilts range 50 ft/mi (9.5 m/km) to 15 ft/mi (3 /km) E. Fields are hydrodynamically tilted stratigraphic/structural traps. In the south-central basin, tilts range from 20 ft/mile (2 m/km) E to 30 ft/mi (5.6 m/km) ENE, from Lone Butte to Elkhorn Ranch fields. Fields range from hydrodynamically enhanced stratigraphic to pure hydrodynamic traps. On the west side of the basin, Mondak field, a fractured carbonate trap, occurs on the edge of, and is hydrodynamically enhanced by, the invading lume of fresher Madison water.

Geochemical analysis of oil from Madison reservoirs in Cedar Creek anticline document a Bakken source. Thus, the southwest part of the basin was charged with Bakken oil but later flushed by early Tertiary hydrodynamic flow. The Cedar Creek fault created a local potentiometric "shadow" which preserves this locally unique Bakken oil.

Hydrodynamic flow was active during (or immediately after) oil generation in the Eocene and profoundly modified primary and/or secondary migration. Oil migration maps constructed assuming hydrodynamic conditions accurately match the observed distribution of shallow oil the northeast side of the basin, and a flushed southwest side.

Early to middle Tertiary hydrodynamic flow implies the Rocky Mountains were topographically high throughout the Tertiary. New, recently published paleobotanical and apatite fission track data independently corroborate Rocky Mountain paleotopography since the Oligocene.

A thorough understanding of basin-wide hydrodynamic flow on the Bakken-Madison petroleum system provides a valuable analog for frontier basins of the world.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994