Hydrodynamic salinity gradients occur in aquifers where lateral salinity changes are caused by regional water flow. Hydrodynamic salinity gradients are highly favorable for oil entrapment in areas where less-saline waters flow downdip to replace more-saline waters because the “tilt amplification factor” increases in updip areas where the oil–water contact tilt may exceed the regional structural dip and induce basinward oil displacement. This can concentrate oil by downdip remigration. Downdip barriers, such as monoclines, may be the dominant structural control. Composite hydrodynamic accumulations consist of oil-productive areas that may not be interconnected but have a common, hydrodynamically tilted, free-water level. They form in regions where the oil–water contact tilt is similar in magnitude and direction to the regional dip. In the southwestern part of the Williston Basin, structurally modified, composite hydrodynamic accumulations that lie within brackish-water to saline-water hydrodynamic salinity gradients occur in the Mississippian Madison Group and Ordovician Red River Formation reservoirs. These oil accumulations have average oil–water contact tilts that range from 22 to 80 ft/mi (4 to 15 m/km) toward the northeast. Individual composite oil accumulations can cover areas larger than 300 mi² (777 km²) and hold at least 1.6 billion bbl of oil-in-place.