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AAPG Bulletin, Preliminary version published online Ahead of Print 1 August 2023.

Copyright © 2023. The American Association of Petroleum Geologists. All rights reserved.


Hydrocarbon trapping in hydrodynamic salinity gradients: Williston Basin case studies

David M. Petty

Ahead of Print Abstract

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 southwest portion of the Williston basin, structurally modified, composite hydrodynamic accumulations that lie within brackish-water to saline-water hydrodynamic salinity gradients occur in Mississippian Madison and Ordovician Red River reservoirs. These oil accumulations have average oil-water contact tilts that vary from 22-80 ft/mi (4-15 m/km) toward the northeast. Individual composite oil accumulations can cover areas larger than 300 sq mi (777 sq km) and hold at least 1.6 billion barrels of oil-in-place.

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Please cite this AAPG Bulletin Ahead of Print article as:

David M. Petty: Hydrocarbon trapping in hydrodynamic salinity gradients: Williston Basin case studies, (in press; preliminary version published online Ahead of Print 01 August 2023: AAPG Bulletin, DOI:10.1306/02242322092.