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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

MTGS-AAPG

Seventh International Williston Basin Symposium, July 23, 1995 (SP12)

Pages 235 - 251

Resolution of Williston Basin Oil System Paradoxes Through Basin Modelling

Jean Burrus, Institut Francais du Petrole, Rueil-Malmaison, France
Kirk Osadetz, Geological Survey of Canada, Calgary, Canada
Sylvie Wolf, Institut Frangais du Petrole, Rueil-Malmaison, France
Brigitte Doligez, Institut Frangais du Petrole, Rueil-Malmaison, France
Kees Visser, Petro-Canada Resources, Calgary, Canada
David Dearborn, Petro-Canada Resources, Calgary, Canada

ABSTRACT

Recently described Williston Basin oil systems show two paradoxical relationships. First, the rich uppermost Devonian-Mississippian Bakken Formation source has contributed much less than the leaner Mississippian Lodgepole Formation source to known Mississippian Madison Group oil pools. Second, the rich and mature Upper Ordovician Bighorn Group sources have not fed many oil pools in the northeastern part of the basin. Bakken shales have an overpressure due to hydrocarbon generation and low vertical permeabilities. Low Bakken permeabilities accompany low porosities, which can be shown to be saturated with residual oil. The small storage capacity of Bakken shale porosities, but the large amounts of hydrocarbon generation in Bakken shales requires the 'forced expulsion' of much Bakken-sourced oil. This oil remains generally dispersed in the drainage system and unlike Lodgepole-sourced oils, seldom accumulates to form significant pools; this explains the paradox of Bakken and Lodgepole effectiveness. Models suggest that hydraulic fracturing thresholds were not reached without the addition of an extensional tectonic stress. In contrast, hydrostatic pressures in Ordovician sources indicate oils were expelled into Bighorn and Inter lake Group strata. These oils migrated into northeastern Williston Basin prior to the Nesson anticline becoming a significant barrier to migration. These oils have probably accumulated in undiscovered pools.

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