Institut Francais du Petrole's two-dimensional model, temispack, is used to discuss the functioning of petroleum systems in the Williston basin along a 330-km-long section, focusing on four regional source intervals: Ordovician Yeoman formation, Lower Devonian Winnipegosis Formation, Upper DevonianuLower Mississippian Bakken Formation, and Mississippian Lodgepole formation. Thermal history calibration against present temperature and source rock maturity profiles suggests that the Williston basin can be divided into a region of constant heat flow of about 55 mW/m² away from the Nesson anticline, and a region of higher heat flow and enhanced thermal maturity in the vicinity of the Nesson anticline. Original kinetic parameters used in the calibration were derived or each of the four source rocks from Rock-Eval yield curves. Bakken overpressures are entirely due to oil generation, not compaction disequilibrium. Very low Bakken vertical permeabilities range from 0.01 to 0.001 nd are matched against observed overpressures, whereas Bakken porosities based on the model and confirmed by measurements are inferred to be also unusually low, around 3%. Mature Bakken shales do not seem to have reached hydraulic fractionation thresholds, except perhaps locally in regions of extensional tectonic stress. Hydraulic fracturing cannot be viewed as a pervasive mechanism driving Bakken oil expulsion. Our expulsion model confirms the high residual Bakken oil saturations and explains the low residual S¹/TOC by the low Bakken shale porosities. Approximately 85% of the oil generated in the Bakken shales is predicted to have been expelled out of Bakken strata, which agrees with observed S¹/TOC trends. Madison Group accumulations at the subcrop edge are found to be fed by Lodgepole-sourced oil only, in particular by the rich middle Lodgepole interval. These accumulations probably result from a three-dimensional migration pattern. Madison accumulations in the Nesson anticline are found to be fed mostly by Lodgepole-sourced oil mixed with minor amounts of Bakken-sourced oil. The vast majority of the expelled Bakken oils are lost in the Madison Group at very low saturations. This explains the low conventional oil resource associated with Bakken sources by recent geochemical studies. Expulsion and migration occurred no earlier than the atest Cretaceous-Paleocene in the Yeoman and no earlier than the Eocene in other source rocks, consistent with temporal controls on oil migration. Flow models show no restriction to expulsion and migration of Yeoman oil. This suggests a potential undiscovered oil resource in Ordovician and Silurian strata of Williston basin, northeast of the Nesson anticline.