A 2-DIMENSIONAL REGIONAL BASIN MODEL OF WILLISTON BASIN HYDROCARBON SYSTEMS

Institut Francais du Petrole's two-dimensional model, TEMISPACK, is us ed to discuss the functioning of petroleum systems in the Williston ba sin along a 330-km-long section, focusing on four regional source inte rvals: Ordovician Yeoman formation, Lower Devonian Winnipegosis Format ion, Upper Devonian-Lower Mississippian Bakken Formation, and Mississi ppian Lodgepole formation. Thermal history calibration against present temperature and source rock maturity profiles suggests that the Willi ston basin can be divided into a region of constant heat flow of about 55 mW/m(2) away from the Nesson anticline, and a region of higher hea t flow and enhanced thermal maturity in the vicinity of the Nesson ant icline. Original kinetic parameters used in the calibration were deriv ed for each of the four source rocks from Rock-Eval yield curves. Bakk en overpressures are entirely due to oil generation, not compaction di sequilibrium. 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 infer red to be also unusually low, around 3%. Mature Bakken shales do not s eem to have reached hydraulic fractionation thresholds, except perhaps locally in regions of extensional tectonic stress. Hydraulic fracturi ng cannot be viewed as a pervasive mechanism driving Bakken oil expuls ion. Our expulsion model confirms the high residual Bakken oil saturat ions and explains the low residual S-1/TOC by the low Bakken shale por osities. Approximately 85% of the oil generated in the Bakken shales i s predicted to have been expelled out of Bakken strata, which agrees w ith observed S-1/TOC trends. Madison Group accumulations at the subcro p edge are found to be fed by Lodgepole-sourced oil only, in particula r by the rich middle Lodgepole interval. These accumulations probably result from a three-dimensional migration pattern. Madison accumulatio ns in the Nesson anticline are found to be fed mostly by Lodgepole-sou rced oil mixed with minor amounts of Bakken-sourced oil. The vast majo rity of the expelled Bakken oils are lost in the Madison Group at very low saturations. This explains the low conventional oil resource asso ciated with Bakken sources by recent geochemical studies. Expulsion an d migration occurred no earlier than the latest Cretaceous-Paleocene i n the Yeoman and no earlier than the Eocene in other source rocks, con sistent with temporal controls on oil migration. Flow models show no r estriction to expulsion and migration of Yeoman oil. This suggests a p otential undiscovered oil resource in Ordovician and Silurian strata o f Williston basin, northeast of the Nesson anticline.