The Bakken shale oil play in the Williston Basin of North Dakota and Montana is one of the most prolific resource plays in the U.S. Here we describe play-wide geological and petrophysical characterization of the Bakken petroleum system as a context for determination of oil in place and geologic drivers of productivity. We mapped Bakken structure and stratigraphy using geophysical well logs and core-based lithologies. The Bakken structural configuration is relatively simple, forming a bowl-shaped depression. Several large, north- to northwest-trending anticlines are the main structures affecting Bakken production. The Bakken petroleum system (Devonian to Mississippian) encompasses the Bakken Formation, upper part of the underlying Three Forks Formation, and lower part of the overlying Lodgepole Formation. The source rocks of the Bakken petroleum system are the Lower and Upper Bakken shales, which have exceptionally high organic matter content. The Middle Bakken is the primary reservoir interval. Laminated sandstone/siltstone forms the best reservoir facies in the Middle Bakken. The Upper Three Forks, which is also an important reservoir interval, is composed of silty to sandy dolostone and finely laminated green mudstone. The Bakken unconventional petroleum system coincides geographically with the oil window (thermally mature area). Within the oil window, Bakken shales generate oil that either stays in adjacent Middle Bakken and upper Three Forks reservoirs (unconventional) or migrates out of the oil window (conventional). Our study focuses on the Bakken unconventional petroleum system. We developed interpretive petrophysical models using an optimized multilog solver calibrated with core data. The log models provide computed mineral volumes, kerogen, porosity, and water saturation. Results of geologic mapping and petrophysical analysis were combined to quantify and map hydrocarbon pore volume in preparation for mapping oil in place. We also constructed a three-dimensional (3D) geocellular model that incorporate Bakken geology, petrophysics, and well histories. The 3D model yields insights on basin-wide geology as well as per-well production behavior.

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