Abstract The Bakken Formation is currently regarded primarily as a self-contained, unconventional petroleum system. While previously viewed as a source for oil occurring in overlying formations, it is now predicted that resources of more than 3.5 billion barrels of oil are trapped intraformationally. New insights into the formation's open v. closed nature are presented here using the physical properties of natural petroleum, source rock characteristics and the numerical modelling of phase behaviour. In the mature western part of the basin petroleum accumulations have been postulated to be continuous in nature, characterized by very short migration distances of indigenous hydrocarbons. This necessitates that the composition and therefore physical properties of the generated hydrocarbons must be controlled by the maturity of the source rock in the immediate vicinity. This assumption is not supported by the clustering of higher gas–oil ratios and lighter oil gravities along the locations of the anticlines in the basin. We have used open and closed system pyrolysis techniques to predict the bulk composition of the petroleum generated at different transformation stages, both cumulatively and instantaneously. Based on these predictions the Bakken would contain dominantly undersaturated fluids throughout the basin. Differences in predicted GORs of cumulative and instantaneous models support the conclusion that the reported hydrocarbon compositions cannot completely be explained by a tight self-contained petroleum system. The observed variability of in-place hydrocarbon compositions is readily explained by lateral migration of petroleum in the main middle Bakken carrier, and vertical leakage of emplaced hydrocarbons from the fractured reservoir at anticline locations. This has resulted in the loss of the early generated petroleum, and led to a present-day dominance of late generation products. These results reveal that the Bakken Formation is a partly open petroleum system, at least along the major anticlines of the Williston Basin.

Abstract Plate tectonics forms and destroys sedimentary basins, accumulating organic carbon and converting it into mobile petroleum which may be concentrated in reservoir traps in which, if temperatures are below 80°C, it may become biologically degraded (biodegraded). The biodegradation process produces altered, denser, heavy oils and methane as a primary product. Much of the world’s oil is biodegraded under anaerobic conditions, with methane being a major by-product of the action of the deep biosphere on petroleum when sulphate is not present as an oxidant. A review of the literature relating to destruction of wet gas and the systematics of methane generation during subsurface oil biodegradation concludes that large biodegrading oil fields may be major source systems of dry gas.