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Petrofacies and Depositional Systems of the Bakken Formation in the Williston Basin, North Dakota Anne Steptoe Late Devonian and Early Mississippian Bakken Formation in the Williston basin of North Dakota is a large emerging unconventional oil play that taps into reserves previously thought to be uneconomical to produce. The hydrocarbon source rocks and unconventional oil reservoir are widespread across the intracratonic basin with an estimated 3.7 billion barrels of undiscovered, recoverable oil, and has significant economic potential in portions of North Dakota, South Dakota, Montana, Saskatchewan, and Manitoba. Although the Bakken interval is the target of numerous horizontal wells throughout the basin, several uncertainties remain including the environment and distribution of depositional facies, especially in the shale members. An improved understanding of the dynamics of Bakken depositional facies is important to further explore and develop this unconventional reservoir. The Bakken Formation consists of a mixed clastics and carbonate middle member bounded by two black, organic-rich shale units. Generally, the upper and lower shale members are considered to have been deposited under relatively deep marine anoxic conditions (>200 meters depth). However, the underlying Sanish sand unit and the middle member of the Bakken have been interpreted as deposited in an epicontinental sea under shallow-water high-energy conditions (< 10 meters depth). The middle member of the Bakken is a complex stacked interval of dolomitic and calcareous siltstone, as well as oolitic or calcareous sandstone. To evaluate the inferred rapid changes in sea level, and the influence on depositional environments and production, subunits in the shale members were correlated for approximately 100 wells. The upper and lower Bakken shale members are divided into transgressive-regressive subunits that can be recognized by changes in lithology, mineralogy and sedimentologic properties, and are attributed to fluctuating depositional conditions. Log based solutions were also used to generate RHOmaa-Umaa cross plots and lithology composition graphs were generated for selected wells across the study area. In addition regional cross-sections and thickness maps were generated to further enhance the understanding of the vertical and lateral continuity of the depositional facies within the Bakken Formation. The result is an improved understanding of depositional patterns and basin evolution for the entire Bakken interval with emphasis on the shale units. Implementation of a transgressive-regressive stratigraphic framework in the Bakken subunits harbors a better understanding of the changing depositional conditions such as fluctuating sea level and complicated sediment accumulation patterns. Thus the improved understanding of the dynamics of the depositional facies in the Bakken Formation, including the shale units, is important to further explore and develop this unconventional reservoir to its best potential.