The Bakken petroleum system is an unconventional tight oil play containing both source and reservoir rock, with continuous saturation throughout much of the Williston basin in North Dakota and eastern Montana. With a continuous-type accumulation, and increased thickness of a source and/or reservoir rock can mean
an increase in hydrocarbons generated and expulsed from the source rock, and/or
an increase in storage capacity within the reservoir rock.
For this, and other reasons yet to be presented, thickness anomalies within the Bakken and Three Forks formations are of great interest to hydrocarbon exploration and production in the Williston basin.
Thickness anomalies have long been recognized in Devonian and Mississippian strata of the Williston basin in North Dakota and Montana, and the Canadian provinces of Saskatchewan and Manitoba. Dissolution of Prairie salt, collapse of overlying beds, and infill of the resultant accommodation space has been cited as one mechanism for the creation of thickness anomalies in the Williston basin (Anderson and Hunt, 1964; Demille et al., 1964; Holter, 1969; Langstroth, 1971; LeFever and LeFever, 2005; Oglesby, 1988; Parker, 1967). Several of these collapse structures form productive hydrocarbon traps. Fields such as Hummingbird field (Saskatchewan) and Outlook field (Montana) are examples of hydrocarbon traps within dissolution-collapse structures. Additionally, over-thickening of reservoir rock at Elm Coulee and Dickinson fields has been attributed to Prairie salt dissolution (LeFever et al., 1995; Sonnenberg and Pramudito, 2009). These examples not only highlight the importance of thickness anomalies for hydrocarbon exploration and production, but also the importance of Prairie salt-dissolution collapse structures.
The study area is located in north central North Dakota (Figure 1.1), and covers approximately 460 townships in Renville, Ward, Bottineau, McHenry, Rolette, Pierce, Towner, Benson, Sheridan, and McLean counties. Well logs were used from 186 wells within the study area that penetrate the Bakken or deeper strata to construct subsurface maps and cross-sections. This study area was chosen because it encompasses a cluster of thickness anomalies in the Bakken and Three Forks formations at or near their eastern zero edges.