STRATIGRAPHIC AND PETROGRAPHIC ANALYSIS OF THE LATE DEVONIAN-EARLY MISSISSIPPIAN LOWER AND UPPER BAKKEN SHALE MEMBERS, FOR THE DEVELOPMENT OF A SEQUENCE STRATIGRAPHIC FRAMEWORK

A definitive sequence stratigraphic framework has not been established for the Lower and Upper Bakken Shale members of the Bakken Formation, North Dakota, despite its extreme economic importance as the top unconventional source for oil production in the United States. The lower and upper shales are dark gray, organic-rich fine mudstones. Historically, sequence boundaries have been difficult to identify in fine-grained sedimentary rocks and this study provides new perspectives with regard to the sedimentary features that characterize these surfaces in the Bakken shales. The Bakken Formation was deposited in the intracratonic Williston Basin of North Dakota, South Dakota, Montana, and Saskatchewan. Three primary members make up the Bakken Formation, the Lower Bakken Shale, the coarser grained Middle Bakken Member, and the Upper Bakken Shale. At different locations in North Dakota, the Lower Bakken Shale unconformably overlies two older members of the Bakken - the Lower Bakken Silt and the Pronghorn Member. The Lower Bakken Silt is similar to the Bakken shales, but is more clay-rich and has much lower TOC.

Separate facies within the shale and silt members are not commonly described, but through sedimentologic analysis of the Bakken Formation from drill cores, subtle and distinct facies transitions are observed. Facies shifts are significant since they represent a change in depositional and diagenetic processes. Abrupt facies changes occur at erosional surfaces and flooding surfaces, the key surfaces that bound sequences and parasequences. Silt, pyrite, and conodont lag deposits are helpful indicators of significant surfaces. Recording changes in rock properties, such as hardness, was found to be a useful proxy for detecting subtle shifts in facies. Rock hardness changes in cyclical fashion and reflects changes in the amount early diagenetic siliceous cement and detrital clay. Application of above criteria helped to identify sequences and parasequences in the Lower Bakken Silt and Lower and Upper Bakken Shales. Further development of these concepts will aid in the development of a regional sequence stratigraphic framework and a better understanding of the depositional and diagenetic conditions that resulted in the accumulation of the finest grained strata of the Bakken Formation.