High-Resolution Chemostratigraphy of the Bakken Formation, Williston Basin

Rowe, Harry¹ and Ruppel, Stephen C.
¹[email protected]

The Bakken Formation of North Dakota and Montana preserves a highly-resolved record of sea-level changes and water column redox variability in the Williston Basin during Devonian-Mississippian time. The Bakken is subdivided into upper and lower organic-rich mudrock members (TOC reaching 25%), and a middle dolomitic-calcitic siltstone unit (TOC <1%). It is difficult to differentiate sub-facies within the upper and lower members because of the fine-grained texture of the rock, the homogeneity in mineralogy, and the high organic content. However, high-resolution (~1-inch sample spacing) elemental analysis within the organic-rich mudrock units helps define large-scale shifts in the concentration of redox-sensitive trace elements, and major changes in mineralogy.

A drill core from northwest North Dakota, containing approximately ten feet of lower Bakken and twelve feet of upper Bakken, preserves a record of severe deep-basin water column reduction during relative sea-level highstands. The lower Bakken is characterized by a wide range in nickel (300 to 800 ppm), and molybdenum (200 to 800 ppm) concentrations, and punctuated, abrupt increases in concentrations of copper, zinc, and uranium (several hundred ppm). Slightly lower concentrations of these elements characterize the upper Bakken. Well-defined oscillation in the trace elements are reflected in TOC values, and are also observed in the major element concentrations. Results will be presented for three cores from the central Williston Basin.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013