The Williston Basin Famennian (Devonian) Shale Succession: Stratigraphic Units and Facies of the Lower Bakken and Upper Pronghorn Shales

Abstract

The Devonian succession typified by the Lower Bakken Shale and Upper Pronghorn Shale comprises two stacked, unconformity-bounded sequences. The Upper Pronghorn sequence represents the initial accumulation of Famennian shales in the Williston Basin and culminates with the organic-rich Lower Bakken sequence representing the final accumulation. The Lower Bakken sequence can be further subdivided into three system tracts separated by two major flooding surfaces. Detailed analyses within this stratigraphic framework have identified geographic and stratigraphic variations in source rock facies within and between these three units. Incorporating detailed mudrock litho-stratigraphy, invertebrate and conodont biostratigraphy, palynology, source rock geochemistry, XRF chemo-stratigraphy, thin section petrography, cores and well logs; a Williston Basin-derived Famennian black shale facies model was developed. This facies model is largely governed by onlap of the deposits upon the flanks of the basin. Consequently, facies belts reflect a proximal to distal transition from siltier, less organic-rich deposits with cement-filled Tasmanites that characterize the flanks of the basin to more organic-rich, pyritic, radiolaria bearing deposits that characterize the basin center.

The timing of major depositional events in the Williston Basin Famennian shale systems corresponds with other North American and global events. This agreement between event stratigraphy in the Williston Basin, Sappington Basin, South Alberta Basin, Appalachian Basin, and European carbonate successions strengthens the confidence in the stratigraphic framework and depositional models. Additionally, the repeatability of facies stacking patterns in the Famennian shales from the Sappington Basin and the Appalachian basin also suggest the robust applicability of the Williston basin-derived black shale facies model in other Devonian shale basins.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018