PALYNOMORPH-BASED CHRONOSTRATIGRAPHY OF THE PRONGHORN MEMBER, BAKKEN PETROLEUM SYSTEM FROM MOUNTRAIL COUNTY, ND CORE

The Bakken petroleum system of the Williston Basin in North Dakota and Montana includes the Late Devonian-Early Mississippian Bakken Formation and the Late Devonian Three Forks Formation. Palynological analyses of this system have focused predominantly on the upper, middle, and lower Bakken, reaffirming Late Famennian and Tournaisian ages (Pocknall and Hohman 2015). Further, they chronicled timing of successions in depositional environment and basin morphology. As the field has developed and subsurface explored, the economic potential of the Three Forks reservoirs became apparent and the lower Bakken was further subdivided to differentiate basal Bakken and the Pronghorn Member. Biostratigraphic assessment of the Pronghorn Member and Three Forks Formation has been difficult due to the low abundance of macrofossils and apparent absence of microfossils. However, high density sampling of the Pronghorn Member and Three Forks Formation from Petrogulf Corp. Three Tribes 151-93-3H core in Mountrail County, ND, in the center of the Williston Basin, has yielded palynological data from laminated black shale and siltstone. Preliminary palynological data from the Pronghorn member of the Mountrail County core reveals a large population of acritarchs, although preservation is variable. Identifiable acritarchs from the top of the Pronghorn member include Leiosphaeridia and Gorgonisphaeridium. The lower Pronghorn member, although sparsely populated, contains well preserved acritarchs such as Gorgonisphaeridium ohioense. This palynoflora population has taxa found in the lower and middle Sappington Formation through the Trident Member of the Three Forks Formation of west-central Montana. The collections correlate to the upper Famennian VCo palynozone (lower expansa conodont zone).