Year of Award


Document Type


Degree Type

Master of Science (MS)

Other Degree Name/Area of Focus


Department or School/College

Department of Geosciences

Committee Chair

Marc S. Hendrix

Commitee Members

Michael H. Hofmann, Michael D. DeGrandpre


Bakken Formation, Williston Basin, sedimentology, eustacy, paleogeography


University of Montana

Subject Categories



The Late Devonian-Early Mississippian Sappington Formation in Montana is a marine unit comprised of lower and upper organic-rich shale members and a middle calcareous siltstone member. The Sappington Formation was deposited during a period of complex paleogeography in Montana, characterized by deposition in sub-basins and onlap onto structural highs, and eustatically- and tectonically-driven transgressive-regressive cycles. Detailed outcrop analysis was conducted on the Sappington Formation across the Bridger Range in southwestern Montana to better understand the Sappington Formation depositional system and changing regional paleogeography. The Sappington Formation is further interpreted in a stratigraphic architectural framework to improve the ability to predict hydrocarbon reservoir heterogeneity within Late Devonian-Early Mississippian strata more regionally.

Fourteen facies within the Sappington Formation are identified: 1) organic-rich mudstone and siltstone; 2) silty mudstone; 3) clay-rich, calcareous siltstone; 4) quartzose siltstone, 5) interlaminated siltstone and mudstone; 6) lenticular siltstone and mudstone; 7) wavy siltstone and mudstone; 8) combined flow siltstone; 9) ripple laminated siltstone; 10) convoluted siltstone; 11) tabular siltstone; 12) low-angle-stratified sandstone; 13) fossiliferous dolomite; and 14) oncoid-bearing floatstone. Genetically related facies are assigned to facies associations that generally represent deposition along a wave-storm-dominated prograding shoreface-shelf system sourced from the Beartooth Shelf to the south. Stratigraphic sequences, surfaces, and systems tracts are interpreted based on facies relationships, depositional processes, and regional stratal stacking patterns. The sequence stratigraphic framework includes two full depositional sequences, the oldest including a TST and HST, the second including a TST, HST and FSST, and a third sequence containing a TST and HST continuing into the overlying Lodgepole Formation. Depositional sequences are interpreted to be controlled by glacioeustatic, third-order sea level fluctuations, whereas basin geometry and configuration is inferred to be tectonic in origin.

Analysis of facies stacking and stratigraphic architecture indicate significant lateral lithologic heterogeneity on the field and reservoir scale. Observed facies heterogeneity and architectural complexity of the Sappington Formation may help explain hydrocarbon production heterogeneity of the contemporaneous Bakken Formation in the Williston Basin and might have strong implications for new development and secondary recovery for the Bakken Formation in the Williston Basin.

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© Copyright 2015 Anna S. Phelps