ABSTRACT

This study describes the sedimentology and stratigraphy of the Devonian–Mississippian Sappington Formation in exceptional outcrop exposures in southwestern Montana. The goal was to assess the extent to which these outcrops could be used to define stratigraphic heterogeneity of time-equivalent Bakken Formation reservoir and source rocks of the Williston Basin.

Facies analysis of the Sappington Formation shows three depositional sequences deposited along a storm- and wave-influenced coastline. The first sequence encompasses the lower Sappington shale, dominated by organic-rich mudstones that represent an anoxic to dysoxic lower-shelf environment. The second sequence is marked by a basal transgressive surface and initial siltstone deposition. These siltstones are abruptly overlain by basinward-dipping, low-angle clinoforms (<1°) that represent a storm- and wave-influenced shoreface. Facies distributions along clinoforms change along depositional dip over approximately 17 km (∼11 mi) from proximal to distal across the study area. The best reservoir facies are present in the upper foresets of these clinoforms. The upper shale of the Sappington Formation forms the lowest part of the third depositional sequence that continues into the overlying Lodgepole Limestone.

The outcrop-based stratigraphy of the Sappington Formation provides insights into the scale of stratigraphic heterogeneity that is likely to be present in the time-equivalent Bakken Formation over the length of a typical approximately 3-km-long (∼2-mi-long) horizontal well. Geosteering horizontal wells in the Bakken along bedding surfaces, that is, along gently dipping clinoforms, is likely to cause variations in rock properties from the toe to heel of a horizontal well.

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