The Devonian-Mississippian Sappington Formation in the Bridger Range, Montana: An Outcrop-Based Reservoir Analog for the Bakken Formation in the Williston Basin

Abstract

Deposition of the Devonian-Mississippian Sappington Fm. is contemporaneous with the Bakken Fm. in the Williston Basin. Despite good drilling success rates, hydrocarbon production rates across the Williston Basin are highly variable and at present, many of the geologic controls behind this heterogeneity are poorly understood. The Sappington Fm. in the Bridger Range (BR), MT offers an opportunity to study the architecture and facies heterogeneity of Bakken-equivalent strata at the surface. Here we present results from an outcrop study along a 17 km transect in the BR to discern the lateral lithologic heterogeneity and architecture on the reservoir and field scale. The Sappington Fm. is divided into three members: the lower and upper organic-rich shale members and a middle calcareous, siltstone to very fine-grained sandstone member. Facies in the lower and upper shale members include: organic-rich mudstones; muddy, organic-rich siltstones; and silty, bioturbated, organic-rich mudstones. These members are interpreted as being deposited in a dysoxic (anoxic?), semi-restricted offshore environment. Middle Sappington member facies include cross-stratified and rippled sandstones, and bioturbated sandstones and siltstones. Normal-marine assemblages of Cruziana ichnofauna and cross- and ripple-stratification suggest deposition in a normal-marine shoreface environment. The Sappington Fm. thickens from 16 m in the south to 23 m in the northern BR. Lithofacies relationships change laterally across the BR and contribute to a complex stratigraphic architecture. The middle member of the Sappington Fm. in the southern BR displays more proximal facies with larger, planar, and high angle cross-stratified bedforms and the highest abundance of the coarsest grained sand in the system. In the northern BR, facies of the middle Sappington member contain smaller ripple lamination and are more heavily bioturbated. Conversely, the upper and lower shales thin in a southerly direction and spectral GR data suggests a lower organic content in the southern locations. The facies distribution and thickening trends, combined with paleoflow analysis, suggest Sappington Fm. shoreface deposition in the BR, with sediment transport directed in a northerly direction. Heterogeneity of facies and architecture of sedimentary elements observed along this depositional dip section provide insight into the geologic controls of reservoirs and the fate of preserved organic carbon on a development scale.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015