Stratigraphy and Sedimentology of the Sappington Formation: Validating Sappington Outcrops as an Exposed Analog for the Bakken Subsurface

Abstract

Widely exposed in south-central Montana; the Sappington Formation comprised of lower and upper shale members that bracket a middle, sandy siltstone member; has long been considered generally correlative to the similarly partitioned Bakken Formation of the Williston Basin. Leveraging over 50 outcrop exposures and 30 nearby wells in addition to 5 cored boreholes, a comprehensive sequence stratigraphic framework for the Sappington has been constructed along with an analysis of the facies architecture. Extensive regional work ties this new Sappington framework to a Bakken sequence stratigraphic framework, detailing the stratigraphic relationships between these two formations and their component facies. Comparing the Sappington and Bakken reveals a succession of four correlative sequences. In ascending stratigraphic order, these sequences include the Lower Sappington and Lower Bakken sequences dominated by black shale, overlain by two stacked, sandy siltstone-dominated sequences corresponding to 1) the lower Middle Sappington and lower Middle Bakken sequences, and 2) the upper Middle Sappington and upper Middle Bakken sequences. Capping the succession are Upper Sappington and Upper Bakken sequences consisting of black shale and localized siltstone. Although exposures of the upper Middle Sappington sequence have not yet been identified and to date remains relegated to the subsurface, the lower Middle Sappington sequence and its three component systems tracts are well exposed and exhibit facies representative of those found in both the Middle Bakken sequences. The similarity between the Sappington and Bakken facies includes grain size, composition, texture and diagenesis across a depositional profile characterized by marine offshore, lower and upper shoreface, and foreshore deposits with localized backshore eolian deposits. The significance of our studies of the Bakken-equivalent Sappington Formation is that it provides detailed insights into the subsurface Bakken that are not possible with subsurface data alone. Only outcrop exposure can provide the continuous, detailed, lateral and vertical observations necessary to document the many levels of heterogeneity that typify this reservoir at regional and DSU scales. Incorporating this heterogeneity into STOOIP calculations and reservoir performance modeling is a critical future need for the Bakken play.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016