FACIES CHARACTERIZATION AND STRATIGRAPHIC ARCHITECTURE OF THE DEVONIAN/MISSISSIPPIAN SAPPINGTON FORMATION (BAKKEN EQUIVALENT STRATA) IN WESTERN MONTANA – AN OUTCROP BASED PERSPECTIVE

The Bakken/Three Forks systems is a the largest source rock continuous oil play in North America, with estimated undiscovered recoverable resources in the Williston Basin of 7.59 BBO. Despite the vast amounts of hydrocarbons in the basin, production results are highly variable throughout the Williston Basin, indicating that these rocks, like conventional reservoirs, display considerable geological heterogeneity.

Outcrop analog studies provide a unique opportunity to contribute to the understanding of the types, controls, and distribution of these facies heterogeneities. In western Montana, rocks of the Bakken equivalent Sappington Formation preserve a wide variety of sedimentologic characteristics and facies reflecting complicated and interrelated processes of sediment production and dispersal. The Sappington Formation contains fourteen depositional facies ranging from organic-rich, fine -grained mudrocks (max. 14.8% TOC) to silty, very fine-grained sandstones in the middle Bakken member, the main producing interval in the Bakken/Three Forks petroleum system. Taking into account the diachroneity of facies belts, we infer that coarser grained siltstone and very fine-grained sandstone facies of the middle Bakken member represent proximal deposits related to prograding shoreface systems whereas finer-grained silt- and mudstones were deposited contemporaneously in more distal environments sourced from local sources. Abundance of hummocky cross stratification in siltstone and very fine grained sandstone from several localities suggest that storms were an important mechanism for dispersing siliciclastic sediment across the shelf, whereas a diverse suite of trace fossils suggests subsequent reworking by a robust infauna in a largely open marine environment.

The results from our outcrop work provide a framework for the facies heterogeneity and reservoir architecture of the Sappington Formation and as such allowing the development of predictive models of the distribution of key reservoir properties for the Bakken Formation in the Williston Basin.