2014 Rocky Mountain Section AAPG Annual Meeting

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Facies Model and Sequence Stratigraphic Framework of the Devonian-Mississippian Sappington and Bakken Formations in Montana


The Devonian-Mississippian Bakken/Three Forks Formations in the Williston Basin, in Montana and North Dakota, are one of the most prolific petroleum systems in the United States, but production results are highly variable, indicating that these rocks, like conventional reservoirs, display considerable geological heterogeneity. In western Montana, rocks of the Bakken equivalent Sappington Formation - the middle and lower Bakken members and the Sappington Formation contain conodonts indicative of the Upper Devonian Expansa Zone and Lower Mississippian Praesulcata Zone - preserve a wide variety of sedimentologic characteristics and facies reflecting complicated and interrelated processes of sediment production and dispersal. The Sappington Formation contains facies ranging from organic-rich, fine-grained mudrocks (max. 14.8% TOC) to silty, very fine-grained sandstones in the middle Sappington member. Abundance of hummocky cross stratification in siltstone and very fine-grained sandstone from several localities suggest that storms were an important mechanism for dispersing sediment across the shelf, whereas a diverse suite of trace fossils suggests subsequent reworking by a robust infauna in a largely open marine environment. Taking into account the diachroneity of those facies belts, we identified four genetically related sequences in the Sappington Formation. Each sequence is bound by sequence stratigraphic significant surfaces that were also identified in well log data throughout Montana. The results from our outcrop work provide a framework for the facies heterogeneity and reservoir architecture of the Sappington Formation and as such allowed for the development of a sequence stratigraphic framework also applicable in other parts of Montana and the Williston Basin.