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Sedimentology and Petroleum Potential of the Devonian/Mississippian Three Forks and Bakken Formations and Equivalent Strata in Central and Western Montana

Hofmann, Michael H.; Hendrix, Marc S.; Nagase, Tetsuro

The Devonian/Mississippian Bakken Formation is a prolific producer of hydrocarbons across much of the Williston Basin in western North Dakota and eastern Montana. The success of the Bakken play in the Williston Basin has sparked more recent interest in exploring time equivalent strata to the west in frontier areas in central and north-central Montana.

To evaluate the hydrocarbon potential of these upper Devonian and lower Mississippian rocks and to predict their distribution in Montana, we integrated results from detailed analysis of outcrops, core, geochemistry, mineralogy, and well log character.

In Montana, rocks of the Bakken and Three Forks Formations preserve a wide variety of sedimentologic characteristics that reflect complex local changes in depositional environment. Sediments are commonly intensely bioturbated, contain abundant siliciclastic detritus, and are characterized by highly variable weight percentages of organic matter and carbonate.

In general, lithofacies variability is interpreted to reflect deposition of a mixed carbonate-clastic sedimentary system within a generally oxic foreland basin to the west and a well aerated shelfal environment to the east. Taking into account the diachroneity of facies belts, we infer that coarser grained siltstone facies represent proximal deposits related to prograding shoreface systems whereas finer grained 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 in the foreland basin and across the shelf, whereas a diverse suite of trace fossils suggests subsequent reworking by a robust infauna.

Our analysis of Devonian/Mississippian outcrops in Montana suggest the presence of several sub-basins that contain organic rich mudrock intervals genetically related to coarser grained lithofacies in other parts of the basin. Based on preliminary thin-section and mineralogic analyses, some of these sub-basins are interpreted to have a potential for hydrocarbon generation in areas outside the Williston Basin and southern Alberta Basin.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013