Upper Devonian — Lower Mississippian Stratigraphy of Northwestern Montana: A Petroleum System Approach
Schietinger, Paul; Sonnenberg, Stephen
The Upper Devonian - Lower Mississippian stratigraphy of northwestern Montana is the current focus of exploration for several petroleum companies. The Exshaw Formation of southern Alberta and northern Montana is Late Devonian - Early Mississippian and is stratigraphically equivalent to the Bakken Formation of the Williston Basin. The Exshaw and Bakken are all lithologically similar; each interval is composed of basal organic-rich shale, middle dolomitic siltstone to sandstone and upper organic-rich shale. The exploration into the Exshaw Formation and surrounding strata is largely due to the success of the Bakken Formation of the Williston Basin.
Deposition of the Upper Devonian - Lower Mississippian strata occurred in the tectonically active Antler foreland basin in northwestern Montana and appears to have been partially controlled by basement structures. Supratidal to open marine, continental shelf and possibly carbonate ramp environments characterize the deposition of the Three Forks, Exshaw and Lodgepole, respectively. The presence of each of these facies is highly variable and may be affected by erosional events associated with transgressions, regressions and tectonic uplift.
The organic-rich black shale within the Exshaw Formation is likely the primary source rock for the Upper Devonian - Lower Mississippian petroleum system while the Three Forks, middle Exshaw siltstone and lower Lodgepole carbonates are the most likely reservoirs. The Exshaw may reach thermal maturity to the west near thrust sheets within the Disturbed Belt. Successful exploration wells drilled in northern Montana in the past two years indicate potential for the petroleum system.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013