Upper Devonian and Lower Mississippian stratigraphy of Northwestern Montana: a petroleum system approach

Abstract

The Upper Devonian and Lower Mississippian stratigraphy in the Antler foreland basin of northwestern Montana is the current focus of exploration for several petroleum companies. Conodont zonation indicates that the Exshaw Formation of southern Alberta and northern Montana is Late Devonian and Early Mississippian in age, and is stratigraphically equivalent to the Bakken Formation in the Williston Basin and the Sappington Member of the Three Forks in southwestern Montana. The Exshaw, Bakken and Sappington are lithologically similar. Each formation is comprised of basal organic-rich shale, middle dolomitic siltstone and sandstone, and upper organic-rich shale. The Three Forks Formation in northwestern Montana consists of two members: the older Logan Gulch Member and the younger Trident Member. The Logan Gulch is 100 ft to over 450 ft thick and was subdivided in this study into three facies that were deposited in salina to sabkha environments. The Logan Gulch is largely composed of massive anhydrite and supratidal facies with nodular to bedded anhydrite. Locally, the Logan Gulch is termed the Potlatch Aanhydrites. The Three Forks Formation in northwestern Montana is equivalent to the Stettler and Big Valley formations in southern Alberta. The younger Trident Member was subdivided into two facies that are transgressive, open marine deposits. The Trident is absent to over 100 ft thick, and is largely comprised of open marine, fossiliferous shale. The Exshaw Formation in the study area consists of three members: an upper shale, middle siltstone unit, and a lower shale. The upper and lower shales are black, organic-rich, and siliceous. The lower shale ranges from one ft to over 20 ft thick, while the upper shale is absent to approximately 12 ft thick. In portions of the study area, a detrital siltstone and very fine-grained sandstone member of the Banff Formation (Lower Lodgepole) conformably overlies the upper shale. The middle siltstone was deposited along a shallow shelf in bathymetries similar to lower shoreface environments. This unit forms a north-to-south depositional thick in northern Toole County and southern Alberta. The middle siltstone is absent to over 120 ft thick in the study area. The Lower Exshaw Shale contains Type I/II kerogen and this shale is the most prominent source rock in the study area. It is the thickest and laterally most continuous of the two shales. Total organic carbon (TOC) of the lower shale ranges from 1.5 wt% to 17 wt%. The lower shale is mature over much of the study area with the highest Tmax values in the western part of the study area near the Disturbed Belt thrust sheets. Reservoir models were constructed for each lithofacies using core plug data and wireline well logs. The most prospective reservoir facies are the dolomitized peritidal facies at the top of the Logan Gulch Member and the lower shoreface facies of the Middle Exshaw. Porosity in the dolomitized peritidal facies ranges from 0.3% to 12.5% with permeability ranging from 0 md to 0.315 md. Lower shoreface facies have porosities from 1.5% to 10% with permeability ranging from 0 md to 0.06 md. Because of the low permeability, over-pressuring is required for reservoir development. In northern Montana, sparse pressure data indicate that potential reservoir facies are normal- to under-pressured. Middle Exshaw upper shoreface sands may exist to the north and east of Toole County, and reservoir permeability may be great enough for conventional reservoir development. The complex tectonic history of the basin produced extensive faulting and folding. Fractures associated with tectonism enhance reservoir development, promoted the migration of hydrocarbons, and possibly compromised seals allowing for depressurization of the reservoirs.