ABSTRACT

Poplar Dome, in western Roosevelt County, Montana, is a major structural feature of the western Williston Basin. Approximately 150 feet of structural closure exists at the Mississippian Ratcliffe horizon. Minor structural movement occurred in the area of Poplar Dome during Mississippian through Cretaceous time. Much of the present structural relief is the result of Laramide uplift.

Beds of salt in the Mississippian Charles Formation (Poplar interval) abruptly pinch out at Poplar Dome. Over 300 feet of salt from regionally extensive beds is missing on and near the structure. Nonsalt beds, however, extend across the dome with little thickness change and few facies changes. The stratigraphic evidence along with the presence of breccia in cores indicate that the salt beds were originally present on Poplar Dome but were subsequently dissolved. The dissolution was probably a Tertiary event caused by the introduction of fresh water along fractures induced by uplift. The removal of the salt caused a collapse over the dome which affected the structural configuration of beds younger than the Charles Formation.

In addition to the dissolution of salt beds, pore-filling salt has been dissolved from limestone of the Charles "A" and "B" units. The dissolution of this salt from Charles "A" porosity is related to the removal of bedded salt, while the flushing of salt from the Charles "B" porosity is related to a regional dissolution front encroaching from the southwest.

Poplar Dome is the site of a major oil field which has yielded more than 45 million barrels of oil, primarily from the Charles "B" porosity. The removal of pore-filling salt from the Charles "B" was a critical factor in creating this oil accumulation. The removal of salt beds also allowed the upward migration of oil into reservoirs of the Charles "A" and the Pennsylvanian Tyler Formation.