ABSTRACT

As one of the transgressive-regressive cycles in the Western Interior Basin, the Judith River-Claggett sequence created potential reservoirs for shallow biogenic gas. Across the Fort Peck Reservation in northeastern Montana, a distance of 75 mi (121 km), the Judith River Formation varies in thickness form 350 ft (105 m) in the west to 130 ft (39 m) in the east.

Three distinct stratigraphic prisms are recognized in this cycle. A thick basal sandstone in the west formed in a shore environment with some deltaic sedimentation. This unit is time regressive, grading into a thin shelf sandstone, properly placed in the older, Claggett Formation. The two sandstone intervals of the main Judith River Formation are nonmarine in the west and become progressively younger as they prograde eastward into the Bearpaw (Pierre) shale. The lower unit is transitional to a shore facies and then to a shelf facies in the central reservation where it represents the maximum regression as an inner shelf ridge complex. Sand lenses reach 140 ft (43 m) in thickness. In the northeast another ridge complex formed farther out on the shelf. The upper sandstone is only a well developed shelf sand ridge in the north-central part of the reservation. Maximum sand thickness is 40 ft (12 m). At least three deltaic channel systems developed farther to the west. This unit may have signalled the last Cretaceous transgression which deposited the Bearpaw shale.

Stratigraphic traps exist in the three sandstone units due to variation in grain size, clay content, and porosity associated with the progradational facies changes. In addition, structures associated with the Poplar Dome and the Wolf Creek Nose have created local trapping mechanisms. Judith River gas has been produced in the East Poplar Field for operational use. Shows have been reported in several reservation wells, although high mud weights and deeper exploration targets have prevented adequate evaluation of the Judith River gas resource.