Cretaceous System Stratigraphy and Shallow Gas Resources on the Fort Peck Reservation, Northeastern Montana

MONSON, LAWRENCE M., and DUANE F. LUND, Minerals Resource, Fort Peck Tribes, Poplar, MT

Five shallow gas-bearing Cretaceous intervals have been identified on the Fort Peck Reservation of northeastern Montana. They include the Lower Judith River Sandstone and shaly sandstone intervals in the Gammon, Niobrara, Greenhorn, and Mowry Formations. Stratigraphic correlations have been carried from southwestern Saskatchewan through the Bowdoin gas field to the reservation. Sparse yet widely distributed gas shows confirm this relatively untested resource.

Each of these gas-bearing intervals belongs to a recognized stratigraphic cycle characterized by thick shales overlain by progradational shaly sandstones and siltstones. The bottom cycle (Skull Creek to Mowry) contains considerable nonmarine deposits, especially within the Muddy Sandstone interval, which is thickly developed in the eastern part of the reservation as a large valley-fill network. The reservation lies within transition zones for many of the facies associated with sea level changes along the western margin of the Cretaceous Interior seaway. Some individual sandstone units are not continuous across the reservation. These, and those that correlate, appear to be related to paleotectonic features defined by northwest-trending lineament zones, and by lineament zone intersectio s. Northeast-trending paleotectonic elements exert secondary influence on stratigraphic isopachs. Circular tectonic elements, which carry through to basement, also have anomalous stratigraphic expression.

Conventional drilling has not been conducive to properly testing the Cretaceous gas potential on the reservation, but empirical well-log analysis suggests that gas can be identified by various crossover techniques. The Judith River Formation did produce gas for field use at East Poplar.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)