Petrography and Permeability of Lower Cretaceous Muddy Sandstone, Bell Creek Area, Montana

Robert R. Berg and David K. Davies

The Cretaceous Muddy Sandstone at Bell Creek field is an elongate sandstone body that has an average thickness of 20 ft, a mean quartz grain size of 0.16 mm (fine-grained), and a quartz content of 85 percent. In vertical sequence, the sandstone can be subdivided on the basis of internal structure into the following depositional units (in ascending order): lower shoreface, middle shoreface, upper shoreface-beach, and eolian. Grain size increases upward from lower shoreface through upper shoreface but decreases slightly in the eolian unit. In horizontal sequence, the sandstone can be subdivided on the basis of dominant rock type into the following facies (from west to east): forebar, bar, and backbar.

Based on petrography, bedding, and unit morphology, the Muddy Sandstone appears to have originated as part of a barrier-bar complex in the main Bell Creek producing area. However, on the southwest at Ranch Creek, the barrier pattern is modified by a local divergent trend. Petrologically, the sandstone at Ranch Creek is indistinguishable from that at Bell Creek and probably was deposited under similar environmental conditions.

Core analyses demonstrate that permeability is related to mean grain size and content of quartz. Therefore, permeability plots indicate vertical variations in grain size and quartz content which, in turn, may be interpreted in terms of depositional environment. Permeability values in the thousands of millidarcys and quartz content in excess of 90 percent are common in upper shoreface-beach sediments of the bar facies at Bell Creek. Permeability values and quartz content are significantly reduced in lower shoreface or lagoonal sediments of the forebar and backbar facies, respectively.

AAPG Search and Discovery Article #91051©2012 AAPG Rocky Mountain Section Meeting, Albuquerque, New Mexico, 23-26 February 1969