Geologic and Seismic Models, Muddy Sandstone, Lower Cretaceous, Bell Creek-Rocky Point Area, Powder River Basin, Montana and Wyoming

Robert J. Weimer, Claudia A. Rebne, Thomas L. Davis

The Muddy Sandstone in the northern Powder River basin consists of two distinct members (genetic units) that are separated by a widespread subaerial surface of erosion. The older sandstone member, the reservoir rock at the giant Bell Creek field, was deposited in shoreline and associated nearshore marine environments. It is genetically related by intertonguing to the underlying Skull Creek Shale. The younger member is a valley-fill deposit of fluvial, estuarine (bay), and tidal-flat environments.

The deposition and distribution of these two genetic units were controlled primarily by relative sea level changes. The Bell Creek sandstone was deposited as a widespread regressive sandstone during a highstand of sea level. A following sea level lowstand caused valley cutting, erosion of all or portions of the Bell Creek sandstone, and paleosoil development causing early diagenesis, especially of kaolinite, siderite, and quartz. A rising sea level resulted in valley filling, coastal onlap, and finally a transgressive surface of erosion that is overlain by black marine shale (Nefsy or Shell Creek).

Recurrent activity on basement-controlled fault blocks appears to have controlled distribution of the Muddy members, drainage incisement patterns, present structure, heat flow, and possibly petroleum migration.

Seismic modeling indicates that it may be possible to seismically recognize the distribution of Muddy reservoir facies that occur in the area. Seismic modeling suggests that the amplitude response of the Muddy is influenced by Muddy facies and thickness changes. The lowest Muddy amplitude observed occurs when the Bell Creek sandstone is prevalent. An increase in the Muddy amplitude is noted when thick valley-fill deposits are present.

Seismic modeling results compare well to actual seismic data. Amplitude changes noted in the Muddy reflector in the Bell Creek-Rocky Point area may help explorationists in the future distinguish between nonprospective and reservoir quality Muddy Sandstone.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988