The Lower Cretaceous Muddy Formation in the northern Powder River basin of Wyoming and Montana was deposited during a marine transgression across a stream-dissected surface of the underlying Skull Creek Shale. The transgression occurred over most of the area, but was limited on the northeast by a prograding delta, which supplied most of the sand.

The Muddy Formation is divided into lower and upper units. The lower Muddy was restricted to a system of dendritic channels incised into the Skull Creek Shale during a period of emergence. The sands from the delta source were transported south by longshore currents. They were deposited principally in a transitional marine and estuarine environment, and are composed of fine-grained, moderately well-sorted, partially clay-filled quartzose grains.

By the time of deposition of the upper Muddy, the incised depressions in the Skull Creek topography had largely been filled. The upper Muddy sands were deposited in a complex marine shoreline environment, which resulted in offshore bars, barrier islands, beaches, and tidal deposits. Several shoreline trends are recognizable in the upper Muddy. They are progressively younger eastward and reflect the overall west to east transgression. These trends were controlled by the remnant Skull Creek topography and changing conditions of sediment supply.

Production from the Muddy Formation is principally from stratigraphic traps; however, structure has been important in localizing some of the oil and gas accumulations.

Lower Muddy pools are restricted to updip channel boundaries and are localized by structural noses and updip channel reentrants. Upper Muddy production is controlled chiefly by porosity development and lateral facies changes.

Exploration for Muddy sandstone reservoirs is aided by the use of an isopach map of the total Muddy Formation. This map shows the configuration of the Skull Creek channels and, therefore, the distribution of the lower Muddy sandstone. It also is helpful in predicting the orientation of the upper Muddy shoreline trends where they were related to remnant Skull Creek highs, and in showing an increased Muddy thickness due to sand buildups in nonchannel areas. Electric log maps, combined with zonal sandstone isopachs, provide a means of visualizing the rapid changes in sandstone geometry and also aid in the interpretation of depositional environments.

Exploration must be focused on the location of primary stratigraphic traps which have not been strongly altered by later structural movements. The widespread clay-filled porosity has resulted in large areas being nonproductive. It is believed that the clay fill is largely diagenetic and occurred subsequent to accumulation of primary oil. The lower percentage of clay fill in the oil-filled primary traps suggests that the presence of the oil inhibited clay diagenesis.

In the last 3 years, nearly 3,000 wells have been drilled in the study area in the search for Muddy oil. Every year new fields of significant size are discovered. Detailed stratigraphic work is called for as well as courage to use the drill as an exploration tool.

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