The application of geologic science to oil exploration requires achievement of sufficient understanding to make predictions. One of the most important predictions geologists are called upon to make is the distribution of rock stratigraphic units. Prediction may be enhanced by interpreting the vertical profile of associated

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environments within an overall framework of deposition.

The Muddy Formation (Lower Cretaceous) in northeastern Wyoming is a clastic unit approximately 150 ft thick composed of sandstone, siltstone, shale, and carbonaceous deposits. Detailed core studies and selected isopach maps suggest that the unit was deposited in a manner very similar to the modern Mississippi deltaic complex. Oil productive distributary channels, crevasse splays, and marginal barrier bars are recognized by the vertical distribution of sedimentary structures observed in cores and by comparing the distribution in space of the interpreted elements to a Mississippi model.

In considering the Mississippi model, one must realize that different processes are taking place side by side at different elevations relative to sea level, hence the unidirectional current-flow processes of a distributary may occur 100 ft or more below sea level at the same time that the wave processes of a beach occur at and near sea level. The Muddy Formation must be understood as a total depositional system; detailed vertical subdivision will only obscure genetic relations.

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