The interpretation of depositional environments for reservoir sandstones requires a knowledge of primary rock properties: composition, texture, sedimentary structure, and morphology. Each property has special significance in interpretation. Compositional and textural changes in vertical sequence are the most important criteria, but because these properties are interdependent, composition alone may be a key indicator of environment. The use of compositional criteria is illustrated by the Lower Cretaceous Muddy Sandstone in the Powder River basin, Wyoming and Montana, where fluvio-deltaic and marine-bar sandstones are clearly separated by compositional differences. Sedimentary structures are also significant. Largely on the basis of bedding, Muddy barrier island strata can e divided into 4 distinct subenvironments even though the unit is only 25 ft thick. These facies represent lower shoreface, middle shoreface, beach-upper shoreface, and dune environments. Morphology of sandstone bodies, commonly suggests environment of deposition, but this criterion is the least reliable unless it is applied with a knowledge of other rock properties.

The interpretation of morphology is commonly the principal exploration problem in stratigraphic traps. Where details of rock character are absent, the secondary properties of porosity and permeability may reflect compositional and textural changes because these

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properties are largely dependent on the primary properties.

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