ABSTRACT

The interpretation of depositional environments for reservoir sandstones requires a knowledge of primary rock properties: composition, texture, sedimentary structures, and morphology. Each of these properties has its special significance in interpretation. Compositional and textural changes in vertical sequence are the most important criteria, but because these two properties are interdependent, composition alone may be a key indicator of environment. The use of compositional criteria is illustrated by Lower Cretaceous Muddy sandstones 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 indicators of sedimentary processes. Morphology of sandstone bodies commonly suggests environment of deposition, but this criterion is the least reliable unless it is used with a knowledge of other rock properties.

The interpretation of morphology is often the principal exploration problem in stratigraphic traps. When details of rock character are lacking the secondary properties of porosity and permeability may reflect compositional and textural changes because these properties are largely dependent on the primary properties.