Thin section, sieve, and clay-mineral analyses indicate that many of the clay-rich sandstones in Wyoming were deposited as clean, well-sorted sands. After deposition and burial these sands were filled with either hydrocarbons or secondary clay cement. It is proposed that hydrocarbon migration took place shortly after burial. Those sands not filled with hydrocarbons were subjected to continued precipitation of clay from the formation waters until all effective porosity and permeability were eliminated. Later tectonic movements may position these "sealed traps" in an off-structure position. A thorough understanding of the geologic and

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tectonic development of the basin is necessary to prospect for this type of trap.

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