Twelve sections of the uppermost part of the Mancos Shale and the lower part of the Mesaverde Group ( I l e s Formation) of Late Cretaceous age have been measured in a linear distance of about 50 m i (83 km) in northwestern Colorado as part of a study of depositional environments of impermeable ("tight"', <O. 1 md) gas-bearing reservoirs in the Sand Wash basin of the Greater Green River Basin. This study focused on interpretation of facies based on lateral and vertical associations of rock types, textures, sedimentary structures , contact relations , and faunas. The depositional model derived from this study (Kiteley, 1979) indicates that most reservoirs-shallow shelf sand bars, barrier beaches, delta margin sands, distributaries and fluvial channels-are discontinuous but nevertheless systematic and predictable, and that reservoir quality is related to depositional environment*