Abstract Exploration and development in the Green River basin of southwestern Wyoming have resulted in discovery of gas reserves which total about 1.5 trillion cu ft in the western part of the basin alone. Important gas-producing units in the western area include sandstone beds in the Frontier and Mesaverde Formations of Late Cretaceous age, and lenticular sandstone bodies of early Tertiary age which appear to have developed near the western shore of an ancient lake occupying the basin. Combinations of structural and stratigraphic factors cause the trapping of gas in most instances. The gas in both the Frontier and Tertiary beds in the Big Piney-La Barge area in the western part of the basin characteristically has a high methane content and no reported noncombustible gases or hydrogen sulfide. Geologic factors such as the basin's 30,000-ft-thick sedimentary rock section; its thrust- faulted, structurally complex borders; the presence of several regional fold trends which may have compound geological histories; and the notable facies changes, especially in the Mesozoic and Tertiary beds, all suggest that much more extensive gas reserves can be developed in the Green River basin.