Lenticular bedding is prominent in reservoir units in the Green River (Eocene) Formation of the Red Wash-Walker Hollow field and adjacent areas. Shale beds are well indurated. Folding is gentle, and faulting is a minor feature of the area. These relationships suggest a short migration distance from source beds to reservoir beds. If the migration distance was small, then the kinds of rocks near the reservoir beds are important in determining the source of the oil. This note summarizes data showing the abundance of the various kinds of lutite within the productive intervals, and suggests that gray shale, and not oil shale, may have been the source beds of the oil.