A geochemical genesis pattern for the lacustrine oil shales of Colorado's Green River Formation is postulated to explain 7 requirements made on depositional conditions by present day observations. The deposition of hydrogen- rich, uniform organic matter in relatively large amounts through more than 2,000ft. of continuous oil shale is shown to be a natural consequence of the development of a chemically stratified sodium carbonate lake. Authigenic formation of the minerals present in the oil-shale matrix stems from conditions in the carbonate lake and from effects of the continuously large proportion of organic matter in the lake's sediment.