Specific gravity of oil retorted from oil shales of the Eocene Green River Formation, northwestern Colorado, decreases uniformly with increasing sample depth. Statistical evaluation of oil specific gravity-depth data from 18 sample sets representing 2000 ft of the formation in an area of 325 sq mi shows this correlation to be uniform stratigraphically and geographically. Mineral profiles did not correlate with this change. Decrease in oil specific gravity is associated with a progressive decrease in O content of organic matter. This stratigraphic change probably resulted from the loss of carboxyl groups from the organic matter under heat and pressure of increasing depth of burial.