Stratigraphic Change in Organic Composition Demonstrated by Oil Specific Gravity-Depth Correlation in Tertiary Green River Oil Shales, Colorado

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 2,000 feet of the formation in an area of 325 square miles 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 oxygen 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.