Abstract: Environments of Deposition of Oil Shale

John R. Donnell

Oil shale deposits are present throughout the world in marine, brackish-water, and freshwater sedimentary sequences that range in age from Cambrian to Holocene. Oil shales were deposited on shallow-marine platforms, in geosynclines, and in lagoons that were marginal to both. They also were deposited in lacustrine environments and in environments transitional between paludal and lacustrine.

Organic material in oil shales varies from humic to sapropelic. Usually during destructive distillation less than one-third of the humic material but as much as two-thirds of the sapropelic material will convert to oil. The humic oil shale is usually high, and the sapropelic oil shale usually low, in trace metals.

The Chattanooga Shale and its lateral equivalents are excellent examples of marine-platform deposits containing mostly humic matter in the organic fraction. The oil shale, thin and low grade, underlies hundreds of thousands of square miles in the eastern and midwestern part of the United States and contains as much as 0.006 percent uranium.

The Green River Formation of Colorado, Utah, and Wyoming is a lacustrine oil shale deposit that contains mostly sapropelic material in the organic fraction. The oil shale, thick and rich, underlies an area of thousands of square miles and contains trace metals generally in amounts less than the average abundance in the earth's crust. However, the Green River oil shale contains the sodium minerals trona, nahcolite, and dawsonite that have a potential by-product value for soda ash and aluminum.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA