Publications

Survey of Energy Resources 2007

Classification of Oil Shales

Oil shales, until recent years, have been an enigmatic group of rocks. Many were named after a locality, mineral or algal content, or the type of product the shale yielded. The following are some names that have been applied to oil shales, a few of which are still in use today:

• algal coal
• alum shale 
• bituminite 
• boghead coal 
• cannel coal 
• gas coal 
• kerosene shale 
• kukersite 
• schistes bitumineux 
• stellarite 
• tasmanite 
• torbanite 
• wollongongite

A.C. Hutton (1987) developed a workable scheme for classifying oil shales on the basis of their depositional environments and by differentiating components of the organic matter with the aid of ultraviolet/blue fluorescent microscopy (Fig. 3-1 ). His classification has proved useful in correlating components of the organic matter with the yields and chemistry of the oil obtained by retorting.

Hutton divided the organic-rich sedimentary rocks into three groups. These groups are (1) humic coals and carbonaceous shales, (2) bitumen-impregnated rock (tar sands and petroleum reservoir rocks), and (3) oil shale. On the basis of the depositional environment, three basic groups of oil shales were recognised: terrestrial, lacustrine, and marine. Terrestrial oil shales include those composed of lipid-rich organic matter such as resins, spores, waxy cuticles, and corky tissue of roots and stems of vascular terrestrial plants commonly found in coal-forming swamps and bogs. Lacustrine oil shales are those containing lipid-rich organic matter derived from algae that lived in freshwater, brackish, or saline lakes. Marine oil shales are composed of lipid-rich organic matter derived from marine algae, acritarchs (unicellular microorganisms of questionable origin), and marine dinoflagellates (one-celled organisms with a flagellum).

Hutton (1987) recognised three major macerals in oil shale: telalginite, lamalginite, and bituminite. Telalginite is defined as structured organic matter composed of large colonial or thick-walled unicellular algae such as Botryococcus and Tasmanites. Lamalginite includes thin-walled colonial or unicellular algae that occur as distinct laminae, but displays little or no recognisable biologic structures. Under the microscope, telalginite and lamalginite are easily recognised by their bright shades of yellow under ultraviolet/blue fluorescent light. The third maceral, bituminite, is another important component in many oil shales. It is largely amorphous, lacks recognisable biologic structures, and displays relatively low fluorescence under the microscope. This material has not been fully characterised with respect to its composition or origin, although it is often a quantitatively important component of the organic matter in many marine oil shales. Other organic constituents include vitrinite and inertinite, which are macerals derived from the humic matter of land plants. These macerals are usually found in relatively small amounts in most oil shales.