Oil Shale Geology - Composition

Composition

As a sapropel fuel, oil shale differs from humus fuels in its lower content of organic matter. The organic matter has an atomic ratio of hydrogen to carbon of about 1.5 – approximately the same as that of crude oil and four to five times higher than coals. The organic matter in oil shales forms a complex macromolecular structure which is insoluble in common organic solvents. It is mixed with varied amounts of mineral matter. For commercial grades of oil shale, the ratio of organic matter to mineral matter is about 0.75:5 to 1.5:5.

The organic portion of oil shale consists largely of prebitumen bituminous groundmass, such as remains of algae, spores, pollen, plant cuticles and corky fragments of herbaceous and woody plants, and cellular debris from other lacustrine, marine, and land plants. While terrestrial oil shales contain resins, spores, waxy cuticles, and corky tissues of roots and stems of vascular terrestrial plants, lacustrine oil shales include lipid-rich organic matter derived from algae. Marine oil shales are composed of marine algae, acritarchs, and marine dinoflagellates. Organic matter in oil shale also contains organic sulfur (about 1.8% on average) and a lower proportion of nitrogen.

Three major types of organic matter (macerals) in oil shale are 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 display few or no recognizable biologic structures. Under the microscope, telalginite and lamalginite are easily recognized by their bright shades of yellow under ultraviolet/blue fluorescent light. Bituminite is largely amorphous, lacks recognizable biologic structures, and displays relatively low fluorescence under the microscope. 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.

Mineral matter in oil shale contains fine-grained silicate and carbonate minerals such as calcite, dolomite, siderite, quartz, rutile, orthoclase, albite, anorthite, muscovite, amphipole, marcasite, limonite, gypsum, nahcolite, dawsonite and alum. Some oil-shale deposits also contain metals such as vanadium, zinc, copper, uranium among others.