Oil shale geology

Geology

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Outcrop of Ordovician kukersite oil shale, northern Estonia.

Classification and varieties

Oil shale does not have a specific chemical formula. Different types of oil shales vary in their chemical composition, type of kerogen, age, and depositional history, including the organisms from which they were derived.

Oil shales have been divided into three categories depending on their composition. These three categories are carbonate-rich shale, siliceous shale and cannel shale. Carbonate-rich shales derive their name from the large amount of carbonate minerals such as calcite and dolomite that can be found in it. As many as 20 carbonate minerals have been found in oil shale, the majority of which have thought to have been deposited at the time of deposition. One type of carbonate-rich shale that is valued quite highly is lacustrine-sourced deposits, because of the frequent occurrence of organic-rich shale layers sandwiched between carbonate-rich layers of oil shale. These deposits are hard formations which could be difficult to process using ex-situ methods. Siliceous shales derive their names as they are not rich in carbonate deposits but rather detrital minerals. Detrital minerals that have been known to be present are quartz, feldspar, clay, chert and opal. Younger siliceous shales can be good sources for hydrocarbons, but as the deposits get older and deeper the hydrocarbons that might have been present have generally migrated. Siliceous shales are not as hard or as weather-resistant as carbonate-rich shale and may be better suited for extraction via ex-situ methods.

Based upon environment of deposition, oil shales are characterized as terrestrial, lacustrine, and marine.

Classification of oil shales by environment of deposition

Terrestrial	Lacustrine	Marine
cannel coal	lamosite; torbanite	kukersite; tasmanite; marinite

Cannel coal is brown to black coal (sometimes with shaly texture) composed of resins, spores, waxes, cutinaceous and corky materials derived from terrestrial vascular plants as well as varied amounts of vitrinite and inertinite. Lamosite is pale brown and grayish-brown to dark-gray to black oil shale in which the chief organic constituent is lamalginite derived from lacustrine planktonic algae. Marinite is a gray to dark-gray to black oil shale of marine origin in which the chief organic components are lamalginite and bituminite derived from marine phytoplankton with varied admixtures of bitumen, telalginite, and vitrinite. Torbanite, named after Torbane Hill in Scotland, is a black oil shale whose organic matter is telalginite derived from lipid-rich Botryococcus and related algal forms. Tasmanite, named after Tasmania, is a brown to black oil shale whose organic matter consists of telalginite derived chiefly from unicellular tasmanitid algae of marine origin. Kukersite, named after Kukruse in Estonia, is a light-brown marine oil shale whose principal organic component is telalginite derived from the green alga, Gloeocapsomorpha prisca.

Composition

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Fossils in Ordovician kukersite oil shale, northern Estonia.

Organic matter in oil shale consists of the 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 contains resins, spores, waxy cuticles, and corky tissue of roots and stems of vascular terrestrial plants, lacustrine oil shales include lipid-rich organic matter derived from algae, and marine oil shales are composed of marine algae, acritarchs, and marine dinoflagellates. Some oil-shale deposits may also contain non-organic minerals and metals including alum, nahcolite (NaHCO3), dawsonite, sulfur, ammonium sulfate, vanadium, zinc, copper, uranium, and others.