Oil shale geology

Oil shale geology is a branch of geologic sciences which studies the formation and composition of oil shales–fine-grained sedimentary rocks containing significant amounts of kerogen, and belonging to the group of sapropel fuels. Oil shale formation takes place in a number of depositional settings and has considerable compositional variation. Oil shales can be classified by their composition or by their depositional environment. Much of the organic matter in oil shales is of algal origin, but may also include remains of vascular land plants. Three major type of organic matter in oil shale are telalginite, lamalginite, and bituminite. Some oil shale deposits also contain metals which include vanadium, zinc, copper, and uranium.
Most oil shale deposits were formed during Middle Cambrian, Early and Middle Ordovician, Late Devonian, Late Jurassic, and Paleogene times through burial by sedimentary loading on top of the algal swamp deposits, resulting in conversion of the organic matter to kerogen by diagenetic processes. The largest deposits are found in the remains of large lakes such as the deposits of the Green River Formation of Wyoming and Utah, USA. Oil-shale deposits formed in the shallow seas of continental shelves generally are much thinner than large lake basin deposits.

Classification and varieties

Oil shale belongs to the group of sapropel fuels. It does not have a definite geological definition nor a specific chemical formula, and its seams do not always have discrete boundaries. Oil shales vary considerably in their mineral content, chemical composition, age, type of kerogen, and depositional history and not all oil shales would necessarily be classified as shales in the strict sense. Their common feature is low solubility in low-boiling organic solvents and generation of liquid organic products on thermal decomposition.
There are varying classifications of oil shales depending on their mineral content, type of kerogen, age, depositional history, and organisms from which they are derived. The age of the known oil shale deposits ranges from Cambrian to Paleogene age. Lithologies comprise shales and marl and carbonate rocks, all of which form a mixture of tightly bound organic matter and inorganic components.
Oil shales have been divided into three categories based on mineral composition – 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. As many as twenty carbonate minerals have been found in oil shales, the majority of which are considered authigenic or diagenetic. Carbonate-rich oil shales, particularly that of lacustrine-sourced deposits, have usually the organic-rich layers sandwiched between carbonate-rich layers. These deposits are hard formations that are resistant to weathering and they are difficult to process using ex-situ methods. Siliceous oil shales are usually dark brown or black shales. They are not rich in carbonates but rather in siliceous minerals such as quartz, feldspar, clay, chert and opal. Siliceous shales are not as hard and weather-resistant as carbonate-rich shales, and may be better suited for extraction via ex-situ methods. Cannel shales are usually dark brown or black shales, which consist of organic matter that completely encloses other mineral grains. They are suitable for extraction via ex-situ methods.
Another classification according to the type of kerogen, is based on the hydrogen, carbon, and oxygen content of the original organic matter in the oil shale. This classification is using the Van Krevelen diagram. The most used classification of oil shales was developed between 1987 and 1991 by Adrian C. Hutton of the University of Wollongong, adapting petrographic terms from coal terminology. According to this classification, oil shales are designated as terrestrial, lacustrine, or marine, based on the environment where the initial biomass was deposited. Hutton's classification scheme has proven useful in estimating the yield and composition of the extracted oil.

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

Cannel coal is a type of terrestrial shale, which is hydrogen-rich 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. Lacustrine shales consist of lamosite and torbanite. Lamosite is a pale-brown and grayish-brown to dark-gray to black oil shale whose chief organic constituent is lamalginite derived from lacustrine planktonic algae. 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. Marine shales consist of three varieties, namely kukersite, tasmanite, and marinite. 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. 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. 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.

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 a pre-bitumen 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 and a lower proportion of nitrogen.
Three major types of organic matter 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, and uranium among others.

Inorganic matrix	Bitumens	Kerogens
quartz; feldspars; clays ; pyrite and others	soluble in CS₂	insoluble in CS₂; containing uranium, iron, vanadium, nickel, molybdenum, etc.

Formation

Most oil shale deposits were formed during Middle Cambrian, Early and Middle Ordovician, Late Devonian, Late Jurassic and Paleogene times. These were formed by the deposition of organic matter in a variety of depositional environments including freshwater to highly saline lakes, epicontinental marine basins and subtidal shelves and were restricted to estuarine areas such as oxbow lakes, peat bogs, limnic and coastal swamps, and muskegs. When plants die in such an anaerobic aquatic environment, low oxygen levels prevent their complete bacterial decay.
For undecayed organic matter to be preserved and to form oil shale, the environment must remain uniform for prolonged periods of time in order to build up sufficiently thick sequences of algal matter. Eventually, the algal swamp or other restricted environment is disrupted and oil shale accumulation ceases. Burial by sedimentary loading on top of the algal swamp deposits converts the organic matter to kerogen by the following normal diagenetic processes:

Compaction due to sediment loading on the coal, leading to compression of the organic matter.
With ongoing heat and compaction, removal of moisture in the peat and from the intracellular structure of fossilized plants, and removal of molecular water.
Methanogenesis—similar to treating wood in a pressure cooker— results in methane being produced, removing hydrogen, some carbon, and some further oxygen.
Dehydration, which removes hydroxyl groups from the cellulose and other plant molecules, resulting in the production of hydrogen-reduced coals or oil shales.

Though similar in their formation process, oil shales differ from coals in several distinct ways. The precursors of the organic matter in oil shale and coal differ in a sense that oil shale is of algal origin, but may also include remains of vascular land plants that more commonly compose much of the organic matter in coal. The origin of some of the organic matter in oil shale is obscure because of the lack of recognizable biologic structures that would help identify the precursor organisms. Such materials may be of bacterial origin or the product of bacterial degradation of algae or other organic matter.
Lower temperature and pressure during the diagenesis process compared to other modes of hydrocarbon generation result in a lower maturation level of oil shale. Continuous burial and further heating and increased pressure over time could result in the conventional production of oil and gas from the oil shale source rock. The largest deposits are found in the remains of large lakes such as the deposits of the Green River Formation of Wyoming and Utah, USA. Large lake oil shale basins are typically found in areas of block faulting or crustal warping due to mountain building. Deposits such as the Green River may be as much as and yield up to 40 gallons of oil for each ton of shale.
Oil shale deposits formed in the shallow seas of continental shelves generally are much thinner than large lake basin deposits. These are typically a few meters thick and are spread over very large areas, extending up to thousands of square kilometers. Of the three lithologic types of oil shales, siliceous oil shales are most commonly found in such environments. These oil shales are not as organically rich as lake-deposited oil shales, and generally do not contain more than 30 gallons of extractable oil per ton of oil shale. Oil shales deposited in lagoonal or small lake environments are rarely extensive and are often associated with coal-bearing rocks. These oil shales can have high yields– as much as 40 gallons per ton of oil shale. However, due to their small areal extent, they are considered unlikely candidates for commercial exploitation.

Country	Location	Type	Age	Organic carbon	Oil yield	Oil conversion ratio
Australia	Glen Davis, New South Wales	torbanite	Permian	40	31	66
Australia	Tasmania	tasmanite	Permian	81	75	78
Brazil	Irati Formation, Irati	marinite	Permian		7.4
Brazil	Paraíba Valley	lacustrine shales	Permian	13–16.5	6.8–11.5	45–59
Canada	Nova Scotia	torbanite; lamosite	Permian	8–26	3.6–19	40–60
China	Fushun	cannel coal; lacustrine shales	Eocene	7.9	3	33
Estonia	Estonia Deposit	kukersite	Ordovician	77	22	66
France	Autun, St. Hilarie	torbanite	Permian	8–22	5–10	45–55
France	Creveney, Severac		Toarcian	5–10	4–5	60
South Africa	Ermelo	torbanite	Permian	44–52	18–35	34–60
Spain	Puertollano	lacustrine shale	Permian	26	18	57
Sweden	Kvarntorp	marinite	Lower Paleozoic	19	6	26
United Kingdom	Scotland	torbanite	Carboniferous	12	8	56
United States	Alaska		Jurassic	25–55	0.4–0.5	28–57
United States	Green River Formation in Colorado, Wyoming and Utah	lamosite	Early to Mid Eocene	11–16	9–13	70
United States	Mississippi	marinite	Devonian