Oil shale geology

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Main article: Oil shale

Oil shale geology is the field of the geologic sciences to study formation and composition of oil shales—fine-grained sedimentary rocks containing significant amounts of kerogen, and belonging to the group of sapropel fuels.^[1] Oil shales are formed in a number of depositional settings and have considerable compositional variation. Oil shales can be classified by their composition (carbonate minerals such as calcite or detrital minerals such as quartz and clays) or by their depositional environment (large lakes, shallow marine, and lagoon/small lake settings). Their organic content also varies widely and contains remnant matter that can include freshwater and marine algae, spores, resin, and corky material from roots. Oil shales have also been mined for metals such as uranium.

[edit] Classification and varieties

Outcrop of Ordovician kukersite oil shale, northern Estonia.

Oil shales vary considerably in mineral content, type of kerogen, age, depositional history, and organisms from which they were derived. Oil shale deposits range in age from Cambrian to Tertiary. Lithologies range from shales to marl and carbonate rocks, all of which form a mixture of tightly bound organic and inorganic materials. ^[2]

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 shale, the majority of which are considered authigenic or diagentic. One type of carbonate-rich shale that is valued 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 that could be difficult to process using ex-situ methods. ^[3] Siliceous shales are not rich in carbonates but rather in siliceous minerals such as quartz, feldspar, clay, chert and opal. Siliceous shales are not as hard or as weather-resistant as carbonate-rich shales, and may be better suited for extraction via ex-situ methods. ^[3] Oil shales could be classified also according to the type of kerogen they contain, made as a function of the relative hydrogen, carbon and oxygen content of the fossilized biomass. ^[2]

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

• Cannel coal (also called candle coal) is a 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.
• 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.
• 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.
• 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.^[5]

[edit] Composition

Fossils in Ordovician kukersite oil shale, northern Estonia.

Photomicrograph showing detail of the varves in a rich Colorado oil shale specimen. The organic laminae are themselves finely laminated. The mineral laminae contain considerable organic matter, but they are readily distinguished by their coarser grain and greater thickness. Note sand grains (white). Enlarged 320 diameters.

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 for crude oil and four to five times higher than for coals. The organic matter in oil shales forms a complex macromolecular structure that is insoluble in common organic solvents.^[1][6] 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.^[7]

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.^[8][9] 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.^[9] Organic matter in oil shale also contains organic sulfur (about 1.8 % on average) and a low proportion of nitrogen.^[1]

Three major type 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 displays 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.^[4]

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 contain metals including vanadium, zinc, copper, uranium, and others.^[1][5]

[edit] Formation

Most oil shale deposits were formed during Middle Cambrian, Early and Middle Ordovician, Late Devonian, Late Jurassic, and Paleogene times.^[1] They are 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 restricted estuarine areas such as oxbow lakes, peat bogs, limnic and coastal swamps, and muskegs.^[5] When plants die in such anaerobic aquatic environments, low oxygen levels prevent their complete decay by bacteria.^[10]

For masses of undecayed organic matter to be preserved and to form oil shale the environment must remain uniform for prolonged periods of time 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 loading of the sediments on the coal which compresses the organic matter
• Removal of the water in the peat
• With ongoing compaction, removal of water from the intracellular structure of fossilized plants
• With heat and compaction, removal of molecular water
• Methanogenesis; similar to treating wood in a pressure cooker, methane is produced, removing hydrogen, some carbon, and some further oxygen (as water)
• Dehydration, which removes hydroxyl groups from the cellulose and other plant molecules, resulting in the production of hydrogen-reduced coals or oil shales.^[10]

Although the formation process is similar, oil shales differ from coals in several distinct ways. The precursors of the organic matter in oil shale and coal also differ. Much of the organic matter in 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.^[5]

Lower temperature and pressure during the diagenesis process compared to other modes of hydrocarbon generation result in a lower maturation level of oil shale. Continued burial and further heating and pressure could result in the production of oil and gas from the oil shale source rock.^[11] 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 can be as much as 2,000 feet (610 m) and yield up to 40 gallons of oil for each ton (166 l/t) of shale.^[5]

Oil-shale deposits formed in the shallow seas of continental shelves generally are much thinner than large lake basin deposits. They are typically on the order of a few metres thick. They are, however, 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 this environment. These oil shales are not as organic rich as lake-deposited oil shales, and generally do not contain more than 30 gallons per ton of oil shale.^{[citation needed]} 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 (170 l/t) of oil shale. However, due to their small areal extent, they are considered unlikely candidates for commercial exploitation.

[edit] Formations in the United States

The United States has two significant oil-shale deposits which are suited for commercial development due to their size, grade and location. The Eocene Green River Formation covers parts of Colorado, Wyoming and Utah; the second significant deposit is Devonian oil shales in the eastern United States. In both of these places there are sub-basins in which the volume and quality of the reserves vary. Oil shale in the Green River Formation is found in five sedimentary basins: Green River, Uinta, Piceance Creek, Sand Wash and Washakie. The first three have undergone some significant investigation and attempts to commercialize the oil shale reserves since the 1960s. The Green River Formation includes deposits from two large lakes which covered an area of over 65,000 square kilometres (25,100 sq mi) during early to middle Eocene time. These lakes were separated by the Uinta uplift and the Axial Basin anticline.^[5] For significant periods during their 10 Ma life, the lakes became closed systems allowing for many changes in size, salinity and sediment deposition. Oil shale is a result of abundant blue-green algae that thrived in the lakes.^[5]

The oil shale that underlies almost 750,000 square kilometres (289,580 sq mi) in the eastern United States was formed in a marine depositional environment very different from the Green River Basins. These deposits have also undergone commercialization attempts; they are also resources for natural gas and have been mined for low-grade oil shale. These oil shales were formed during the Late Devonian and Early Mississippian periods. During this time much of the eastern United States was covered by a large shallow sea. The oil shale is thought to have been the result of slow deposition of planktonic algae under quiet conditions. In parts of the basin close to the shoreline, the organic mixture that helped form the oil shale contains organic-rich sediment from the rising Appalachian mountains.^[5]

[edit] Formations in Brazil

Brazil has nine significant oil shale deposits. The size, location and quality of shale oil deposits in the Paraiba Valley and the Irati Formation have attracted the most attention. These two contain an estimated 1.4 billion barrels of in-situ shale oil with total resources as much as 3 or more billion barrels. While the Irati Formation deposit is the smaller of the two, containing an estimated 600 million barrels in-situ compared to 840 million in the Paraiba Valley formation, it is the more economically viable.^[5]

The Irati Formation consists of two beds of oil shale separated by 12 metres (40 ft) of limestone and shale. The upper layer is thicker (9 metres (30 ft)) but the thinner lower bed (4 metres (10 ft)) is of greater value; the weight percent of shale oil yield is around 12 % for the lower, compared to 7 % for the upper. The oil shale yield varies laterally, and may be as little as 7 % for the lower and 4 % for the upper layer. The formation is a very fine grained and laminated deposit ranging in color from dark gray to brown to black. 60-70 % of the shale is clay minerals; organic matter makes up the rest.^[5]

No consensus has been reached on the exact depositional nature of the Irati oil shale. One theory is that the organic material is from algae deposited in a freshwater to brackish lacustrine environment. Alternatively, the organic sediment may have been deposited in a shallow, partially restricted marine basin. Hutton has described it as a marine source shale.^[5]

[edit] Formation in Estonia

The kukersite oil shale of Ordovician age in Estonia is part of the Baltic Oil Shale Basin and was deposited in shallow marine basins. The deposit is one of the world’s highest-grade deposits with more than 40 % organic content and 66 % conversion ratio into shale oil and gas. The oil shale is located in a single calcareous layer 2.5-3 metres in thickness and is buried at depths from 7 to 100 m.^[2] The total area of the basin is about 3,000 km².^[1] Matrix minerals includes dominantly low-Mg calcite, dolomite, and siliciclastic minerals. It is not enriched in heavy metals.^[5]

[edit] See also

• Oil shale industry
• Oil shale reserves

[edit] Footnotes

[edit] References

• (2004) Survey of energy resources (PDF), 20, World Energy Council (WEC). ISBN 9780080444109. Retrieved on 2007-11-07. 
• (2007) Survey of energy resources (PDF), 21, World Energy Council (WEC). ISBN 0946121265. Retrieved on 2007-11-13.