Oil shale geology
[1][3] 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.
[7] 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.
Lithologies comprise shales and marl and carbonate rocks, all of which form a mixture of tightly bound organic matter and inorganic components.
[9] 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.
[6] 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 (lake-bottom-deposited), or marine (ocean bottom-deposited), based on the environment where the initial biomass was deposited.
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.
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.
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.
Bituminite is largely amorphous, lacks recognizable biologic structures, and displays relatively low fluorescence under the microscope.
[3] When plants die in such an anaerobic aquatic environment, low oxygen levels prevent their complete bacterial decay.
Large lake oil shale basins are typically found in areas of block faulting or crustal warping due to mountain building.
[3] 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.
The Green River Formation includes deposits from two large lakes which covered an estimated area of over 65,000 square kilometres (25,100 sq mi) during the Early to Middle Eocene.
[3] 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.
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.
The size, location and quality of oil shale deposits in the Paraíba Valley and the Irati Formation have attracted the most attention.
One theory suggests that the organic material in the Irati oil shale originated from algae deposited in a lacustrine environment with salinity varying from that of freshwater to brackish water.
Most of the organic matter is derived from the fossil green alga, Gloeocapsomorpha prisca, which has affinities to the modern cyanobacterium, Entophysalis major, an extant species that forms algal mats in inter-tidal to very shallow subtidal waters.
[3][6] A 2008 estimate set the total world resources of oil shale at 689 gigatons — equivalent to yield of 4.8 trillion barrels (760 billion cubic metres) of shale oil, with the largest reserves in the United States, which is thought to have 3.7 trillion barrels (590 billion cubic metres), though only a part of it is recoverable.
[3] Professor Alan R. Carroll of University of Wisconsin–Madison regards the Upper Permian lacustrine oil-shale deposits of northwest China, absent from previous global oil shale assessments, as comparable in size to the Green River Formation.
