Oil Shale Geology - Formation

Formation

Most oil shale formations took place during mid-Cambrian, early and middle Ordovician, late Devonian, late Jurassic and Paleogene periods. 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 pressure could result in the 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 2,000 feet (610 m) and yield up to 40 gallons of oil for each ton (166 l/t) 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 environment. These oil shales are not as organically rich as lake-deposited oil shales, and generally do not contain more than 30 gallons 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 (166 l/t) of oil shale. However, due to their small areal extent, they are considered unlikely candidates for commercial exploitation.