Shale Coal Petroleum Gas Notes

Expansion Tectonic small Earth models provide a means to investigate the global distribution and association of shale oil/gas, coal, petroleum, and natural gas across adjoining continents at various times throughout Earth history. Recognition of their inter-relationships and distributions over time provides an important means for furthering international and regional search, as well as understanding genetic associations beyond their known occurrences.

Data used in this study are based on a number of published global maps. It is acknowledged that information on the location and global distribution of these natural resources may be incomplete or overly simplistic. Shale oil/gas data were sourced from the 2013 U.S. EIA report "Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries outside the United States". Coal data was sourced from the "Major Coal Deposits of the World" map (2010) showing the global distribution of anthracite and bituminous coal, and lignite. Petroleum and gas data were sourced from the "World Oil and Gas Map" (4th edition) produced by the Petroleum Economist (2013).

Oil shale

Oil shale, also known as kerogen shale, is an organic-rich fine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds) from which shale oil can be produced. The general composition of oil shales constitutes an inorganic matrix, bitumen, and kerogen. Oil shale formation takes place in a number of depositional settings and has considerable compositional variation. The three major types of organic matter in oil shale are telalginite, lamalginite, and bituminite. Some oil shale deposits also contain metals which include vanadium, zinc, copper, uranium.

The organic components of oil shale derive from a variety of organisms, such as the remains of algae, spores, pollen, plant cuticles and corky fragments of herbaceous and woody plants, and cellular debris from other aquatic and land plants. Some deposits contain significant fossils. The mineral matter in oil shale includes various fine-grained silicates and carbonates including quartz, feldspars, clays (mainly illite and chlorite), carbonates (calcite and dolomites), pyrite and some other minerals.

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 where low oxygen levels prevented their complete bacterial decay.

Shale Gas

Shale gas is natural gas that is found trapped within shale formations. Shale gas is one of a number of unconventional sources of natural gas. Others include coalbed methane, tight sandstones, and methane hydrates. Shale has low matrix permeability, so gas production in commercial quantities requires fractures to provide permeability. Shale gas has been produced for years from shales with natural fractures. The shale gas boom in recent years has been due to modern technology in hydraulic fracturing (fracking) to create extensive artificial fractures around well bores.

Shales that host economic quantities of gas have a number of common properties. They are rich in organic material (0.5% to 25%), and are usually mature petroleum source rocks in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas. They are sufficiently brittle and rigid enough to maintain open fractures.

Like oil and coal, natural gas in shales has essentially formed from the remains of plants, animals, and micro-organisms. Though there are different theories on the origins of fossil fuels, the most widely accepted is that they are formed when organic matter is buried, compressed and heated in the earth´s crust. In the case of natural gas, this is referred to as thermogenic methane generation. These shales are present worldwide in the sedimentary sequence where sediments accumulated over millions of years.

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperatures and pressures. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulphur, oxygen, and nitrogen.

There are several different types of coal which are ranked according to their carbon and volatile matter content.

•	Anthracite is 86 to 98% pure carbon and 8 to 3% volatile matter. It is an excellent fuel that is still used to heat homes.
•	Bituminous coal contains 70 to 86% carbon and 46 to 31% volatile matter. It is used to make coke, used in metallurgy.
•	Sub-bituminous coal is 70 to 76% carbon and 53 to 42% volatile matter. It is burned in industrial boilers.
•	Lignite is 65 to 70% carbon and 63 to 53% volatile matter. It is a low-grade fuel with a high moisture content that is used in industrial boilers.
•	Peat consists of partially decomposed vegetation. It has a carbon content of less than 60% and is composed entirely of volatile matter.

The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Vast swaths of forest covered the land, which would eventually be laid down and become the coal beds characteristic of the Carboniferous system. The atmospheric content of oxygen also reached their highest levels in history during the period, 35% compared with 21% today. This increased the atmospheric density by a third over today's value. A minor marine and terrestrial extinction event occurred in the middle of the period, caused by a change in climate. The latter half of the period experienced glaciations, low sea level, and mountain building.

The large coal deposits of the Carboniferous may owe their existence primarily to two factors. The first of these is the appearance of wood tissue and bark-bearing trees. The evolution of the wood fibre lignin and the bark-sealing, waxy substance suberin variously opposed decay organisms so effectively that dead materials accumulated long enough to fossilise on a large scale. The second factor was the lower sea levels that occurred during the Carboniferous as compared to the preceding Devonian period. This promoted the development of extensive lowland swamps and forests. It has been proposed that large quantities of wood were buried during this period because animals and decomposing bacteria had not yet evolved enzymes that could effectively digest the resistant phenolic lignin polymers and waxy suberin polymers. Fungi that could break those substances down effectively only became dominant towards the end of the period, making subsequent coal formation much rarer.

Lignite

Lignite, often referred to as brown coal, is a soft brown combustible sedimentary rock formed from naturally compressed peat. It is considered the lowest rank of coal due to its relatively low heat content. It has a carbon content around 25-35%. It is mined all around the world and is used almost exclusively as a fuel for steam-electric power generation. Lignite deposits are typically younger than higher-ranked coals, with the majority of them having formed during the Tertiary period.

Lignite begins as an accumulation of partially decayed plant material, or peat. Burial by other sediments results in increasing temperature, depending on the local geothermal gradient and tectonic setting, and increasing pressure. This causes compaction of the material and loss of some of the water and volatile matter (primarily methane and carbon dioxide). This process, called coalification, concentrates the carbon content, and thus the heat content, of the material. Deeper burial and the passage of time result in further expulsion of moisture and volatile matter, eventually transforming the material into higher rank coals such as bituminous and anthracite coal.

Petroleum

Oil, also referred to as petroleum is a naturally occurring, yellow-to-black liquid found in geological formations beneath the Earth's surface, which is commonly refined into various types of fuels. It consists of hydrocarbons of various molecular weights and other organic compounds. The name petroleum covers both naturally occurring unprocessed crude oil and petroleum products that are made up of refined crude oil.

Petroleum is a fossil fuel derived from ancient fossilized organic materials, such as zooplankton and algae. Vast quantities of these remains settled to sea or lake bottoms, mixing with sediments and being buried under anoxic conditions. As further layers settled to the sea or lake bed, intense heat and pressure build up in the lower regions. This process caused the organic matter to change, first into a waxy material known as kerogen, which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis. Formation of petroleum occurs from hydrocarbon pyrolysis in a variety of mainly endothermic reactions at high temperature and/or pressure.

Three conditions must be present for oil reservoirs to form: a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil, a porous and permeable reservoir rock for it to accumulate in, and a cap rock (seal) or other mechanism that prevents it from escaping to the surface. Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are less dense than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.

Natural Gas

Natural gas is a fossil fuel formed when layers of decomposing plant and animal matter are exposed to intense heat and pressure over millions of years. Natural gas is a hydrocarbon gas mixture consisting primarily of methane, but commonly includes varying amounts of other higher alkanes, it is also the main source of helium (2-7%) and sometimes a usually lesser percentage of carbon dioxide, nitrogen, and/or hydrogen sulphide.

Natural gas is found in deep underground rock formations or associated with other hydrocarbon reservoirs in coal beds and as methane clathrates. It is often found in close proximity to petroleum. Most natural gas was created over time by two mechanisms: biogenic and thermogenic. Biogenic gas is created by methanogenic organisms in marshes, bogs, landfills, and shallow sediments. Deeper in the earth, at greater temperature and pressure, thermogenic gas is created from buried organic material.