16.1: Types of Fossil Fuels and Formation

Fossil fuels are nonrenewable sources of energy formed from the organic matter of plants and microorganisms that lived millions of years ago. This energy was originally captured via photosynthesis by living organisms such as plants, algae, and photosynthetic bacteria. Sometimes this is known as fossil solar energy since the energy of the sun in the past has been converted into the chemical energy within a fossil fuel. As discussed in Food Chains and Food Webs and Matter, the organic molecules store chemical energy, which is released when the higher energy (less stable) bonds in these molecules are broken to form lower energy (more stable) bonds. Fossil fuels are nonrenewable because their formation took millions of years. Furthermore, higher productivity in the ancient environment allowed for more fossil fuel accumulation, meaning that the fossil fuel reserves available now could not necessarily be regenerated millions of years in the future.

Fossil fuels are composed primarily of hydrocarbons (molecules of just carbon and hydrogen), but they contain lesser amounts nitrogen, sulfur, oxygen, and other elements as well. The precise chemical structures vary depending on the type of fossil fuel (coal, oil, or natural gas). The molecules in coal tend to be larger than those in oil and natural gas. Coal is thus solid at room temperature, oil is liquid, and natural gas is in a gaseous phase. Specifically, coal is a black or dark brown solid fossil fuel found as coal seams in rock layers formed from ancient swamp vegetation. Both oil and natural gas are fossil fuels found underground that formed from marine microorganisms. Oil (petroleum) is a liquid fossil fuel and consists of a variety of hydrocarbons while natural gas is a gaseous fossil fuel that consists of mostly methane and other small hydrocarbons.

Coal

Coal is the product of fossilized swamps, although some older coal deposits that predate terrestrial plants are presumed to come from algal buildups. Coal was formed when plant material is buried, heated, and compressed in oxygen-poor conditions over a long period of time (figure \(\PageIndex{a}\)). Millions of years ago, continents were in different locations with different climates, and swamp-like vegetation covered many regions. When the vegetation died, it could not fully decompose due to oxygen-poor conditions. Instead, it formed peat (a brown substance high in organic content). The peat was buried and formed coal after millions of years of high pressure and temperature. The pressure was from the weight of sediments as well as from continental collisions.

Figure \(\PageIndex{a}\): Coal was formed when large plants in swamps died 300 million years ago (before the dinosaurs). Over millions of years, this vegetation was buried under water and dirt (100 million years ago). Eventually, heat and pressure turned the dead plants into coal, which is found under layers of rock and dirt. Image from U.S. Energy Information Administration/National Energy Education Development Project (public domain).

There are several different types of coal ranging in quality (figure \(\PageIndex{b}\)). The more heat and pressure that coal undergoes during formation, the greater is its fuel value and the more desirable is the coal. The general sequence of a swamp turning into the various stages of coal are as follows:

Swamp → Peat → Lignite → Subbituminous coal → Bituminous coal → Anthracitic coal → Graphite

Figure \(\PageIndex{b}\): Coal rankings depend on energy content, measured as gross calorific value (how much energy is released from combustion) and carbon content that can be burned (percentage of fixed carbon). Anthracitic coal (orange) is the highest quality coal, with high energy and carbon content. Next in quality is bituminous coal (gray), subbituminous coal (green), and lignite (yellow). All three have less carbon content than anthracitic coal. Bituminous coal retains high energy content, but subbituminous coal and lignite have lower energy content. Image by USGS (public domain).

Specifically, peat compacts to form solid rock through a process called lithification, producing lignite (brown coal, a low-quality form of coal). With increasing heat and pressure, lignite turns to subbituminous coal and bituminous coal. Lignite, subbituminous coal, and bituminous coal are considered sedimentary rocks because they from from compacted sediments. At very high heat and pressure, bituminous coal is  transformed to anthracite, a high-grade coal that is the most desirable coal since it provides the highest energy output (figure \(\PageIndex{c}\)). Anthracite is considered a metamorphic rock because it has been compacted and transformed to the extent that it is denser than the other forms of coal and no longer contains sheet-like layers of sediments. With even more heat and pressure driving out all the components that evaporate easily and leaving pure carbon, anthracite can turn to graphite. 

Figure \(\PageIndex{c}\): Anthracitic coal, the highest grade of coal.

Oil and Gas

Oil and natural gas formed from ancient marine microorganism (plankton). When plankton died, they were buried in sediments. As with coal, oxygen-poor conditions limited decomposition. As sediments continued to accumulate, the dead organisms were further buried. High temperature and pressure over millions of years ultimately produced oil and natural gas from these dead organisms.

Figure \(\PageIndex{d}\): Petroleum (oil) and natural gas were formed from marine microorganisms. (The image text mentions tiny marine plants, but they were primarily algae and photosynthetic bacteria rather than plants.) These were covered by layers of silt and sand 300-400 million years ago. Over millions of years, the remains were buried deeper and deeper. They are pictured 100 million years ago. The enormous heat and pressure turned the remains into oil and natural gas. Now, oil and natural gas deposits are found underground and can be extracted via drilling through the layers of sand, silt, and rock. Image from U.S. Energy Information Administration/National Energy Education Development Project (public domain).

As the rock forms from the sediments that originally trapped the plankton, the oil and gas leak out of the source rock due to the increased pressure and temperature, and migrate to a different rock unit higher in the rock column. If the rock is porous and permeable rock, then that rock can act as a reservoir for the oil and gas. Petroleum is usually found one to two miles (1.6 – 3.2 km) below the Earth’s surface, whether that is on land or ocean.

A trap is a combination of a subsurface geologic structure and an impervious layer that helps block the movement of oil and gas and concentrates it for later human extraction. Traps pool the fluid fossil fuels into a configuration in which extraction is more likely to be profitable, and such fossil fuels are called conventional oil and natural gas (figure \(\PageIndex{e}\)). Extraction of oil or gas outside of a trap (unconventional oil and natural gas) is less efficient and more expensive; sometimes it is not economically viable at all (does not produce a profit). Examples of unconventional fossil fuels include oil shale, tight oil and gas, tar sands (oil sands), and coalbed methane.

Figure \(\PageIndex{e}\): Conventional oil and natural gas deposits are trapped beneath impervious rock (gray). Conventional natural gas may be associated with oil or nonassociated. Coalbed methane and tight gas found in shale and sandstone are examples of unconventional fossil fuels. Image from USGS/EIA (public domain)

Oil Shale

Oil shale is a fine-grained sedimentary rock that sometimes contains kerogen, a solid material from which petroleum products can ultimately be manufactured. In order extract the fossil fuels, the material has to be mined and heated, which is expensive and typically has a negative impact on the environment. 

Tight Oil and Natural Gas

Tight oil and natural gas are also trapped in shale rock, fine-grained sedimentary rocks with relatively high porosity and low permeability. They differ from oil shale in that they can be extracted through a process called hydraulic fracturing (fracking). 

Similarly, fracking can be used to extract natural gas from tight sands, which are gas-bearing, fine-grained sandstones or carbonates (rocks made of minerals containing carbonate, CO₃^2-) with a low permeability.

Tar Sands

Tar sands, or oil sands, are sandstones that contain petroleum products that are highly viscous (like tar), and thus, can not be drilled and pumped out of the ground, unlike conventional oil (figure \(\PageIndex{f}\)). The fossil fuel in question is bitumen, which can be pumped as a fluid only at very low rates of recovery and only when heated or mixed with solvents. Thus, injections of steam and solvent or mining of the tar sands for later processing can be used to extract the tar from the sands. (See related information about strip mining with respect to coal in Mining, Processing, and Generating Electricity.) Alberta, Canada is known to have the largest reserves of tar sands in the world.

Figure \(\PageIndex{f}\): Tar sandstone from the Miocene Monterrey Formation of California.

Coalbed Methane

Some natural gas is also found associated with coal deposits (coalbed methane), consisting of methane produced during coal formation.

Attributions 

Modified by Melissa Ha from the following sources:

• Challenges and Impacts of Energy Use and Non-Renewable Energy Sources from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)
• Fossil Fuels from An Introduction to Geology by Chris Johnson et al. (licensed under CC-BY-NC-SA)
• Shale Gas 101. Office of Fossil Energy. U.S. Department of Energy. Accessed 01-12-2021. (public domain)
• The Process of Unconventional Natural Gas Production. 2021. United States Environmental Protection Agency. Accessed 01-12-2021. (public domain)