10.3: Fossil Fuels- Formation and Mining

Fossil fuels are nonrenewable sources of energy formed from the organic matter of plants and microorganisms that lived millions of years ago. The natural resources that typically fall under this category are coal, oil (petroleum), and natural gas. This energy (and CO₂) 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. Of course, as the energy is used, just like respiration from photosynthesis that occurs today, carbon can enter the atmosphere, causing climate consequences. 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.

The conversion of living organisms into fossil fuels is a complex process.  As organisms die, most organic matter is oxidized back to CO₂ relatively quickly (within weeks or years in most cases), but any of it that gets isolated from the oxygen of the atmosphere (for example, deep in the ocean or in a stagnant bog) may last long enough to be buried by sediments and, if so, may be preserved for tens to hundreds of millions of years and the chemical energy within the organisms’ tissues is added to surrounding geologic materials. Higher productivity in the ancient environment leads to a higher potential for fossil fuel accumulation, and there is some evidence of higher global biomass and productivity over geologic time [8]. Lack of oxygen and moderate temperatures seem to enhance the preservation of these organic substances [9; 10]. Heat and pressure that is applied after burial also can cause transformation into higher quality materials (brown coal to anthracite, oil to gas) and/or migration of mobile materials [11].

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, the first fossil fuel to be widely used, forms mostly on land in swampy areas adjacent to rivers and deltas in areas with humid tropical to temperate climates. 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 \(\PageIndex{1}\)). 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). This situation, where the dead organic matter is submerged in oxygen-poor water, must be maintained for centuries to millennia in order for enough material to accumulate to form a thick layer. At some point, the peat was buried — typically because a river changes its course or sea level rises — and formed coal after millions of years of high pressure and temperature.

Figure \(\PageIndex{1}\): 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 \(\PageIndex{2}\)). 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 \(\PageIndex{2}\): 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 subbitminous 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 \(\PageIndex{3}\)). 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 \(\PageIndex{3}\): Anthracitic coal, the highest grade of coal.

Mining Coal

Coal is extracted by two principal methods, of which there are many variants: surface mining or subsurface mining. Surface mining uses large machines to remove the soil and layers of rock known as overburden to expose coal seams that are close to the Earth’s surface (figure \(\PageIndex{4}\)). Strip mining is a type of surface mining in which overburden is sequentially removed from each stretch (strip) of land. Once overburden is removed from the first strip, coal is removed. Overburden from the second strip is then deposited into the first strip, and coal is removed from the second strip. Overburden from the third strip is then placed in the first strip, and so on.

Figure \(\PageIndex{4}\): Surface mining of coal in Wyoming. Image by Bureau of Land Management (public domain).

Mountaintop removal is a more destructive type of surface mining in which all of the overburden is removed with explosives, revealing the entire coal seam at once (figure \(\PageIndex{5}\)). The large mass of overburden (the mountaintop) is dumped into a nearby valley, and the coal is them removed.

Figure \(\PageIndex{5}\): Left: Mountaintop removal uses explosives to loosen overburden at the top of a mountain, which is then displaced into a nearby valley (valley fill). The underlying coal seam is then extracted. A sediment pond collects soil that erodes from the valley fill. Image by EPA (public domain). Right: A mountaintop removal site. Image by JW Randolph (public domain)

Subsurface mining (deep mining) employs underground tunnels to access deeper deposits (figure \(\PageIndex{6}\)). Some underground mines are thousands of feet deep, and extend for miles. Miners ride elevators down deep mine shafts and travel on small trains in long tunnels to get to the coal. The miners use large machines that dig out the coal. In drift mines, a tunnel is dug horizontally into the side of a mountain. In slope mines, this tunnel is diagonal. In shaft mines, elevators are used to move coal through vertical tunnels.

Figure \(\PageIndex{6}\): Subsurface mining involves digging tunnels to access coal deposits that are deep underground. Image by National Energy Education Development Project/EIA (public domain)

There are also significant health effects and risks to coal miners and those living in the vicinity of coal mines. Traditional underground mining is risky to mine workers due to the risk of entrapment or death. Over the last 15 years, the U.S. Mine Safety and Health Administration has published the number of mine worker fatalities and it has varied from 18-48 per year. Twenty-nine miners died on April 6, 2010 in an explosion at the Upper Big Branch coal mine in West Virginia, contributing to the uptick in deaths between 2009 and 2010. In other countries, with less safety regulations, accidents occur more frequently. In May 2011, for example, three people died and 11 were trapped in a coalmine in Mexico for several days. There is also risk of getting black lung disease (pneumoconiosis). This is a disease of the lungs caused by the inhalation of coal dust over a long period of time. It causes coughing and shortness of breath. If exposure is stopped the outcome is good. However, the complicated form may cause shortness of breath that gets increasingly worse.  Mountaintop mining (MTM), while less hazardous to workers, has particularly detrimental effects on land resources. MTM is a surface mining practice involving the removal of mountaintops to expose coal seams, and disposing of the associated mining waste in adjacent valleys. This form of mining is very damaging to the environment because it literally removes the tops of mountains, destroying the existing habitat. Additionally, the debris from MTM is dumped into valleys burying streams and other important habitat.

Figure \(\PageIndex{7}\): Mountaintop Removal Coal Mining in Martin County, Kentucky Photograph shows mountaintop coal removal mining in Martin County, Kentucky. Source: Flashdark.

 

Oil and Gas

Petroleum, with the liquid component commonly called oil and gas component called natural gas (mostly made up of methane), is principally derived from organic-rich shallow marine sedimentary deposits [12] formed from ancient marine microorganisms (plankton) (Figure \(\PageIndex{8}\)). 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.  As the depth of burial increases, so does the temperature—due to the geothermal gradient—and gradually the organic matter within the sediments is converted to hydrocarbons. The first stage is the biological production (involving anaerobic bacteria) of methane. Most of this escapes back to the surface, but some is trapped in methane hydrates near the sea floor. At depths beyond about 2 km, and at temperatures ranging from 60° to 120°C, the organic matter is converted by chemical processes to oil. This depth and temperature range is known as the oil window. Beyond 120°C most of the organic matter is chemically converted to methane.  (Some natural gas is also found associated with coal deposits (coalbed methane), consisting of methane produced during coal formation.)

Figure \(\PageIndex{8}\): 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 \(\PageIndex{9}\)). 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 \(\PageIndex{9}\): 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)

Extraction of Conventional Oil and Natural Gas

Conventional oil and natural gas are contained under a trap (cap rock). Because natural gas consists of lighter molecules that are in a gaseous form at moderate temperatures, it is found on top of the oil, which may be floating on groundwater. To access conventional oil and natural gas, the trap is first pierced. Initially, they are under high enough pressure, and this drives them out of the well (primary recovery). Next, water (or gas) is injected to force more fossil fuels out (secondary recovery). Finally, enhanced oil recovery (tertiary recovery) may be used to extract further oil by applying heat (injecting steam) or injecting carbon dioxide, other gases, or larger molecules. For example, carbon dioxide causes the oil to thin and expand, making it easier to remove from the rocks. Note that secondary recovery simply increases pressure inside the reservoir whereas tertiary recovery changes the properties of the oil, making it easier to extract (figure \(\PageIndex{10}\)). Each stage of recovery is increasingly expensive, and extraction from a well continues as long as it remains profitable.

Figure \(\PageIndex{10}\): Injection wells transfer water, carbon dioxide, or other substances to an oil deposit increase pressure or change the oil's properties, facilitating extraction. On the right is the production well, through which extracted oil flows. This is facilitated by an injection well for enhanced recovery. On the left is an injection well for disposal through with waste water (produced water) is stored underground. Several confining formations trap substances underground. Closer to the top is the base of underground sources of drinking water, meaning that all drinking water is extracted from above this point. Image by Government Accountability Office (public domain).

Oil is mainly obtained by drilling either on land (onshore) or in the ocean (offshore). Early offshore drilling was generally limited to areas where the water was less than 300 feet deep. Oil and natural gas drilling rigs now operate in water as deep as two miles. Floating platforms are used for drilling in deeper waters (figure \(\PageIndex{11}\)). These self-propelled vessels are attached to the ocean floor using large cables and anchors. Wells are drilled from these platforms which are also used to lower production equipment to the ocean floor. Some drilling platforms stand on stilt-like legs that are embedded in the ocean floor. These platforms hold all required drilling equipment as well as housing and storage areas for the work crews. Offshore production is much more expensive than land-based production.

Figure \(\PageIndex{11}\): A platform for offshore oil drilling. Image by Pixabay/keridjackson (Pixabay license).

Extraction of Unconventional Oil and Natural Gas

Tight oil and natural gas trapped in shale (fine-grained sedimentary rocks with relatively high porosity and low permeability) as well as natural gas in tight sands (gas-bearing, fine-grained sandstones or carbonates with a low permeability) are extracted via hydraulic fracturing, informally referred to as “fracking”. This process uses explosives to create new fractures in these low-permeability rocks as well as increase the size, extent, and connectivity of existing fractures and then applies high-pressure fluid. First, a drill permeates the rock layers and then proceeds horizontally. Explosives then fracture rocks, freeing oil and natural gas. Finally, water, sand, and chemicals and injected, which flush out oil and natural gas (figure \(\PageIndex{12}\)).

Figure \(\PageIndex{12}\): Two diagrams of fracking. Water is mixed with sand and chemicals and then injected into shale trapping tight oil and natural gas or tight sand. This flushes fossil fuels from the fissures that were previously created by explosives. Top: This section shows four underground layers. From top to bottom, they are the shallow aquifer, aquiclude (impermeable layer), deep aquifer, and another aquiclude. Fracking fluid is injected underground through a well surrounded by casing into the gas-bearing formation. Methane (red arrows) might escape from hydraulic fractures in this formation. Additionally, when the fractures intersect with a pre-existing fault, induced seismicity (earthquakes) is possible. Aboveground, fracking fluid is stored in wastewater ponds. Blue arrows and question marks indicate places where toxic wastewater may escape and contaminate groundwater such as from storage containers, wastewater ponds, casing, or faults. Bottom: Proper treatment and disposal of the wastewater is needed to limit environmental impact. 1: Water is acquired. 2: Chemicals are mixed. 3: Fracking fluid is injected into the well. Here, natural gas flows from fissures into the well. 4: Fracking results in flowback and produced water (wastewater). 5: Wastewater undergoes treatment and disposal. Bottom image by USGS (public domain).

Fracking causes more environmental damage than conventional extraction. The considerable use of water (figure \(\PageIndex{13}\)) may affect the availability of water for other uses in some regions, and this can affect aquatic habitats. In fact, fracking consumes more water than the use of nuclear energy, coal, or conventional oil and natural gas. If mismanaged, hydraulic fracturing fluid can be released by spills, leaks, or various other exposure pathways that contaminate land and groundwater (figure \(\PageIndex{13}\)). Fracking fluid flowback – the fluid pumped out of the well and separated from oil and gas – not only contains the chemical additives used in the drilling process but also contains heavy metals, radioactive materials (which release radiation), volatile organic compounds, benzene (a carcinogen), toluene, ethylbenze, xylene, and other toxic air pollutants. Volatile organic compounds (VOCs) can react with the atmosphere to form ground-level ozone, which is associated with respiratory disease. Toulene can cause dizziness, confusion, headaches, and miscarriages. Ethylbenzene is a possible carcinogen that also causes dizziness, eye irritation, and hearing loss. Xylene also causes dizziness and headaches and furthermore can be fatal at high concentrations. In some cases, this contaminated water is sent to water treatment plants that are not equipped to deal with some of these classes of contamination. Finally, injecting wastewater for disposal can even induce earthquakes.

Figure \(\PageIndex{13}\): Fracking influences the water cycle. Water acquisition is the withdrawal of groundwater or surface water to make hydraulic fracturing fluids. Chemical mixing involves combining a base fluid, sand, and additives at the well site to create hydraulic fracturing fluids. During well injection, hydraulic fracturing fluids move through the oil and gas production well and in the targeted rock formation. Produced water handling refers to the onsite collection and handling of water that returns to the surface after hydraulic fracturing and the transportation of that water for disposal or reuse. Finally, the disposal and reuse of wastewater occurs. If this is not done properly, wastewater can pollute surrounding areas. Image and caption (modified) by EPA (public domain).

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 \(\PageIndex{14}\)). They are “unconventional” because the oil is exposed near the surface and is highly viscous because of microbial changes that have taken place at the surface. 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.   Tar sands can mined through strip mining or open-pit mining, a type of surface mining that involves forming a progressively deeper hole. The walls of the pit are as steep as can safely be managed. A steep wall means there is less waste overburden to remove and is an engineering balance between efficient mining and mass wasting.  Alberta, Canada is known to have the largest reserves of tar sands in the world.

Figure \(\PageIndex{14}\): Tar sandstone from the Miocene Monterrey Formation of California.

The oil sands are controversial primarily because of the environmental cost of their extraction. Since the oil is so viscous, it requires heat to make it sufficiently liquid to process. This energy comes from gas; approximately 25 m³ of gas is used to produce 0.16 m³ (one barrel) of oil. (That’s not quite as bad as it sounds, as the energy equivalent of the required gas is about 20% of the energy embodied in the produced oil.)

Oil shale (or tight oil) is a fine-grained sedimentary rock that sometimes contains kerogen, a solid material from which petroleum products can ultimately be manufactured.   Shale is a common source of fossil fuels with high porosity but it has very low permeability. 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.  Oil shale is extracted by strip mining, creating subsurface mines, or open-pit mining. Oil shale can be burned directly like coal or baked in the presence of hydrogen to extract liquid petroleum (figure \(\PageIndex{15}\)).  In order to get the oil out, the material has to be mined and heated, which, like with tar sands, is expensive and typically has a negative impact on the environment [17].

Figure \(\PageIndex{15}\): Underground mining of oil shale in Estonia.

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Contributors and Attributions

Modified by Kyle Whittinghill and Melissa Ha from the following sources:

• Challenges and Impacts of Energy Use, Water Pollution, and Non-Renewable Energy Sources from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)
• 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)
• Non-Renewable Energy Sources from Environmental Biology by Matthew R. Fisher (licensed under CC-BY)
• Fossil Fuels and Mining from An Introduction to Geology by Chris Johnson et al. (licensed under CC-BY-NC-SA)
• Chapter 4: Non-Renewable Energy from Introduction to Environmental Science: 2nd Edition (2018) Biological Sciences Open Textbooks by Zehnder, Caralyn; Manoylov, Kalina; Mutiti, Samuel; Mutiti, Christine; VandeVoort, Allison; and Bennett, Donna (licensed under CC-BY-NC-SA).
• Fossil Energy Study Guide: Natural Gas. 2014. U.S. Department of Energy. Accessed 01-12-2021.
• Fossil Energy Study Guide: Oil. 2013. U.S. Department of Energy. Accessed 01-12-2021.
• Fossil Fuels from Physical Geology by Steven Earle (licensed under a Creative Commons Attribution 4.0 International License)
• Environmental Challenges in Energy, Carbon Dioxide, Air, Water and Land Use  and Fossil Fuels from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors. Textbook content produced by Tom Theis and Jonathan (CC-BY). Download for free at CNX.