14.3: Fossil Fuels

Figure \(\PageIndex{1}\): Coal power plant in Helper, Utah.

Fossils fuels are extractable sources of stored energy created by ancient ecosystems. The natural resources that typically fall under this category are coal, oil (petroleum), and natural gas. This energy was originally formed via photosynthesis by living organisms such as plants, phytoplankton, algae, and cyanobacteria. 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 account for a large portion of the energy used in the world.

Figure \(\PageIndex{1}\): Modern coral reefs and other highly-productive shallow marine environments are thought to be the sources of most petroleum resources.

Oil and Gas

Figure \(\PageIndex{1}\): World Oil Reserves in 2013. Scale in billions of barrels.

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]. As the rock (which is typically shale, mudstone, or limestone) lithifies, 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. Similar to the discussion of good aquifers in the chapter covering groundwater, if the rock is sandstone, limestone, or other porous and permeable rock, then that rock can act as a reservoir for the oil and gas.

Figure \(\PageIndex{1}\): A structural or anticline trap. The red on the image represents pooling petroleum. The green layer would be a non-permeable rock, and the yellow would be a reservoir rock.

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 [13; 14]. The development of a trap could be a result of many different geologic situations. Common examples include an anticline or domal structure, an impermeable salt dome, or a fault-bounded stratigraphic block (porous rock next to non-porous rock). The different traps have one thing in common: they pool the fluid fossil fuels into a configuration in which extraction is more likely to be profitable. Oil or gas in strata outside of a trap renders extraction is less viable.

Oil Shale

Figure \(\PageIndex{1}\): Global production of Oil Shale, 1880-2010.

Oil shale (or tight oil) is a fine-grained sedimentary rock that has a significant quantity of petroleum or natural gas. Shale is a common source of fossil fuels with high porosity but it has very low permeability. In order to get the oil out, the material has to be mined and heated which is expensive and typically has a negative impact on the environment [17].

Fracking

Figure \(\PageIndex{1}\): Schematic diagram of fracking.

Another process which is used to extract the oil and gas from shale and other unconventional tight resources is called hydraulic fracturing, better known as fracking [18]. In this method, high-pressure injections of water, sand grains, and added chemicals are pumped underground, creating and holding open fractures in the rocks, which aids in the release of the hard-to-access fluids, mostly natural gas. This is more useful in tighter sediments, especially shale, which has a high porosity to store the hydrocarbons but low permeability to transmit the hydrocarbons. Fracking has become controversial due to the potential for groundwater contamination [19] and induced seismicity [20] and represents a balance between public concerns and energy value.

Coal

Figure \(\PageIndex{1}\): USGS diagram of different coal rankings.

Coal is the product of fossilized swamps [21], though some older coal deposits that predate terrestrial plants are presumed to come from algal buildups [22]. It is chiefly carbon, hydrogen, nitrogen, sulfur, and oxygen, with minor amounts of other elements [23]. As this plant material is incorporated into sediments, it undergoes a series of changes due to heat and pressure which concentrates fixed carbon, the combustible portion of the coal. In this sense, the more heat and pressure that coal undergoes, 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:

Swamp => Peat => Lignite => Sub-bituminous => Bituminous => Anthracite => Graphite.

As swamp materials collect on the floor of the swamp, they turn to peat. As lithification occurs, peat turns to lignite. With increasing heat and pressure, lignite turns to sub-bituminous coal, bituminous coal, and then, in a process like metamorphism, anthracite. Anthracite is the highest metamorphic grade and most desirable coal since it provides the highest energy output. With even more heat and pressure driving out all the volatiles and leaving pure carbon, anthracite can turn to graphite.

Figure \(\PageIndex{1}\): Anthracite coal, the highest grade of coal.

Coal has been used by humans for at least 6000 years [23], mainly as a fuel source. Coal resources in Wales are often cited as a primary reason for the rise of Britain (and later, the United States) in the Industrial Revolution [24; 25; 26]. According to the US Energy Information Administration, the production of coal in the US has decreased due to cheaper prices of competing for energy sources and recognition of its negative environmental impacts, including increased very fine-grained particulate matter, greenhouse gases [27], acid rain [28], and heavy metal pollution [29]. Seen from this point of view, the coal industry is unlikely to revive.

References

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