15.2: Fossil Fuels

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. These resources were originally formed via photosynthesis by living organisms such as plants, phytoplankton, algae, and cyanobacteria. This energy is actually fossil solar energy, since the Sun’s ancient energy was converted by ancient organisms into tissues that preserved the chemical energy within the fossil fuel. Of course, as the energy is used, just like respiration from photosynthesis that occurs today, carbon enters the atmosphere as CO₂, causing climate consequences. Today, fossil fuels account for a large portion of the energy used in the world.

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

Converting solar energy by living organisms into hydrocarbon fossil fuels is a complex process. As organisms die, they decompose slowly, usually due to being buried rapidly, and the chemical energy stored within the organisms’ tissues is buried within surrounding geologic materials. All fossil fuels contain carbon that was produced in an ancient environment. In environments rich with organic matter such as swamps, coral reefs, and planktonic blooms, there is a higher potential for fossil fuels to accumulate. Indeed, there is some evidence that over geologic time, organic hydrocarbon fossil fuel material was highly produced globally. [8]. Lack of oxygen and moderate temperatures in the environment seem to help preserve these organic substances [9; 10]. Also, the heat and pressure applied to organic material after it is buried contribute to transforming it into higher quality materials, such as brown coal to anthracite and oil to gas. Heat and pressure can also cause mobile materials to migrate to conditions suitable for extraction [11].

Figure \(\PageIndex{2}\): Modern coral reefs and other highly-productive shallow marine environments are thought to be the sources of most petroleum resources.

Oil and Gas

Petroleum is principally derived from organic-rich shallow marine sedimentary deposits where the remains of micro-organisms like plankton accumulated in fine-grained sediments. [12]. Petroleum's liquid component is commonly called oil and its gas component called natural gas, which is mostly made up of methane (CH₄). As rocks such as shale, mudstone, or limestone lithify, increasing pressure and temperature cause the oil and gas to be squeezed out and migrate from the source rock to a different rock unit higher in the rock column. Similar to the discussion of good aquifers, if the rock is sandstone, limestone, or other porous and permeable rock, and involved in a suitable stratigraphic or structural trapping process, then that rock can act as an oil and gas reservoir.

Figure \(\PageIndex{3}\): World Oil Reserves in 2020. Scale in billions of tons. (By Our World in Data; CC BY via ourworldindata.org)

A trap is a combination of a subsurface geologic structure, a porous and permable rock, 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, which is 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 it less viable to extract.

Figure \(\PageIndex{4}\): A structural or anticline trap. Gas and oil can get trapped under an impermeable rock layer. (By MagentaGreen; CC BY-SA 4.0 via Wikimedia Commons.)

Sequence stratigraphy is a branch of geology that studies sedimentary facies both horizontally and vertically and is devoted to understanding how changing sea level creates organic-rich shallow marine muds, carbonates, and sands in areas that are close to each other [15]. For example, shoreline environments may have beaches, lagoons, reefs, nearshore and offshore deposits, all next to each other. Beach sand, lagoonal and nearshore muds, and coral reef layers accumulate into sediments that include sandstones—good reservoir rocks— next to mudstones, next to limestones, both of which are potential source rocks. As sea level either rises or falls, the location of the shoreline changes and the locations of sands, muds, and reefs with it. This places oil and gas producing rocks such as mudstones and limestones next to oil and gas reservoirs such as sandstones and some limestones. Understanding the lithology and the facies/stratigraphic relationships interplay is very important in finding new petroleum resources. Using sequence stratigraphy as a model allows geologists to predict favorable locations of source rocks and reservoirs.

Figure \(\PageIndex{5}\): The rising sea levels of transgressions create onlapping sediments, while regressions create offlapping.

Tar Sands

Conventional oil and gas, which is pumped from a reservoir, are not the only way to obtain hydrocarbons. There are a few fuel sources known as unconventional petroleum sources. However, they are becoming more important as conventional sources become scarce. Tar sands, or oil sands, are sandstones that contain petroleum products that are highly viscous, like tar, and thus, cannot be drilled and pumped out of the ground readily like conventional oil. The unconventional fossil fuel is bitumen, which can be pumped as a fluid only at very low rates of recovery and only when heated or mixed with solvents. So, using injections of steam and solvents, or directly mining tar sands for later processing are ways to extract the tar from the sands. Alberta, Canada is known to have the largest reserves of tar sands in the world [16].

Figure \(\PageIndex{6}\): Tar sandstone from the Miocene Monterey Formation of California.

An energy resource becomes uneconomic once the total extraction and processing costs exceed the extracted material's sales revenue. Environmental costs may also contribute to a resource becoming uneconomic.

Oil Shale

Oil shale, or tight oil, is a fine-grained sedimentary rock that has a significant quantity of petroleum or natural gas locked tightly in the sediment. Shale is a common source of fossil fuels with high porosity but it has very low permeability. To extract the oil directly from the shale, 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 \(\PageIndex{7}\): Global production of oil shale, 1880-2010.

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. Under high pressure, this creates and holds open fractures in the rocks, which aids in the release of the hard-to-access fluids, mostly natural gas. Fracking 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 because its methods contaminate groundwater [19] and induce seismic activity [20]. This has created much controversy between public concerns, political concerns, and energy value.

Figure \(\PageIndex{8}\): Schematic diagram of fracking.

Coal

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]. Coal is chiefly carbon, hydrogen, nitrogen, sulfur, and oxygen, with minor amounts of other elements [23]. As plant material is incorporated into sediments, heat and pressure cause several changes that concentrate the fixed carbon, which is the combustible portion of the coal. So, the more heat and pressure that coal undergoes, the greater its carbon concentration and fuel value, and the more desirable is the coal.

The general sequence of a swamp progressing through the various stages of coal formation and becoming more concentrated in carbon is:

Swamp \(\rightarrow\) Peat \(\rightarrow\) Lignite \(\rightarrow\) Sub-bituminous Coal \(\rightarrow\) Bituminous Coal \(\rightarrow\) Anthracite \(\rightarrow\) Graphite

As swamp materials collect on the swamp floor and are buried under accumulating materials, they first turn to peat. Peat itself is an economic fuel in some locations like the British Isles and Scandinavia. 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 \(\PageIndex{9}\): USGS diagram of different coal rankings.

Humans have used coal 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 during 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 energy sources and due to society recognizing its negative environmental impacts, including increased very fine-grained particulate matter as an air pollutant, greenhouse gases [27], acid rain [28], and heavy metal pollution [29]. Seen from this point of view, the coal industry as a source of fossil energy is unlikely to revive.

Figure \(\PageIndex{10}\): Left: Peat (also known as turf) consists of partially decayed organic matter. The Irish have long mined peat to be burned as fuel though this practice is now discouraged for environmental reasons. (By David Stanley; CC BY 2.0 via Wikimedia Commons.) Right: Anthracite coal, the highest grade of coal.

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