Fossil Fuels

Plants have an amazing ability. A towering redwood tree, or any other plant, is just carbon dioxide (CO₂) and water (H₂O) plus a few trace elements, put together with the sun’s energy. All plants that photosynthesize do this, releasing oxygen (O₂) in the process. The chemical composition of plants is, more-or-less, CH₂O, so an approximate chemical formula for photosynthesis is:

$C O_2+ H_{2}O + s u n^{\prime }s e n e r g y \to C H_{2}O + O_2$

Most of the rest of us—animals, fungi, many bacteria—as well as forest fires run this reaction backwards, combining plant material with oxygen to release energy, carbon dioxide, and water. Done rapidly, this is the “burning” of a fire; done slowly, it is still burning of a sort, which you might call "respiration." Plants usually include a little nitrogen and traces of other elements that we didn't write in the simplified formula above, and animals often use the plant material with its nitrogen and trace elements to make proteins that make animals, but after the animals die, they are almost always "burned" by bacteria or fungi or other animals that eat them to release the energy, and the equation is pretty close to what happens.

But, sometimes the dead materials end up in a place without oxygen. Then what? A little more “burning” can be done by microorganisms using certain other chemicals such as sulfate instead of oxygen, but all burning by living things stops when these other chemicals run out, too, and burning by fires does not occur in wet places. Then, the dead things may remain as unburned dead things for a long time. And, if a lot of dead things occur close together, fossil fuels—coal, oil, and natural gas—become possible.

Coal

Coal is formed when bacteria break down dead woody plants (trees, leaves, etc.). When and where there is no free oxygen in the air or water, bacteria remove the oxygen and hydrogen from the plant material, leaving mostly carbon and forming a brown material called peat. (Note that when gardeners talk about “peat”, they usually mean “peat moss”, a specially selected product made mostly of dead sphagnum moss. Peat moss is peat, but most peat would not be satisfactory for gardeners as peat moss.) When peat is buried by more sediment, it gets hotter as that sediment partially traps the Earth’s geothermal heat, which helps drive off more of the remaining oxygen and hydrogen, thus forming coal.

The classification of coal can become quite complex, but in common usage, coal is usually separated into lignite, bituminous, and anthracite. Lignite or brown coal has not been cooked too much; it is common in the western United States. Bituminous coal is formed from lignite by heat and pressure and is common in many places including western Pennsylvania. In a few places including eastern Pennsylvania, closer to the center of the old Appalachian Mountains, the bituminous coal was cooked to metamorphic anthracite coal. Peat is found with loose sediment, lignite with not-too-hard sedimentary rock, bituminous with harder sedimentary rock, and anthracite with metamorphic rock.

Oil and Natural Gas

Oil has similarities to coal, but oil is formed mostly from dead algae buried in mud, usually from marine settings but sometimes from lakes. Other dead things may be involved in oil formation as well, but they were “slimy” rather than woody in life. Please note that dead dinosaurs or other large creatures have never been important in fossil-fuel formation; there just aren’t enough large creatures at any time to supply the immense amounts of carbon needed to make economically exploitable fossil-fuel deposits.

Algae start with more hydrogen and less oxygen than wood, so the fossil fuel they produce ends up with chemicals containing mostly carbon and hydrogen (usually called hydrocarbons), different from the mostly-carbon coal that forms from woody plants. If mud containing dead algae is buried deeper by more sediment, heat breaks down the algae to release liquid oil. More heat breaks down the oil and makes natural gas, which is primarily methane (CH₄). (Some natural gas also is made at low temperatures by bacteria before the algae and mud are buried too deeply, and some natural gas is made during the formation of coal.) While the heat is making oil and gas, the mud is being squeezed to make shale.

Pennsylvania contains some oil and natural gas, and the first modern oil well was drilled in western Pennsylvania in the year 1859. Humans had used petroleum before then, but from natural seeps rather than wells. Indeed, that first Pennsylvania oil well was drilled where oil was leaking out of the ground and had been used by native people for a long time. That first oil well was motivated in part by a looming shortage of whale oil for lamps to light homes on dark nights, because the demand for whale oil far exceeded the ability of the oceans to grow whales. (People also used a biofuel that was a mixture of alcohol and turpentine in lamps, but it was even more explosive than the kerosene that came from the oil, and thus was quite dangerous.)

Where We Find Fossil Fuels

As noted above, early humans found fossil fuels where they leaked out of the ground. At certain oil seeps, after some of the oil evaporated or was used by bacteria, a sticky material was left behind that trapped animals in “tar pits”. Native people probably harvested these trapped animals at what is now the La Brea Tar Pits National Natural Landmark in California, and at other such tar pits elsewhere, in addition to using the oil and tar in other ways. But, to get the huge quantities of fossil fuel we use today, we cannot wait for it to leak out of the ground; we have to go down and get it.

Coal is usually found in layers between layers of other rocks. Early coal mining generally involved digging out the coal from these layers and hauling it to the surface, using supports to prevent roof collapses (which sometimes occurred anyway). Increasingly, coal mining is done by digging giant open pits, stripping off the materials above the coal and dumping them elsewhere, then digging up the coal. Such “strip mining” has large environmental costs.

Most oil and gas are formed in shale, which is made from mud. The shales that are rich in dead algae and thus produce a lot of oil and gas often appear dark in color and are called “black shales”. The mud that makes up the shale has very small particles with very small spaces between them, and oil and gas do not move rapidly through those small spaces unless the shale is broken. Because of this, much of the oil and gas that originally formed in these shale layers is still in them.

We humans have recently become much more efficient at getting oil and gas out of these black shales, in a process generally called “fracking”. An oil company will drill down to a black shale layer, and then use some impressive technologies to turn the drill and bore along inside the layer. Then, the borehole is pressurized enough to break the surrounding shale, and water, sand, and various chemicals are squirted into the new cracks to keep them open and enhance the ability of the oil and gas to move through the new cracks, as shown in the first panel of the diagram below. Oil and gas are then pumped out of the well, as shown in the second panel.

But much of the original oil and gas long ago escaped from where they formed in the shale. Probably most of that original oil and gas has been lost entirely, as described soon, but some of it has been trapped elsewhere. Traditionally, most oil and gas came from oil companies drilling into this oil and gas that migrated to traps.

Gas or oil take up more space than the dead algae they came from, and so as they form, they tend to push on the surrounding shale and help break it, often aided by mountain-building stresses or other stresses in the Earth. Most of these fractures will let some oil and gas out of the shale and then re-seal as the weight of sediments above squeezes them back together and often as new minerals are deposited in the cracks. This keeps much of the oil and gas in the shale and makes it difficult for other oil, gas, and water to move through the shale layers.

Oil and gas are low in density and float on water or vent into the air. So, when they escape from a black shale layer, they tend to rise through water-filled pores in rocks and escape at natural seeps on land or beneath the sea. In some places such as in the Gulf of Mexico, strange biological communities have been found living on oil seeping out of the sea floor. (Oil is natural, and some species “like” oil in small quantities. But if a supertanker wrecks or an oil well blows out, nature cannot use that much oil all at once, and large problems for nearby living things usually result.)

Because of the tendency for oil and gas to escape, a large accumulation of oil outside of black shale can form only if there is a trap of some sort. Many different types of traps exist. For example, the figure below shows one common type of trap.

Deposition of shales often occurred alternating with sandstones, as you saw at the Grand Canyon, so it is common to see a pile of sedimentary layers that goes shale-sandstone-shale-sandstone, and so on. As shown in the diagram, mountain-building processes often folded the layers in such a pile. Then, oil and gas escaping from a deep black shale layer rose into the spaces between grains in the sandstone above, but were trapped by a shale layer above that.

Geoscientists have worked for decades to design better ways to find such special places so that wells can be drilled into them to extract the oil from the sandstone, as shown. Extracting the trapped oil and gas is easier than fracking, and fewer wells can produce more fossil fuels for longer times, thus making more money for the oil company if the geoscientists are really good at finding the special places. If the geoscientists don’t find the special places or the drillers don’t hit the special places, a “dry hole” results and millions of dollars may have been wasted on drilling. Oil companies have hired more geoscientists for decades because the geoscientists have been so efficient at doing their jobs.

At present rates of use, and at costs vaguely similar to what we see today, the oil and gas will last maybe half a century, and the coal for more than a century. If we were willing to pay more for gasoline, say $50 per gallon, more fossil fuels would be available. As discussed below, that would cause very large and very damaging climate changes, as well as other damaging impacts.

Impacts of Fossil Fuels

There are a few references at the end of the Module if you want to follow up on the topics introduced here.

We use fossil fuels for good reasons. Most of our energy is obtained from fossil fuels. We run washing machines, rather than hand-scrubbing our clothes, primarily with fossil-fuel energy. Most of us are freed from the manual labor of hoeing and plowing to grow our food because fossil-fueled tractors plow and plant and cultivate and harvest. Many of us have been freed from freezing to death in the winter, or perishing of heat stroke in the summer, or dying because we can’t get to the hospital in an emergency, because of fossil fuels. In the US in recent decades, slightly more than 80% of our external energy use was supplied by fossil fuels. And, our energy use in that time was about 100 times as much as the energy from our food—what is done for us by external energy, mostly fossil fuels, is 100 times more than we can do for ourselves. This is a HUGE difference, and an important reason why we live as well as we do. (For the world over the same time, the average is about 25 times more external energy than energy from food, with a similar fraction of slightly more than 80% from fossil fuels.)

For humans, and for the few types of domesticated animals and plants that have benefited from our use of fossil fuels (pigs, rice, chickens and soybeans, for example), there is little question that fossil fuels have been good. For other species on Earth, our use of fossil fuels to tame much of the planet has been less beneficial. How much easier did fossil-fueled trains make it for humans to travel west to shoot bison? How much easier is it to cut a tree with a fossil-fueled chain saw than with a stone hatchet? However, this is complicated by the fact that we have let some trees grow back, and we quit burning whales (or whale oil) to light evenings because we switched to burning the long-dead algae and trees that are fossil fuel.

There clearly are other costs of fossil-fuel use. Damage to tundra from oil exploration may last decades or longer. Acid rain, mainly from coal-fired power plants, killed the trout in headwaters streams in the Great Smoky Mountains National Park and in some other places including in Pennsylvania. Smog is not good for us and shortens our lives. A study published in 2021, for example, found that 20% of global deaths were caused by breathing fine particles released by fossil-fuel burning. (Other studies have found somewhat different numbers, and there are ways to clean up much of this pollution without completely stopping the use of fossil fuels, but there is no doubt that burning causes air pollution that hurts health, and that much of our burning is fossil fuels. We will see soon that fossil-fuel use is also making forest fires worse, further increasing the health risks.)

The list could go on. Interstate 95 was closed twice in less than a year for extended periods because bridges were damaged by the resulting fires when fuel tankers crashed. Various insurance and government data coming out in the early 2020s showed that gasoline-powered cars, with their explosive fuel interacting with electrical systems (spark plugs, batteries, lights, …) were 10-100 times more likely to catch on fire than electric-only cars. Leaks from gasoline storage tanks or other fuel and oil leaks have contaminated groundwater in many places. Carbon dioxide released by fossil-fuel burning is acidifying the ocean and endangering many species. Earthquakes are often triggered when salty, somewhat-radioactive fluids that are recovered from fossil-fuel wells are pumped back into the ground for disposal. This is far from a complete list. And, as discussed in the next section, climate change caused by fossil-fuel use is probably the biggest issue, with potentially immense costs.

There can be no doubt that anything we do to get large amounts of energy will have unintended consequences, and that we could make a long list of problems with energy sources we might use to replace fossil fuels, such as nuclear or hydroelectric or geothermal or wind or solar or waves or tides or…. There is an increasingly rich scientific and engineering literature, though, showing that a sustainable energy system can be built that will supply more energy at less total expense and with fewer unintended consequences than the one we have now, which is really good news. We cannot possibly cover this whole topic in this course, but we’ll return to it a little before we finish this Module. Be assured, though, that there can be good outcomes from what may seem like a really bad situation.