Module 3: Oil, Coal & Natural Gas | Drilling, Fracking & Reserves
Module 3 Overview
Each spring, plants grow rapidly on the land and in the ocean. And, each year, enough plants die to approximately balance the new growth. Most of the dead plants are broken down quickly, by bacteria or bison or button mushrooms, or any of the other living things that rely on plants, or by being burned in fires. But, some of the plants are buried without oxygen, and begin the process of being cooked by the Earth to make fossil fuels.
Woody plants eventually may become coal, “slimy” plants may become oil, and both produce natural gas. The fossil fuels now in the Earth accumulated over a few hundred million years. If we keep burning them at modern rates, the fuels will be gone in a few hundred years; if much of the world continues to catch up with the US rate of use, the fossil fuels may become quite scarce late in this century. Nature will make more, but not enough to be helpful until millions of years have passed.
Video: Colyer Lake (4:11)
Goals and Objectives
Goals
- Recognize the natural and human-driven systems and processes that produce energy and affect the climate
- Use numerical tools and publicly available scientific data to demonstrate important concepts about the Earth, its climate, and resources
Learning Objectives:
By the end of this module you should be able to:
- Recall that oil, coal and natural gas are produced naturally by well-understood processes
- Evaluate the effects of technology, economics, and population growth on fossil fuel production using computer models
- Demonstrate that our current consumption of fossil fuels is not sustainable by exploring future scenarios with computer models
Roadmap
| To Read | Materials on the course website (Module 3) | - |
|---|---|---|
| To Do | Complete Summative Assessment [3] Quiz 3 |
Due Following Tuesday Due Sunday |
Questions?
If you prefer to use email:
If you have any questions, please email your faculty member through your campus CMS (Canvas/Moodle/myShip). We will check daily to respond. If your question is one that is relevant to the entire class, we may respond to the entire class rather than individually.
If you prefer to use the discussion forums:
If you have any questions, please post them to Help Discussion. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
The Formation and Future of Fossil Fuels
What the Frack? The Formation and Future of Fossil Fuels
Older residents of the US may recall a television comedy that ran during the years 1962-1971, and much longer in reruns, concerned with the energy system. The show, called The Beverly Hillbillies, was the story of a “man named Jed” from the mountains, who became rich when his hunting bullet struck oil.
The idea was already rather unlikely in the 1960s, but not yet absurd. Before our modern oil industry was developed, oil did seep to the surface at many places around the world. For example, the ice-age bones of saber-toothed cats, mammoths and other creatures of the La Brea Tar Pits of southern California are stuck in the sticky left-overs from oil seeps, after the more-liquid parts evaporated or were “eaten” by microbes (see figure below).
The first modern oil well, the Drake Well in western Pennsylvania, was drilled in 1859 at the site of natural oil seeps. Native people had used the oil, and Europeans were using it in medicine and in lamps.
Note
For more about oil and gas seeps, including dice for games, and weapons, made by native people from “tar” or asphaltum at oil seeps, see USGS [5].
Throughout history, people had relied on springs that produce water, and for millennia had drilled or dug down to get more water. People had even occasionally gotten oil and gas from their water wells, so it is not surprising that someone (Edwin Drake) decided to drill at one of the many natural oil seeps to get more oil (see figure below).
Natural oil seeps still exist, but most are long-gone, except in the most remote places, including far down on special parts of the sea floor. Huge numbers of oil and gas wells since the Drake Well—maybe more than a million—have rapidly pumped out the oil that would have seeped slowly to the surface over millennia and longer. The idea of a modern mountaineer hitting a huge new oil field with a stray bullet really is absurd.
Origins of Oil, Coal and Gas
Where do oil, coal and gas come from?
Short version: Growing plants use the sun’s energy and simple chemicals to make more plants, and animals “burn” the plants to get that stored energy from the sun. Almost everything that grows is burned, but in special cases some plants are buried without oxygen, escaping burning. Time and heat turn these buried plants into fossil fuels.
Friendlier but longer version: Recall that energy is the ability to do things. And, living requires doing things—fighting against randomness to put particular chemicals in particular places to make cells and cell walls, to protect oneself and reproduce.
Living things on Earth could tap into many energy sources. Heat flows out of the Earth beneath our feet, for example. But, the energy from the sun reaching the Earth’s surface exceeds that from inside the planet by more than 2000 times, so it is clear that harnessing the sun gives greater opportunities for living things. (This is also why you will never hear a weather forecaster worrying about the effects of the Earth’s heat!)
Video: Photosynthesis and Respiration (:51)
Photosynthesis is the process by which plants grow more of themselves, using simple chemicals and the sun’s energy to make more-complex chemicals that store energy. Respiration is the process by which animals, fungi, etc. run photosynthesis backward, “burning” plants to release the stored energy for use by the animals, and releasing simple chemicals, ready to be used by plants again.
You probably have seen the equations for photosynthesis, the process by which plants harness the sun. The simplest statement of the commonest type of photosynthesis goes something like this: water + carbon dioxide + solar energy → plants + oxygen. Or, if you prefer chemical symbols saying the same thing:
(Don’t worry if you had a class sometime in which this equation was written with 6 waters plus 6 carbon dioxides making 6 oxygens plus the chemical glucose, ; that’s the same story simplified in a slightly different way, and either way you write it is close enough for our purposes.)
Almost all of the biological activity on the planet depends on this pathway to capture the sun’s energy. When the sun isn’t shining, plants run this backward, and animals and bacteria and fungi all run it backward, combining oxygen with plants to release water, carbon dioxide, and the sun’s energy that the plants stored chemically. Fires do this, too. Depending on whether it happens in a fox or a fire, you may see this energy release called respiration, or burning, or oxidation, or combustion, or perhaps other words, but all serve to combine oxygen with plant material to release carbon dioxide, water and energy.
Video: Respiration (:39)
Averaged around the planet and over a year, roughly 0.1% of the energy from the sun that reaches Earth is stored as chemical energy by plants. (This is called net primary production, if you want the technical term.) Clearly, plants capture more of the sun’s energy in some places and times than in others, and agricultural experts have worked hard to find ways to make plants especially productive for us in our gardens and farms, but plants are still not very efficient. Even so, the world’s plants capture about 10 times as much energy as humans use.
If plants would jump into our fuel tanks and liquefy, we would have far more energy than we needed, but things don’t work that way. And, because everything alive on Earth wants to burn plants for energy, we face large difficulties in harvesting plants and burning them for our use before something else beats us to it. Almost all plant material is burned rather quickly after it grows, sometimes being eaten by caterpillars or cows while still alive, other times by fungi or bacteria after dying.
But, it is a very large and very old world, so even a very small difference between what grows and what is burned will eventually add up to a very large store of energy. And, that is what fossil fuels are.
Earth: The Operators' Manual
Video: Formation of Fossil Fuels: A 2+ minute clip on Fossil Fuels - How they are made and why they are ultimately unsustainable (2:25)
Oxygen in Water
Oxygen in Water
Water doesn’t hold much oxygen, so lakes and the oceans are relatively low in oxygen, especially if the water is warm. Oxygen made in the water by growing plants tends to form bubbles that rise and escape to the air above. Aquariums often need “bubblers” to add air to the water and give the fish enough oxygen to breathe. Running water, or fast currents in the ocean, do this job in nature, picking up a little oxygen at the surface and taking it down to fish and worms and other creatures. But if the currents are slow and a lot of dead plants are sinking, the bottom of the ocean or a swamp or lake may have more plants to be “burned” than oxygen to burn them.
Sometimes “dead zones” form in ocean water above the bottom, where the decay of sinking plants uses up almost all the oxygen so that fish and other large creatures cannot live. Such dead zones are especially associated with places where runoff of human fertilizer from fields on land causes huge blooms of algae.
Video: Gulf of Mexico Dead Zones (1:33)
More commonly, though, oxygen is present in the water but scarce in the sediments beneath. Almost everywhere in lakes and the ocean, sediment is piling up at the bottom. This may include large pieces of rock—sand and gravel—washed into the water by rivers, or carried across by melting icebergs and dropped. Smaller pieces are more common—silt and clay, sometimes just called mud—with most of the small pieces washing into the water in streams, but some blowing in, and even a tiny bit sifting down from meteorites. This sediment also includes organic matter (dead plants and animals).
Strong currents carrying plenty of oxygen tend to carry away the small pieces of mud and the dead plants, leaving sand and gravel without much organic material, and with big spaces between the big grains that oxygen-bearing water can move through. Where currents are slow, mud and dead plants accumulate, and the tiny spaces make it hard for water to move through, carrying oxygen. As worms and bacteria start to burn the dead plants, the oxygen is exhausted and the burning stops. So, where lots of plants grow in still, warm water, dead ones tend to pile up in the mud at the bottom without being burned.
Want to learn more?
Read the Enrichment called More on Oxygen in Water at the end of the module!
How Nature Makes Coal, Oil and Gas
How Picky Eaters and Earth's Cooks Make Coal, Oil and Gas
We humans eat apples and eggplants, but we don’t eat their stems or leaves or roots. Bacteria in water are similarly picky. Even before a plant sinks all the way to the bottom of the ocean, bacteria and other living things are picking off the chemicals they like, either because those chemicals are easier to get or more useful to the bacteria, leaving other chemicals behind. This continues as the plants are buried. Some bacteria in low-oxygen but organic-rich mud make methane, CH4, the main ingredient in natural gas, as described in the Enrichment section More on Oxygen in Water. As more mud accumulates on top, deeper sediments are warmed by the heat of the Earth, “cooking” the dead plants. The result depends on how much cooking occurs, and what the plants were at the beginning.
“Woody” land plants—tree trunks, but also leaves, twigs, roots, etc.—become coal, which is mostly carbon. During the transformation from leaves and twigs to hard, shiny black coal, we change the name, first to peat, and then to coal of different types, lignite, then bituminous, then anthracite. You’ll generally find that as time, heat, and pressure change the organic materials, they also change the rest of the sediment around the coal. Peat occurs in sediments that are not yet hard enough to be called rock, lignite in soft sedimentary rocks, bituminous in harder ones, and anthracite in metamorphic rocks.
Oil is formed from “slimy” water plants (algae, plus things such as cyanobacteria that probably shouldn’t really be called plants, but we’re simplifying a little here). Because oil is primarily made of carbon (C) and hydrogen (H), we sometimes refer to it as a hydrocarbon. Methane is the simplest hydrocarbon, CH4, but oil contains a great range of larger hydrocarbon molecules, such as octane (C8H18). With too much heat, the oil breaks down to make methane. This gas is also produced as coal forms.
Coal, as a solid, mostly sits where it was formed. Eventually, if the rocks above it are eroded so that it is exposed at the Earth’s surface, the coal itself may be eroded away, and either “eaten” by bacteria, or buried in new rocks. And, occasionally, a natural forest fire or a lightning strike may set coal on fire. This burning usually isn’t really fast because after the coal nearest the surface burns away, oxygen doesn’t get to deeper coal very easily. But, a lot of coal has avoided being eroded or burned, and is sitting in the rocks where it formed.
(Humans have also set coal on fire, releasing mercury and other toxic materials, and burning up a valuable resource. A few percent of China’s annual coal production may be burned in such fires, the town of Centralia in Pennsylvania was abandoned because of one such fire (see the figure below), and other impacts occur.)
Mining coal involves either removing the rocks on top, or tunneling into the Earth along the coal layer. Removing the rocks on top of the coal, called “surface mining” or “strip mining”, requires putting those rocks on top of something else, breaking the coal loose with machines or explosives, hauling the coal away to be burned, and then either putting the rocks back on top or just leaving them. (We’ll revisit some of the implications of this later in the semester.) Digging along the coal is often called “deep mining”, and puts miners in a potentially dangerous place. For more information about mountain removal mining, visit the U.S. Environmental Protection Agency [16] for some good resources, and watch the video at NASA's page on Mountaintop Removal [17].
When mud rocks (shale layers) are heated, the buried dead plants break down into the smaller molecules that make up oil and gas. Initially, these are trapped in the shale. However, because many small molecules take up more space than a few big ones, heating and cooking the rocks raises the pressure inside until the oil and gas seep out, often by cracking the rock. After some oil and gas escape, the pressure drops and the cracks close under the weight of rocks above. This may happen multiple times as more cooking occurs.
After oil and gas have escaped from the shale into sandstone or other rocks with bigger spaces, the oil and gas can move through those spaces. Most sediments are deposited under water, or the spaces in them fill up with water later. Natural gas is gaseous (no surprise there!), oil is liquid and floats on water, and so both tend to move upward through the water-filled spaces. The great majority of oil and gas eventually reach the Earth’s surface as oil or gas seeps. Before the industrial revolution, the amount of fossil fuel being formed, and the amount leaking out of seeps, were probably very similar (we’ll give some numbers soon).
However, recall that fluids have more difficulty moving through smaller spaces. If oil and gas are rising through spaces in rock, their motion may be blocked by another shale layer. Especially if the shale has been bent by movements in the Earth associated with mountain-building, so that the oil and gas rise into a “trap”, the fossil fuels may sit there for a long time (see the figure below).
Exploration for Oil and Gas
Oil and Gas Exploration
For over a century, exploration for oil and gas—finding the next big field full of valuable fossil fuels—has involved locating oil and gas traps and drilling into them. Most commonly, this has involved “seismic” exploration (see the figure and explanation below). Nature figured out how to use this technique long before humans did. For example, a bat flying around in the dark “looking” for a moth to eat will make a noise, and listen to the echo off the moth, using the time and direction to locate the flying dinner. Dolphins can find their food the same way.
Video: Air Gun Vessel to find Oil (0:55)
Oil explorers make noises, and listen to the reflections from layers in the Earth, using the time and direction to locate the oil-and-gas-filled traps. Then, drillers drill into the traps, and pump the oil and gas out. (Sometimes, the pressure is so high in the trap at the start that the oil comes out of the hole without being pumped, as a “gusher,” see figures below.)
But, soon, the pressure down there is reduced, and a pump is needed. Occasionally, a gusher catches on fire, with sometimes disastrous consequences, see the figure below.
Increasingly, a new technique is being used to recover oil and gas. Shale layers often have a lot of hydrocarbon left in them that did not escape in the past. Drillers have learned how to bore down to a shale layer, then turn the drill and bore along in the layer. When the hole is long enough, the drillers pump fluids at high pressure into the hole, breaking the shale in a process called “fracking” (from “fracturing”) that mimics the natural process by which oil and gas escaped the shale. Human use of this process was apparently first invented by a veteran of the US Civil War, Col. Edward Roberts, who saw the fractures in the ground caused by an exploding Confederate shell, and went on to patent the technique of using explosives to fracture rocks and allow more flow into wells. The technique has been improved in many ways since.
In many ways, fracking is not revolutionary but evolutionary from older techniques for recovering oil and gas. Under best practices, fracking probably isn’t inherently more risky or dangerous than those other methods. The biggest difference is that fracking is used to recover oil and gas that are spread out over large areas rather than having a large quantity concentrated in one place. So, fracking takes lots more drilling and pumping and installing pipelines in more places. Fracking is more likely to be in someone’s backyard, or near it, so there are more people seeing it and hearing it and complaining about it.
The more drilling there is, the more chances there are for mistakes to be made, contaminating groundwater or otherwise causing problems for neighbors. The drilling can also bring other problems, including lots of traffic. For example, back on Sept. 23, 2011, an article by Cliff White in the Centre Daily Times, State College, PA noted “A review of inspections performed by state police on commercial motor vehicles used in support of Marcellus Shale gas drilling operations in 2010 revealed 56 percent resulted in either the vehicle or driver being placed out of service for serious safety violations” but that “Thanks to heavy enforcement, the noncompliance rate has dropped to about 45 percent in the most recent study.” And, in the same article, “…a trooper in gas-rich Bradford County, said during the initial ramp-up of activity in that area a few years ago, almost all of the vehicles used for gas drilling-related purposes that he stopped had “some degree” of noncompliance.”)
Fracking is done with high-pressure fluids to which certain chemicals have been added, as noted above, and some of those chemicals may be dangerous to humans. The fracking fluids plus salty brines from the rocks “flow back” out of the wells, and these flowback fluids must be disposed of in some way. Much of that disposal recently has involved injecting the flowback fluids into the Earth in special deep wells. This has caused numerous earthquakes, some of them damaging. (See, for example, USGS: Induced Earthquakes [24].) Fluid injection for other reasons also has caused earthquakes; fracking is especially important in this only because it generates so much fluid that is being injected. Note that while fracking has probably triggered a few small earthquakes directly, the main cause of earthquakes is this injection of flowback fluids.
Fracking is likely to be with us for a long time. And, it is likely to remain at least somewhat controversial.
Video: Process of Fracking (1:01)
Earth: The Operators' Manual
Fort Worth: Gas, Waste & Water (9:08)
If you want to see a little more on fracking, much of the clip is relevant, but the first 3 minutes and 40 seconds especially fit here.
No Discussion Assignment for Module 3
Unconventional Oil
What is unconventional Oil?
You may also hear about oil shales and tar sands (see image below). These are sometimes called unconventional petroleum or unconventional oil, or something similar, and represent opposite ends of a spectrum: oil shales haven’t been cooked enough to make oil yet, and tar sands are the leftovers after cooking and dining.
Tar sands, such as the huge deposits of Alberta, Canada (see images above), are like the much smaller tar deposits in the pits at La Brea, mentioned earlier. Oil contains many different types of molecules. When oil seeps to the surface, the smaller ones tend to evaporate, or to be used preferentially by bacteria, leaving the larger molecules behind. These larger ones don’t flow as easily, so the result is a thick, almost solid mass of “tar” (technically called “bitumen”). Native Americans were waterproofing their birch-bark canoes with Alberta’s bitumen when the first Europeans arrived, probably with no knowledge that early peoples of the Fertile Crescent of Mesopotamia also used bitumen to waterproof boats.
Because the bitumen is so “thick” (viscous), normal drill-and-pump techniques don’t work well. Many techniques are in use or being tested to separate oil from the sand or gravel in which it occurs. For shallow deposits, the tar-soaked sands can be surface-mined and then heated or mixed with appropriate chemicals to free the oil from the sand. For deeper deposits, injection of steam or hot air or other hot fluids can warm the bitumen enough that it will flow. Oil companies are even experimenting with setting small fires in wells, to make heat and gases that drive liquid hydrocarbon to other wells. All of these techniques have associated costs, including water and energy use. For now, much more energy is obtained from the oil recovered than is used in recovery, but the ratio is not as good as for “normal” oil, and is likely to get worse as the easier-to-recover tar sands are used up.
In contrast to the tar-sand “leftovers” from normal oil after bacteria have eaten a lot, oil shales are undercooked not-yet-oil. In many places, dead plants and mud accumulated, but without being buried deeply enough to get hot enough to break down the dead plants and make oil. The dead plants have typically been changed enough to get a new name (“kerogen”), but not to make oil that can be pumped out easily. This sort of deposit is called oil shale (Figures 13-15). (The names are NOT the easiest to deal with. Oil pumped out of shale may be called shale oil, but the shale from which that oil is pumped is generally not called oil shale. Instead, that shale is an oil source rock. The name “oil shale” is saved for those shales that haven’t been heated enough to make oil, but that could be in the future. Given our choice, most of us who work in these areas would pick clearer names, but no one asked us!)
Oil shale can be burned as-is, but the organic matter is diluted by the clay in the shale, so just burning doesn’t work really well. Most plans for future use involve speeding up the natural process, heating the rock to “pyrolyze” the organic matter, releasing oil and gas while leaving some organic material behind in the rock. This may be done in the ground, or after mining the shale. Because energy is needed to heat the rock, costs tend to be higher, and energy recovered lower, than for conventional oil in which the heat of the Earth acting over millions of years did the cooking for us.
Reserves and Resources
What the difference between reserves and resources?
Short version: If we work hard at recovering fossil fuels, huge amounts remain. We are probably at least decades and perhaps longer from real scarcity of fossil fuels, although with notable uncertainty. But, we may be close to the point at which fossil fuels are scarce enough to start causing problems.
Friendlier but longer version: Experts in the field generally separate fossil-fuel “reserves” from “resources” (and, they have additional technical terms that subdivide these big types). You might say that “reserves” are what you are (almost) sure you can use in the modern economy with modern technology, whereas “resources” are what you think you can have in the future.
For example, the US Energy Information Agency, in defining “proved reserves” for oil (also known as “proven reserves”; similar definitions apply for gas and coal), says that this includes “the estimated quantities of all liquids defined as crude oil, which geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known reservoirs under existing economic and operating conditions.
Their data indicate that, as of 2011 (the last year for which full data were available at the time this was being written; a partial update in 2016 didn’t change these numbers much), the world had 46 years of proved reserves at current rates of use. These numbers were higher for gas (150 years) and coal (120 years). With oil slightly more important than the others in the world economy, these would give most of a century of fossil fuels before we run out. However, use has been rising rapidly. If everyone in the world used fossil fuels at the same rate as in the US, total use would be more than 4 times faster, reducing the life of the proved reserves to perhaps 20-30 years.
The resource is likely much bigger. If you spend a little while looking at the figure "Where is the Carbon?” just below, you’ll see first that the authors from the US NOAA are discussing how much fossil fuel we have, and how much we’re burning, in units of gigatons of carbon. (1 gigaton of carbon = 1 Gt C. A gigaton is a billion tons. Fossil fuels contain some hydrogen and a little bit of other things, but focusing on the carbon is a useful way to calculate.) The authors estimate that we have already burned fossil fuels containing about 244 Gt C, from an original 3700 Gt C, leaving 3456 Gt C to be burned. The figure is a few years old, and the use rate of 6.4 Gt C per year that they show has increased to perhaps 9 Gt C per year. That would leave almost 400 years of fossil fuels at current use rate, or less than a century if everyone reached the US rate (and even less if population continues growing). Some estimates of the resource are even bigger, in the range of 4000 to 6000 Gt C, and even more if we figure out how to use clathrate hydrates, which might have about as much carbon as the other fossil fuels although probably notably less.
Video: Where is Carbon? (2:10)
Proven Reserves
Proven Reserves Explained
But, because of the heating needed to get oil from tar sands and oil shales, and the extra effort to drill deeper and frack rocks, some of the resources will be used up recovering the rest of it, increasing the rate of use. And, we don’t really know very well what the resource is; serious investors and regulators tend to rely on the proved reserves for good reasons.
You may also recall from earlier in the course that peak whale oil production from the US fleet was followed almost immediately by a tripling of the cost, even though oil production continued at fairly high levels during the following decades—impacts of scarcity are felt long before the resource is exhausted. People often assume that production of a resource follows a sort of bell curve, starting slow, then rising rapidly, peaking, declining and tailing off to almost nothing. If that model is accurate, then the peak—peak oil, or peak coal, or peak-whatever, occurs when half of the resource has been used. The whale-oil experience suggests that scarcity shows up and starts to restrain the economy just about then. If so, then we now may be much closer to problems from fossil-fuel shortages than we realize.
Another point worth considering is that some countries have a lot of fossil fuel, and others not much (see the "World's Proven Reserves" figure just below). And, who has fossil fuel and who uses it are not always the same. For oil, for example, for most recent years including 2016, the US was the third-largest producer, although the US was #1 by a small margin in 2015, based on data from the International Energy Agency. But, because the US is the largest user, being third in production isn’t enough, and the US is the largest importer.
Video: Where is Oil, Coal, and Gas? (1:28)
Notice that the sorts of numbers here can be “spun” in many, many ways in the public discussion. Estimates of how much we have already discovered in the ground are at least somewhat uncertain; estimates of how much is in the ground that we haven’t discovered yet, or haven’t learned how to recover, are much more uncertain. Businesses and companies might have reason to report optimistic numbers, or pessimistic ones, depending on what they want to accomplish or sell or buy just now. How long the resource will last depends on how fast we burn. Should we estimate using modern rates of burning, or future ones, and if future, what will they be?
The rise of gas and oil fracking in the US has led to a rapid increase in reserves. You may hear people talking about a century of gas, although others use numbers as small as 25 years for the US reserve. But, at the start of fracking, gas was only about ¼ of the US energy use, so relying on gas as our main fuel could bump the low-end estimates down to only about 6 years. You could find some justification for bragging about a century or more of gas or warning that we may run out in a decade or less, by carefully choosing which estimates to adopt and how to use them. The module is to be very careful about the first numbers you hear, and think and compare before using a number to make decisions on, say, where to invest your retirement fund! (This applies to what you see in this class, too; no one can give you the absolute truth on this topic!)
Future of Fossil Fuels
Will Nature Make More Fossil Fuels?
You can be quite confident that as we use the fossil fuels, nature will produce more, and that this new natural production will be grossly inadequate to help us over the next decades to centuries.
Let's go back to the Where Is the Carbon diagram, which is repeated for your convenience. You’ll see that it shows 0.2 Gt C per year going into “surface sediment” at the bottom of the ocean. Other estimates vary somewhat; one well-known textbook used 0.05 Gt C for this flux. With the figure showing a burn rate of 6.4 Gt C per year, a number that has risen close to 9 Gt C per year, 0.2 is not especially big, but it isn’t completely zero, either. But, you’ll also notice a return flux labeled “weathering” that is also 0.2 Gt C per year. In the natural setting, the amount of dead plants being buried, and the amount of fossil fuel seeping out or otherwise returning to the surface were very similar.
Video: Where is Carbon Going? (1:50)
Note:
Please realize that while you saw this figure on the previous page, the video discussion here focuses on different aspects of this figure.
The black pre-industrial carbon values show the carbon cycle and the balance that existed without human emissions. The red values indicate the effects that the human emissions have had on the carbon cycle. Increased emissions have increased levels of carbon in the atmosphere, pressuring the ocean and land biosphere to accept more carbon and limiting their future effectiveness as carbon dioxide sinks.
The total amount of buried organic carbon, former dead plants, may be 10,000,000 Gt C, but that accumulated over 4.6 billion years, so the rate has averaged only 0.002 Gt C per year, tiny compared to our use. And, almost all of that buried organic carbon is too widely distributed to be used as fossil fuel; we would expend more energy getting it than it would yield when burned. The available resource is shown in the figure as 3500 Gt C, and other estimates are a little higher. Most of the resource accumulated in the last 500 million years, at a rate of roughly 0.00001 Gt C per year.
Thus, we are burning the fossil fuels roughly a million times faster than nature saved them for us. Nature will make more fossil fuels over geologic time, but what we burn is gone forever on the timescale of human economies. We have been given a “bank account” of fossil fuels, but when we spend it, it’s gone, with no significant deposits being made.
Summative Assessment
Reminder!
After completing your Summative Assessment, don't forget to take the Module 3 Quiz. If you didn't answer the Learning Checkpoint questions, take a few minutes to complete them now. They will help your study for the quiz, and you may even see a few of those questions on the quiz!Peak Oil Activity
You have, by now, learned some things about “peak oil”, the notion that the production of oil is at or near a peak and will decline in the future, forcing us to conserve more and shift to other sources for our energy needs in the future. The goal of this activity is to explore this notion of peak oil in a bit more depth, to understand how it is a natural consequence of supplies, demands, prices.
In this activity, we’ll be using computer models created in a program called STELLA. STELLA models are simple computer models that are perfect for learning about the dynamics of systems — how systems change over time. Systems, in this case are sets of related processes that are involved in the transfer and storage of some quantity. For example, the global water cycle is a system that involves processes like evaporation, precipitation, surface water runoff, groundwater flow, moving water from one place to another. Earth’s climate system is set of related processes involved in the absorption, storage, and radiation of thermal energy. In fact, you can think of the whole Earth as one big, complex system. Through the use of computer models, we can learn some important things about how they work, how they react to changes; this understanding can then help us make smart decisions about how to respond and adapt to a changing world.
What is a STELLA model?
A STELLA model is a computer program containing numbers, equations, and rules that together form a description of how we think a system works — it is a kind of simplified mathematical representation of a part of the real world. Systems, in the world of STELLA, are composed of a few basic parts that can be seen in the diagram below:
Terminology
A Reservoir is a model component that stores some quantity — thermal energy in this case.
- A Flow adds to or subtracts from a Reservoir — it can be thought of as a pipe with a valve attached to it that controls how much material is added or removed in a given period of time. In the above example, the Energy Added flow might be a constant value, while Energy Lost would be an equation that involves Temperature. The cloud symbols at the ends of the flows signify that the material or quantity has a limitless source, or sink.
- A Connector is an arrow that establishes a link between different model components — it shows how different parts of the model influence each other. The labeled connector, for instance, tells us that the Energy Lost flow is dependent on the Temperature of the planet.
- A Converter is something that does a conversion or adds information to some other part of the model. In this case, the Temperature converter takes the thermal energy stored in the Thermal Energy reservoir and converts it into a temperature using an equation.
To construct a STELLA model, you first draw the model components and then link them together. Equations and starting conditions are then added (these are hidden from view in the model) and then the timing is set — telling the computer how long to run the model and how frequently to do the calculations needed to figure out the flow and accumulation of quantities the model is keeping track of. When the system is fully constructed, you can essentially press the ‘on’ button, sit back, and watch what happens.
In this course, the models have all been made; you will interact with the models by changing variables with a user interface that has knobs and dials and then running the models to see how they change over time.
We will start with the simplest model we can imagine that represents the consumption of oil and gas, and then we will work with progressively more complex versions of the model.
Instructions
This assessment is broken into five sub-parts, with questions related to each part. Separate web pages have been provided for each part to reduce scrolling. We have also provided the activity as a worksheet that you can download and even print if you prefer. You may find downloading or printing the complete worksheet easier to work with as you prepare your answers to submit to the Mod 3 Summative Assessment (Graded) quiz.
Files to Download
Download the worksheet [33]. Completing the 'Practice' and 'Graded' versions of the exercise, in the following pages or on the attached worksheet, is required before submitting your assignment.
Submitting Your Assessment
Once you have answered all of the questions on the worksheet, go to the Module 3 Summative Assessment (Graded) quiz, in which you will see the worksheet link again and the Graded Assessment. The worksheet has practice questions with answers provided, and then graded versions of similar questions. Use the practice questions to make sure you are running the model correctly and reading the graphs properly, then do the graded questions, writing down your answers. The questions listed in the worksheet are repeated in the Canvas Assessment, so all you will have to do is read the question and select the answer that you have on your worksheet. You should not need much time to submit your answers since all of the work should be done prior to logging into clicking the assessment quiz.
Grading
This assignment is worth a total of 19 points -- the questions are all multiple choice.
Oil and Gas Reserves
Introduction
Oil and gas form at extremely slow rates — 10’s of millions of years — so we can consider the oil and gas present now to be all that is available. We can wait around all we want and there will be no significant increase in the oil and gas. The total amount of oil and gas in existence on Earth is sometimes called the oil in place. We can only guess at this (somewhere around 6 trillion barrels of oil equivalent), but regardless of its size, we can probably only get about 50% of it out of the ground (this recovery factor ranges from 10% to 80% for individual oil fields). The recoverable oil and gas can be divided into two types of reserves — proven and unproven. Proven reserves are the oil and gas that we know about (which means we have a 90% confidence level about them), while unproven reserves are the oil and gas that we are less certain of, but we have some indication of their existence. These reserves are usually expressed in terms of barrels of oil equivalent and include both oil and natural gas.
It is estimated that our proven reserves are on the order of 1.5 trillion barrels of oil, and unproven reserves are thought to be in the range of 3 trillion barrels. Last year, we consumed 31 billion barrels of oil, and at this rate of consumption, we’ve got less than 50 years worth of oil in the proven reserves, and about 97 years worth in the unproven reserves. Now, move onto the first part of this assessment 1. The Simplest Case.
1. The Simplest Case
In this first case, we’ll just consider the proven reserves, and we’ll assume that the oil produced is a constant percentage of how much remains in the proven reserves. The logic here is very simple — if there is more oil, you can produce more in a period of time, while if there is less oil, you produce less in the same time period — but the percentage remains the same.
Here is what the system looks like as a STELLA model:
Since this model is simply meant to illustrate the general pattern of oil/gas production resulting from an assumption of how production works, we’re not going to worry about the actual values, but you can think of the starting amount of Proven Reserves as 100% of what we have. Every year, we produce oil/gas at the rate of 2% of however much remains in the Proven Reserves reservoir. The production flow transfers oil/gas into the Produced Oil reservoir, so we can keep track of the total amount of oil/gas produced over time.
Let’s see if we can predict what will happen by doing a few simple calculations. When the model first begins:
Proven Reserves = 100
production = 100 x 0.02 = 2
This will reduce the Proven Reserves by 2, so it becomes 100-2=98. Then, in the next year:
Proven Reserves = 98
production = 98 x 0.02 = 1.96
This will reduce the Proven Reserves by 1.96, so it becomes 98-1.96=96.04. So, in the next year:
Proven Reserves = 96.04
production = 96.04 x 0.02 = 1.92
Notice that the production is declining as time goes on, and the amount of decline is getting smaller. If this pattern continues, the production will follow an exponential decline curve — like this:
Take a few minutes to watch the following video and learn about the browser-hosted STELLA model interface, before running the model.
Video: Peak Oil Model 1 (2:13)
Now, let’s run the model and see what happens. Follow this link to the Peak Oil Model [35] which should be set up exactly the same as the diagram above. Answer the questions either on the worksheet you downloaded or on a piece of paper to be submitted later to the Module 3 Summative Assessment (Graded) quiz. If you didn't download the worksheet on the main page of this assessment, do it now.
Question
1A. Does the production history agree with our simple calculations (position the cursor on the graph, and it will show you the values at different times)?
- yes
- no
2. Oil Production with Improving Technology
Oil and gas companies have certainly become better at what they do over time. Originally, they drilled near natural oil seeps and hoped for the best, but now, a good team of geoscientists can “see” exactly where the oil/gas is, and engineers can drill with great precision and then “stimulate” the oil/gas-bearing rock formations to squeeze as much oil/gas as possible out of the rocks.
One way to incorporate this into the model is to change the rate of oil/gas production, r, so that it increases as time goes on. To do this, we make a simple equation that says r = 0.0005 x TIME, so then when TIME is 10 years, r will be 0.005 and when TIME is 100 years, r will be 0.05. Other than this change, the model is the same as in experiment 1. The value 0.0005 is called tech rate in the model, and we’ll see what happens if we change it.
Let’s see how this change affects the history of oil/gas production. Click this link to run the model [36] and then answer the following questions on your worksheet or on a sheet of paper to be submitted to a Canvas Assessment later. As you can see, the production of oil peaks in this case. It rises because r is increasing, but as r increases, the Proven Reserves is decreasing and eventually a point is reached where the product of these two numbers (the production) starts to decline.
| - | Practice | Graded |
|---|---|---|
| Tech Rate | 0.0002 | 0.0004 |
Questions
2A. When does the production reach its maximum (peak) value?
2B. What is the magnitude of the peak in production?
3. Including Price in the Production Flow
In addition to technology, economics also plays a role in the production of oil/gas, in the sense that higher prices will motivate greater production. Let’s assume that the as the supply of proven reserves drops, the price will rise. As long as there is a demand for oil and gas, as it becomes more scarce, it will become more valuable. This is a pretty simplistic view of what determines the price of oil and gas — reality is much more complex, which is why prices fluctuate quite a bit over time. But it is hard to escape the basic reality that as a desirable commodity becomes scarce, its value goes up.
To make this change in the model, we need to add something that will calculate the price. This new model looks like this:
As before, production is defined as Proven Reserves x r, and r in this case is defined as price x tech_slope x TIME, so it once again has the increase over time that our previous model had, but it also increases as the price goes up. The tech_slope is just the slope of the increase in technology over time and the default value is 0.0002. Price here is defined as 0.01 + price_slope x (100 – Proven Reserves); price_slope is the slope of price increase relative to change in Proven Reserves, and is originally set to 0.05. At the beginning, Proven Reserves is 100, so this gives a price of 0.01 — very small. But, when Proven Reserves has declined to 50, we get a price of 2.51. This equation is not meant to be anything more than a way to make the price increase as the Proven Reserves get smaller. The value 0.01 at the front end of this equation is just there so that the price is not 0 at the beginning, which would then make r be 0 and no oil would ever get produced.
What we have created here is a system with a feedback mechanism. Here is how it works:
If the production increases, then the proven reserves must decrease; this triggers an increase in price, which in turns triggers an increase in production. Notice that the starting point (production increase) and the ending point (production increase) are the same. In other words, the change at the beginning of the mechanism promotes more of the same — this is what is known as a positive feedback mechanism. Positive feedback mechanisms tend to cause an acceleration of change, sometimes resulting in runaway behavior. In contrast, the are other feedback mechanisms that tend to counteract change, encouraging stability; these are known as negative feedback mechanisms. Note that in this context, positive is not necessarily good, and negative is not necessarily bad.
Take a few minutes to watch the video below to learn more about the positive feedback mechanism the oil production model before running the next model.
Video: Peak Oil Positive Feedback (1:19)
Activity:
This model has two pages of graphs to look at; the first one shows the Proven Reserves, Produced Oil, price, and production, and r (which combines price and tech slope), while the second one shows just the production. The second graph retains the results from previous model runs, allowing you to make comparisons as you make changes to some of the adjustable model parameters. If you want to clear this graph, hit the Restore Graphs button.
| - | Practice | Graded |
|---|---|---|
| Tech slope | 0.0001 | 0.0002 |
| Price slope | .05 | .07 |
Questions
3A. First, run the model as it is, with the price slope set to 0.05 and the tech slope set to 0.0002. Note the time and magnitude of the peak in production. Then alter the tech slope or price slope as prescribed, using the new values provided. Run the model and compare the peak time and magnitude with the original case (use page 2 of the graph pad). Use “sooner” or “later” and “greater” or “smaller” to describe how your alterations changed the timing and magnitude of the peak in production.
Change in time of peak =
Change in magnitude of peak =
- Yes — there are just a few cases in which a peak occurs
- No — it is impossible; the best you can do is a broad, low peak that takes a long time to develop.
4. Production Driven by Demand From a Growing Population
For our next experiment, we’ll try a different assumption about what drives oil/gas production — demand. The demand for oil and gas has risen over time due to an increase in the global population and an increase in the per capita energy consumption. Here is what this modified version of the model looks like:
Here, the population increases according to pop pct, which is the net growth percentage per year derived from historical data and then extrapolated into the future — so it is a graphical function that changes over time. The population starts at the 1800 level of 1 billion; the net growth % drops to 0 in 2100, and at that point, the population will stabilize.
The demand for oil/gas is represented here by per capita demand, which is essentially a percentage of the proven reserves per billion people. The per capita demand is another graphical function of time, patterned after actual history up until 2010 and then extrapolated to 2100 — optimistically assuming that the per capita energy demands will level off at about 2100. Multiplying the population times the per capita demand gives us r, the fraction of the proven reserves produced in a given year, and then r multiplied by the Proven Reserves gives us the production. The fraction r will increase as the population grows and as the per capita demand grows, and if population and per capita demand level off, so will r. Recall from experiment 2 that if r is increasing over time, a peak in production is inevitable.
Because we are using real population values and real values for the per capita demand, it makes sense to use real numbers for the Proven Reserves. At the present time, the best estimates are that there are 1.5 trillion barrels of oil as proven reserves (this number includes natural gas too), and we have consumed about 1.2 trillion barrels from about 1900 to the present. This means that at the beginning of time, our Proven Reserves will be 2.7 trillion barrels.
This model also includes a component called per capita oil that keeps track of how much oil is actually available per person, by taking the production and dividing it by the population. As per capita oil increases, we can use more and more oil for our energy needs, but as it decreases, we will have to either reduce our energy consumption or turn to other sources to meet our energy demands.
Questions
4A. Can you guess what will happen? Remember that r here is just like r in the earlier models, and you’ve seen what happens to the production history when r increases over time. Which of the following represents your approximate prediction?
- Production will increase throughout the model run
- Production will decrease throughout the model run
- Production will peak sometime during the model run
Now run the model by clicking this link [39], and see what happens. We will consider this as the “control” for the next experiment.
| - | Practice | Graded |
|---|---|---|
| Initial proven reserves | 2.0 | 3.5 |
(above numbers refer to trillions of barrels of oil)
4B. How will changing the initial size of the Proven Reserves reservoir affect the history of production? Set the initial Proven Reserves to 2.0 for the Practice Assessment (3.5 for the Graded Assessment) and then run the model and see what happens; choose the response below that best represents how your altered model compares with the control. Page 2 of the graph pad will be useful in making this comparison.
- It peaks at the same time, with a larger peak
- It peaks at the same time, with a smaller peak
- It peaks later, with a smaller peak
- It peaks later, with a larger peak
- It peaks earlier, with a larger peak
- It peaks earlier with a smaller peak
- It does not peak at all
Oil per capita in 2100 = _______ (within 0.1 barrels/person)
Previous time in history with same oil per capita = _______ (within 10 years)
As you can see, this model suggests that our future use of oil will be a little like traveling back in time.
5. Adding Unproven Reserves
For our last experiment, we’ll see what happens when we add two more reservoirs, Unproven Reserves (the oil and gas that, we think, is likely to be discovered in the future) and Unknown Oil (the oil and gas we don’t know about, but might be there). Discovery adds Unproven Reserves to the Proven Reserves reservoir, and another flow called discovery adds Unknown Oil to the Unproven reservoir. An example from the Arctic Ocean region helps us get a grasp of these unknown reserves. In this frontier region, less than half of the offshore sedimentary basins have been explored, but based on what is known from more serious exploration off the coast of Alaska, the USGS estimates that there might be ~130 billion barrels of oil and gas — so this is a resource that, we think, might exist, but not enough is known about it yet to put it into the unproven reserves category, which applies to oil reserves that we know exist, but we don’t know enough about them to put them into the proven reserves. For perspective, this Arctic Ocean oil might represent 10-15% of all the unknown oil/gas that remains, and it would be enough to last for 4 years at the current rate of global use.
The discovery of these new resources is a function of a rate constant that increases over time, dictated by something called the exploration slope. The discovery flow that leads from Unknown to Unproven Reserves is set to be 1/5 the rate of the other discovery flow, reflecting the fact that it is much harder to discover something we know little about. Both of the discovery flows are controlled by switches (they can be turned on or off) and they begin (if the switch is on) at a time that can be set using the explor start time control knob. Here is what this new model looks like:
Question
5A. How will these new sources of oil/gas change the production history? The total amount of produced oil obviously must be greater than in our model from experiment 4, but how about the shape of that production curve? Will there be a peak, as before? If so, what will that peak look like?
- Yes, it will still peak, but the peak will be broader than before
- No, it will not peak — the production will rise and then remain stead
- Yes, it will peak, but the peak will be delayed and it will be bigger
Video: Peak Oil Intro (2:44)
Before launching the model and experimenting with it, take a few minutes and watch the video that explains how to operate the switches that can turn the discovery flows on and off.
Open the model here [41], and first make sure the switches are in the off position (down), disabling the two discovery flows. Run the model and you should see exactly the same thing you saw in experiment 4B, with the difference that it runs for a longer period of time. If you study the graphs #3 and #7 show comparative plots of the production (in billions of barrels per year) and oil per capita (in barrels). Make sure you watch the video above to get a general sense of what happens when you turn on the switches.
You will be presented with one of the following 4 sets of initial conditions. Your answers to the following 3 questions will depend on which case you are presented.
| - | Practice | Graded |
|---|---|---|
| Initial unproven reserves | 1.5 | 3.5 |
| Initial unknown reserves | 2.0 | 2.5 |
| Explore start time | 1980 ± 1 | 2000 ± 1 |
Questions
5B-D. Set the model up using the initial values provided. Use the slider bars at the top to set the initial unproven reserves and the initial unknown oil, and use the dial near the lower right to set the explore start time (the time when we begin to develop and produce the unproven reserves and unknown oil. This dial is a bit hard to adjust precisely, but if you are within a year or two of the specified date, it will be fine. Then run the model with both switches off, then run it again with the unproven switch turned on and then one more time with both switches turned on. Evaluate the differences between these three model runs in terms of the production (graph #3) and the oil per capita (#7). There are many ways to evaluate the effects of adding these new sources of oil, but we’ll focus on the size and timing of the production peak, and the oil per capita in the year 2100.
5B. Oil per capita in 2100 with Unproven Reserve switch on (± 0.1)
5C. Oil per capita in 2100 with both switches on (± 0.1)
5D. Peak in production with both switches on compared to control (with no switches on).
- About the same time (within 10 yrs) and size (within 2 billion barrels/yr)
- About the same time (within 10 yrs), but slightly larger (2-5 billion barrels/yr)
- lightly later (10-20 yrs), and slightly larger (2-5 billion barrels/yr)
- Slightly later (10-20 yrs), and much larger (>5 billion barrels/yr)
- uch later (>20 yrs), and much larger (>5 billion barrels/yr)
5F. Can a peak in oil production be avoided? In other words, is it possible to find some combination of model parameters that results in more of a plateau in oil production? To figure this out, try changing the exploration slope (this will control that rate that the discovery flows increase), and the exploration start time. We’ll leave the unproven and unknown reserves at 3.0 because this is already a very optimistic outlook.
- Yes, a peak can be avoided
- No, a peak cannot be avoided, and no plateau greater than 10 yrs is possible
- No, a peak cannot be avoided, but a ~50 yr plateau is possible
6. In your own words, summarize the effects of 1) improving technology (of oil production); 2) the price-production feedback; and 3) growing population on the history of oil production.
Summary
We’ve just completed quite a few experiments, so it is a good idea to try to summarize a few important points.
- When we talk about “peak oil”, we’re talking about the rate of oil production — how much oil/gas is brought to market in a given year — not the total amount of oil/gas on Earth (which peaked as soon as we started to pump it out of the ground!).
- Improvements in our ability to extract oil/gas (i.e., improving technology) lead to a distinct peak in oil production.
- If we assume that production is motivated by price, and the price goes up as the oil becomes more scarce, this also leads to a peak in oil production.
- If the demand for oil is related to population, and population increases, this also leads to a peak in oil production. This effect is enhanced if the per capita demand for energy increases, as it has during the last 100 years.
- If we include unproven oil reserves and unknown reserves into the system, we can make the decline in oil production more gradual, but there is still a tendency for it to peak.
- At the end of the day, there is a finite amount of oil and gas available to us. This fact, combined with improvements in technology, an increase in demand for production related to price, increasing population, and increased standard of living, makes a peak in oil production inevitable — we cannot have a sustainable supply of oil and gas to fuel our economy. Facing up to this reality is important because it leads us to be more serious about making plans for a future where our energy needs are met without relying on fossil fuels.
Summary and Final Tasks
Module 3 Summary
The sun sends out energy continuously. Plants have figured out how to store that energy for later use, by combining water and carbon dioxide to make more plants. Almost all the other living things on Earth survive by “burning” these plants to get the stored energy.
Usually, plants are burned soon after they die, but occasionally some plants are buried without oxygen and survive for much longer. Time and the Earth’s heat combine to “cook” these old, buried plants, making fossil fuels. We rely on oil—primarily from “slimy” plants (algae, and similar water plants), coal—primarily from “woody” plants, and gas from both.
Most of the coal is found in the rocks where it formed, but most of the oil and gas we are using had migrated upward through spaces in the rock and then been trapped in geologically special places before reaching the surface. Recently, we have begun “fracking” to get oil and gas still trapped in the rocks where they formed. We are also using bitumen from “tar sands”, the leftovers from where oil seeped all the way to the surface and the more-fluid parts were burned by bacteria or else evaporated. We’re trying to learn how to use “oil shale” containing dead plants that would make oil if they were cooked more. And we’re thinking about the possibility of using natural gas that has formed clathrate ice in cold places beneath the sea floor.
The known reserves of these fossil fuels—the ones we’re sure we can use—will be gone in a few decades at the current rate of use. The total resource—including the fuels we think we’ll discover as we search harder, and we think we’ll learn how to use as we invent new ways—would last a few centuries at the current rate of use, but that might drop to less than a century if population and use per person continue to rise. And, sharp increases in price and other problems are likely to start well before the fossil fuels become really scarce.
Nature will make new fossil fuels, but not nearly fast enough to help us. We are burning our way through a “bank account” of fossil fuels supplied by nature, with no income to replace what we use. And, as we will see in the next lessons, our fossil-fuel burning is releasing carbon dioxide that is accumulating in the air and changing the climate.
Reminder - Complete all of the Module 3 tasks!
You have reached the end of Module 3! Double-check the Module Roadmap to make sure you have completed all of the activities listed there before you begin Module 4.
References and Further Reading
Alley, R.B., Earth: The Operators’ Manual, 2011
Organic Origins of Petroleum, United States Geological Survey Energy Resources Program [42]
Enrichments
More on Oxygen in Water
Oxygen in Water Enrichment
We oversimplified slightly in the text above. Even after oxygen is used up burning dead plants in mud beneath an ocean or lake, a little more burning may occur as bacteria use other chemicals in place of oxygen. For example, bacteria may use the sulfate in sea water. The reaction can be written this way:
Sulfuric acid + plant → hydrogen sulfide + carbon dioxide + water + energy
In reality, the sulfate (SO4-2) will also be reacting with other things in the ocean, but this isn't too far off. Hydrogen sulfide (H2S) is the source of “rotten egg smell”. It also readily reacts with iron in mud to make iron sulfide minerals, which initially appear black in the mud but which later may recrystallize to beautiful fools-gold pyrite if they have enough time and a bit of heat and other help. You might have seen this if you have visited a salt marsh. The mud in the shallowest parts of salt marshes is often black just below the surface, and releases rotten-egg smell if stirred up, because the marsh is growing lots of plants, the mud has little oxygen, and bacteria are using sulfate to burn the organic matter.
As mud is deposited at the bottom of lakes, the sea floor and elsewhere, it buries older mud with its organic matter. If you dig a hole, the material farther down was deposited longer ago, and has had more time to run out of oxygen and the other chemicals that are used to burn dead plants. One often sees a sequence going down in the mud in which, at the top, oxygen is used to burn organic matter, and then nitrate, manganese oxides, iron oxides, and then sulfate. If organic matter still remains, the next step is for bacteria to produce methane, CH4, which is the main component of natural gas.
Something really interesting may happen next. At the pressures and temperatures we commonly see under water, methane is usually a gas, although at high pressure it can be liquefied for storage or shipping. But, if the pressure is high enough, the temperature low enough, and there is lots of water around, instead of making bubbles, the gas will combine with the water to make a special kind of ice. This ice is often called methane hydrate or methane clathrate. When samples are brought to the surface, are brought to the surface, they actually will burn (see figures below).
There is a lot of clathrate under the sea floor in many places, and more in the Arctic in permafrost. (Yes, we know that we told you that warmer conditions favor burial of plants without burning, but this burial can happen in cold places as well, and freezing may actually help it happen by keeping worms and other creatures from eating dead plants before they are buried in mud. The frozen soils of the Arctic are rich in dead plants, and much methane is produced from them where thawing occurs without much oxygen.)
As mud is buried deeper and deeper by more sediment, the Earth's heat warms it up. At some depth, the ice melts to release bubbles of methane. When this process was first discovered, some scientists were worried that undersea landslides or other accidents might release giant methane belches that would sink ships (if a huge bubble rose right where a ship was, the ship could fall into the bubble), and change the climate, and cause other problems.
Additional research has reduced these worries, although they haven't gone away entirely. It is still just possible that a bubble might endanger a boat in certain special conditions, but we are fairly confident that huge amounts of gas can't come out really rapidly. As the clathrate is buried by more sediment, trapping the Earth's heat, the deepest ice melts to make bubbles. But, making those bubbles requires pushing water out of the way, which requires that the gas have high pressure. Pushing more water away needs higher pressure. At some high enough pressure, the gas will fracture the icy layer above and bubble out gradually, before enough gas can build up to make a climate-changing belch. Also, as clathrate forms, it uses the water but not the salt in seawater, and that salt may build up in water remaining in mud nearby, lowering the melting point so that some water doesn't freeze even if a lot of methane is supplied, allowing gas to move up through unfrozen regions to leak out at the sea floor.
As we will discuss climate change next chapter, methane in the sea floor may be very important for amplifying warming over decades and centuries, as warmer conditions melt the ice and let methane escape to increase the greenhouse warming. But, conduction of heat through the sediments to cause melting is rather slow, so we don't think that giant methane belches will change the climate even faster than that.