---
title: "The Geology of Coal, Oil and Gas"
book: "High School Earth and Space Sciences"
subject: earth
language: en
chapter: 18
exercises: 15
source: https://one-course.com/books/earth/2/en/chapter/18-the-geology-of-coal-oil-and-gas
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 18 — The Geology of Coal, Oil and Gas

A slab of black shale taken from an old [coal](#def-g11-fossil-fuel-geology-coal) mine in Pennsylvania carries the leaves of fern-like plants, flattened and silvery against the dark rock. They grew in a swamp more than three hundred million years ago, in the Carboniferous; buried before they could rot, they kept their shape, and the carbon of the swamp’s plants became [coal](#def-g11-fossil-fuel-geology-coal). Oil and gas, pumped today from platforms out at sea, have the same kind of origin: the remains of living things that escaped decay and were then buried and heated. This chapter follows that buried carbon from the swamp and the sea floor to the [coal](#def-g11-fossil-fuel-geology-coal) seam and the oil field.

![Left: fossil plant leaves in carbon-rich shale from an old coal mine, Pennsylvania, Carboniferous; the slab is about 10\, cm across. Photograph James St. John (CC BY 2.0). Right: an oil and gas production platform at sea (AI illustration).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/img-a8d783755ed1.jpg)

![Left: fossil plant leaves in carbon-rich shale from an old coal mine, Pennsylvania, Carboniferous; the slab is about 10\, cm across. Photograph James St. John (CC BY 2.0). Right: an oil and gas production platform at sea (AI illustration).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/img-809f5ec2882c.jpg)

*Left: fossil plant leaves in carbon-rich shale from an old [coal](#def-g11-fossil-fuel-geology-coal) mine, Pennsylvania, Carboniferous; the slab is about $10\,\mathrm{cm}$ across. Photograph James St. John (CC BY 2.0). Right: an oil and gas production platform at sea (AI illustration).*

**You already know.**

[Coal](#def-g11-fossil-fuel-geology-coal), oil and natural gas are fossil fuels: oil and gas are mixtures of hydrocarbons, compounds of carbon and hydrogen, and burning a fossil fuel releases energy and carbon dioxide (the chemistry books). Plants and algae make organic matter from carbon dioxide and water by photosynthesis (the biology books). [Coal](#def-g11-fossil-fuel-geology-coal), oil and gas come from organic matter buried in sediments, and they are stored in sedimentary rocks (Book 1, grade 9). A rock is porous when it holds empty spaces, permeable when water can flow through it (Book 1, grade 6). Temperature rises with depth along the [geotherm](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm), by 25 to $30{}^{\circ}\mathrm{C}$ per kilometre in the first kilometres ([Chapter 12](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#ch-g11-lithosphere-asthenosphere)).

## 18.1 Preserving organic matter

Most organic matter never becomes a fuel. A dead leaf or a dead alga is eaten by animals and bacteria or reacts with the oxygen of the air: within weeks or years its carbon has returned to the atmosphere as carbon dioxide. Organic matter survives only where it is cut off from oxygen — at the bottom of a stagnant swamp, or on a sea floor or lake floor whose water holds no oxygen. Buried there under more sediment, it can be kept for tens or hundreds of millions of years.

[Coal](#def-g11-fossil-fuel-geology-coal) comes mostly from land plants that piled up in swamps. Oil and gas come mostly from the microscopic algae and other plankton of the sea: where the surface waters are very productive, dead plankton rains onto the sea floor fast enough for some of it to escape decay, and it is buried in the mud.

**Definition 18.1 (Source rock; kerogen).**

*Kerogen* is the organic matter scattered in a sedimentary rock, in small particles that do not dissolve in organic solvents. A *source rock* is a sedimentary rock rich in kerogen, in which oil and gas form when it is buried and heated.

Most [source rocks](#def-g11-fossil-fuel-geology-source-rock) are shales and mudstones, laid down as muds rich in organic matter. Near the surface their [kerogen](#def-g11-fossil-fuel-geology-source-rock) stays solid: bacteria living without oxygen turn some of the organic matter into methane, but most of that methane escapes back to the surface. Making oil takes heat, and heat takes depth.

## 18.2 Burial and maturation

**Definition 18.2 (Sedimentary basin).**

A *sedimentary basin* is a region of the crust that sinks over millions of years, so that sediments pile up in it, often several kilometres thick.

In a sinking basin, each layer is buried deeper and deeper under the layers that pile up on top of it. As it goes down, it follows the [geotherm](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm): it is heated. Its [kerogen](#def-g11-fossil-fuel-geology-source-rock) is slowly cooked, its large molecules broken into the smaller ones of oil and gas.

**Definition 18.3 (Maturation; oil window; gas window).**

The *maturation* of a [source rock](#def-g11-fossil-fuel-geology-source-rock) is the transformation of its [kerogen](#def-g11-fossil-fuel-geology-source-rock) into oil and gas as it is buried and heated. The *oil window* is the range of temperature, from about 60 to $120{}^{\circ}\mathrm{C}$, in which the [kerogen](#def-g11-fossil-fuel-geology-source-rock) mostly forms oil. Beyond about $120{}^{\circ}\mathrm{C}$, in the *gas window*, it forms mostly methane.

Because the temperature rises with depth, the [oil window](#def-g11-fossil-fuel-geology-oil-window) is also a range of depths. Where the [geotherm](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) is a straight line, it is easy to find.

**Proposition 18.4 (Depth of the oil window).**

Let the temperature be $T_0$ at the surface and rise with depth $z$ at a constant [geothermal gradient](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) $G$: $T = T_0 + G z$. Then the [oil window](#def-g11-fossil-fuel-geology-oil-window) lies between the depths

$$
z_{\text{top}} = \frac{60 - T_0}{G}
  \qquad\text{and}\qquad
  z_{\text{base}} = \frac{120 - T_0}{G},
$$

temperatures in ${}^{\circ}\mathrm{C}$. Its thickness is $60/G$: the steeper the [geotherm](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm), the shallower and the thinner the window.

**Proof.** The top of the window is the depth where $T_0 + G z = 60$, that is $z = (60 - T_0)/G$; the base is where $T_0 + G z = 120$. The difference is $(120 - 60)/G = 60/G$. ∎

With a surface at $20{}^{\circ}\mathrm{C}$ and a gradient of $30{}^{\circ}\mathrm{C}$ per kilometre, the [oil window](#def-g11-fossil-fuel-geology-oil-window) lies between $40/30 \approx 1.3\,\mathrm{km}$ and $100/30 \approx 3.3\,\mathrm{km}$. In drill holes across the United States, the gradient averages $29{}^{\circ}\mathrm{C}$ per kilometre, $25$ in the east and $34$ in the west; a [source rock](#def-g11-fossil-fuel-geology-source-rock) usually has to be buried more than about $2\,\mathrm{km}$ before it makes oil. When no measurement of the [kerogen](#def-g11-fossil-fuel-geology-source-rock) is available, petroleum geologists have taken a [source rock](#def-g11-fossil-fuel-geology-source-rock) buried under more than about $3\,\mathrm{km}$ of younger rocks as mature.

![Left: burial history of the Merrow Basin, an invented basin (computed model, compaction ignored). Each line follows one layer through time, from the day it was laid down at the surface; the thick line is the source rock, laid down 165 million years ago. With the surface at 20 C and a gradient of 30 C per kilometre, the oil window is the band between 1.3 and 3.3\, km. The dots mark when the source rock entered it and when it left it, for the gas window. Right: depth of the top and the base of the oil window against the geothermal gradient (surface at 20 C).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/fig-e592581fc393.svg)

*Left: burial history of the Merrow Basin, an invented basin (computed model, compaction ignored). Each line follows one layer through time, from the day it was laid down at the surface; the thick line is the [source rock](#def-g11-fossil-fuel-geology-source-rock), laid down 165 million years ago. With the surface at $20{}^{\circ}\mathrm{C}$ and a gradient of $30{}^{\circ}\mathrm{C}$ per kilometre, the [oil window](#def-g11-fossil-fuel-geology-oil-window) is the band between $1.3$ and $3.3\,\mathrm{km}$. The dots mark when the [source rock](#def-g11-fossil-fuel-geology-source-rock) entered it and when it left it, for the [gas window](#def-g11-fossil-fuel-geology-oil-window). Right: depth of the top and the base of the [oil window](#def-g11-fossil-fuel-geology-oil-window) against the [geothermal gradient](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) (surface at $20{}^{\circ}\mathrm{C}$).*

A diagram like this one is a *burial-history diagram*: age runs from left to right towards today, depth downwards. It turns a column of rocks, dated layer by layer, into a story.

**Method 18.5 (Reading a burial-history diagram).**

1. Find the line of the [source rock](#def-g11-fossil-fuel-geology-source-rock) : it starts at the surface on the day the rock was laid down.
2. Draw the top and the base of the [oil window](#def-g11-fossil-fuel-geology-oil-window) , as depths, from the [geotherm](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) ( [Proposition 18.4](#prop-g11-fossil-fuel-geology-window-depth) ).
3. Read the age at which the [source rock](#def-g11-fossil-fuel-geology-source-rock) ’s line crosses the top of the window: the [source rock](#def-g11-fossil-fuel-geology-source-rock) begins to make oil. Read the age at which it crosses the base: from then on it makes gas.
4. Compare those ages with the age of the [trap](#def-g11-fossil-fuel-geology-trap) : only oil and gas formed after the [trap](#def-g11-fossil-fuel-geology-trap) exists can be held in it.

In the Merrow Basin, the [source rock](#def-g11-fossil-fuel-geology-source-rock) enters the [oil window](#def-g11-fossil-fuel-geology-oil-window) about 117 million years ago and leaves it about 51 million years ago; its deepest burial, $4.0\,\mathrm{km}$ at $140{}^{\circ}\mathrm{C}$, comes 20 million years ago, before uplift and erosion raise it again. Today it is still in the [gas window](#def-g11-fossil-fuel-geology-oil-window). The weekend problem computes these ages.

## 18.3 From source to trap

Oil and gas are lighter than water, and the pores of buried rocks are full of water. Once formed, they are pushed out of the [source rock](#def-g11-fossil-fuel-geology-source-rock) and creep upwards, through any rock that lets them pass.

**Definition 18.6 (Migration).**

*Migration* is the slow movement of oil and gas out of the [source rock](#def-g11-fossil-fuel-geology-source-rock) where they formed, upwards through permeable rocks, driven by their lower density than the water around them.

**Definition 18.7 (Reservoir rock; seal).**

A *reservoir rock* is a porous and permeable rock in which oil and gas collect: its pores store them, and their links let them flow towards a well. A *seal* is an impermeable rock above the reservoir, through which oil and gas cannot pass.

Sandstones and some limestones make good reservoirs; mudstones, shales and salt make good [seals](#def-g11-fossil-fuel-geology-reservoir). Where nothing stops them, oil and gas reach the surface, where they seep out and are oxidised: their carbon goes back to the atmosphere. They gather only where a [seal](#def-g11-fossil-fuel-geology-reservoir) is shaped so that they cannot slip past it.

**Definition 18.8 (Trap).**

A *trap* is an arrangement of [reservoir rock](#def-g11-fossil-fuel-geology-reservoir) and [seal](#def-g11-fossil-fuel-geology-reservoir) that holds oil and gas in place, closing the way up on every side.

![A petroleum system (a drawing, not to scale). The source rock, deep on the left, lies below the 60 C isotherm and makes oil; oil and gas move out of it into the sandstone above and creep up along it, under the mudstone seal, to the arch of an anticline. There they gather by density: gas at the top, oil beneath, water below. The overburden rocks on top bury the source rock deep enough to mature it.](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/fig-2641085960a1.svg)

*A [petroleum system](#def-g11-fossil-fuel-geology-petroleum-system) (a drawing, not to scale). The [source rock](#def-g11-fossil-fuel-geology-source-rock), deep on the left, lies below the $60{}^{\circ}\mathrm{C}$ isotherm and makes oil; oil and gas move out of it into the sandstone above and creep up along it, under the mudstone [seal](#def-g11-fossil-fuel-geology-reservoir), to the arch of an anticline. There they gather by density: gas at the top, oil beneath, water below. The [overburden](#def-g11-fossil-fuel-geology-petroleum-system) rocks on top bury the [source rock](#def-g11-fossil-fuel-geology-source-rock) deep enough to mature it.*

In a [trap](#def-g11-fossil-fuel-geology-trap), the fluids sort themselves by density: gas on top, oil beneath, water at the bottom. The simplest [trap](#def-g11-fossil-fuel-geology-trap) is an anticline, an arch in the layers (folds are the subject of [Chapter 20](https://one-course.com/books/earth/2/en/chapter/20-building-mountains#ch-g12-mountain-building)). Faults, salt domes and changes in the rocks themselves also make [traps](#def-g11-fossil-fuel-geology-trap).

![Four kinds of trap (drawings, not to scale): reservoir sandstone in yellow, sealing mudstone in grey-green, oil in green, gas in red. (a) The arch of an anticline. (b) A tilted reservoir cut by a fault, against a mudstone on the other side. (c) Layers bent up against a rising salt dome. (d) A sandstone that thins out up-slope within mudstone, a trap made by a change in the rocks themselves.](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/fig-384a589b1f3f.svg)

*Four kinds of [trap](#def-g11-fossil-fuel-geology-trap) (drawings, not to scale): reservoir sandstone in yellow, sealing mudstone in grey-green, oil in green, gas in red. (a) The arch of an anticline. (b) A tilted reservoir cut by a fault, against a mudstone on the other side. (c) Layers bent up against a rising salt dome. (d) A sandstone that thins out up-slope within mudstone, a [trap](#def-g11-fossil-fuel-geology-trap) made by a change in the rocks themselves.*

**Example 18.9 (A trap in an anticline).**

A seismic survey maps the top of a sandstone sealed by mudstone. It forms an arch whose crest lies $1800\,\mathrm{m}$ below the surface. Around the crest, the deepest contour line that still closes all the way round lies at $2050\,\mathrm{m}$; below it, the sandstone opens to the side. A well at the crest finds gas down to $1880\,\mathrm{m}$. How tall a column of oil and gas can the [trap](#def-g11-fossil-fuel-geology-trap) hold, and how thick is the oil?

*Solution.* Oil and gas fill the arch from the crest downwards; any that sinks below the last closed contour spills out to the side, at the *spill point*, and migrates on. The column is at most $2050 - 1800 = 250\,\mathrm{m}$ tall. If the [trap](#def-g11-fossil-fuel-geology-trap) is full, gas fills the top $1880 - 1800 = 80\,\mathrm{m}$ and oil the next $2050 - 1880 = 170\,\mathrm{m}$, with water below.

Each of the pieces — a [source rock](#def-g11-fossil-fuel-geology-source-rock), a reservoir, a [seal](#def-g11-fossil-fuel-geology-reservoir), the rocks on top that bury the [source rock](#def-g11-fossil-fuel-geology-source-rock), a [trap](#def-g11-fossil-fuel-geology-trap) — is necessary, and they must come in the right order.

**Definition 18.10 (Petroleum system).**

A *petroleum system* is a body of mature [source rock](#def-g11-fossil-fuel-geology-source-rock) together with all the oil and gas that came from it, and the elements and processes they need: the [source rock](#def-g11-fossil-fuel-geology-source-rock), the [reservoir rock](#def-g11-fossil-fuel-geology-reservoir), the [seal](#def-g11-fossil-fuel-geology-reservoir) and the *overburden* (the rocks above, which bury the [source rock](#def-g11-fossil-fuel-geology-source-rock)), and the formation of [traps](#def-g11-fossil-fuel-geology-trap), the generation, [migration](#def-g11-fossil-fuel-geology-migration) and accumulation of oil and gas.

The order matters most. A [trap](#def-g11-fossil-fuel-geology-trap) that forms after the oil has gone by catches nothing: the burial-history diagram dates the generation of oil, the history of the folds and faults dates the [trap](#def-g11-fossil-fuel-geology-trap), and the [petroleum system](#def-g11-fossil-fuel-geology-petroleum-system) works only if the [trap](#def-g11-fossil-fuel-geology-trap) came first. Petroleum geologists also call the time that best shows generation, [migration](#def-g11-fossil-fuel-geology-migration) and accumulation together the *critical moment*.

## 18.4 Where oil and gas gather

Where does a [petroleum system](#def-g11-fossil-fuel-geology-petroleum-system) come together? It needs a basin that sinks for a long time, waters poor in oxygen to keep organic matter, and [seals](#def-g11-fossil-fuel-geology-reservoir). Rifts and the continental margins they grow into have all three.

When a continent begins to split, as Africa does today along the East African Rift (Book 1, grade 7), its crust stretches and breaks into blocks along [normal faults](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-normal-fault) ([Chapter 14](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#ch-g11-ocean-ridges)). Some blocks drop, others tilt, and the troughs between them fill with sediments, often several kilometres thick, and with lakes, whose muds rich in organic matter can become [source rocks](#def-g11-fossil-fuel-geology-source-rock). When stretching stops, the thinned crust cools and keeps sinking slowly, so that the basin goes on filling. If the sea then floods into a basin only partly connected to the ocean and evaporates, it leaves thick layers of salt. Once the rift has opened into an ocean, its edges become passive margins: continental edges inside a plate, far from any plate boundary, which go on sinking and receiving sediment from the land.

![A passive margin built on an old rift (a drawing, not to scale, depths exaggerated). In the rift, tilted blocks of continental crust bounded by normal faults hold lake shales, the source rocks. A layer of salt, laid down when the young sea evaporated, seals them; above it, marine sediments pile up as the margin sinks. Salt, lighter than the sediments around it, rises in domes that bend the layers into traps.](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/fig-c1923107f0c7.svg)

*A passive margin built on an old rift (a drawing, not to scale, depths exaggerated). In the rift, tilted blocks of continental crust bounded by [normal faults](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-normal-fault) hold lake shales, the [source rocks](#def-g11-fossil-fuel-geology-source-rock). A layer of salt, laid down when the young sea evaporated, [seals](#def-g11-fossil-fuel-geology-reservoir) them; above it, marine sediments pile up as the margin sinks. Salt, lighter than the sediments around it, rises in domes that bend the layers into [traps](#def-g11-fossil-fuel-geology-trap).*

The margins of the South Atlantic were built this way. Africa and South America began to separate in the Late Triassic or Early Jurassic. Off western Central Africa, the rift troughs filled with several kilometres of sediments, among them lake shales up to $600\,\mathrm{m}$ thick holding up to $20\,\%$ of organic carbon by mass. In the Early Cretaceous the sea began to flood in, again and again, and evaporated, leaving more than a kilometre of salt; where the salt has stayed in place, it [seals](#def-g11-fossil-fuel-geology-reservoir) the oil and gas of the rocks below. In the Gulf of Mexico, salt that is lighter than the sediments around it has risen in domes, and the layers bent against them are [traps](#def-g11-fossil-fuel-geology-trap).

## 18.5 Coal

[Coal](#def-g11-fossil-fuel-geology-coal) forms on land, mostly in swamps beside rivers and deltas, in humid climates from tropical to temperate, where plants grow fast and their remains fall into stagnant water poor in oxygen. Over centuries to millennia they pile up as peat. When a river changes course, or the sea rises, sand and mud bury the peat; buried deeper, it is squeezed and heated.

**Definition 18.11 (Coal; coalification; rank).**

*Coal* is a sedimentary rock made mostly of the remains of land plants, rich in carbon. *Coalification* is the transformation of peat into coal and of coal into richer coal, by burial and heating. The *rank* of a coal is how far coalification has gone: from lignite through subbituminous and bituminous coal to anthracite, the carbon content rises.

Lignite forms at depths from a few hundred metres to about $1.5\,\mathrm{km}$, up to about $50{}^{\circ}\mathrm{C}$; bituminous [coal](#def-g11-fossil-fuel-geology-coal) between 1 and $5\,\mathrm{km}$, up to about $150{}^{\circ}\mathrm{C}$; anthracite beyond $5\,\mathrm{km}$ and $150{}^{\circ}\mathrm{C}$. As the [rank](#def-g11-fossil-fuel-geology-coal) rises, carbon makes up more and more of the [coal](#def-g11-fossil-fuel-geology-coal): about 25 to $35\,\%$ in lignite, up to 86 to $97\,\%$ in anthracite. Methane formed on the way stays in the pores of the [coal](#def-g11-fossil-fuel-geology-coal), which is why a [coal](#def-g11-fossil-fuel-geology-coal) mine can hold explosive gas. Forming a [coal](#def-g11-fossil-fuel-geology-coal) seam takes millions of years.

![Top left: carbon content of the ranks of coal. Data: U.S. Energy Information Administration. Top right: a bed of lignite, the black band under pale sands, exposed in a mine in Romania; photograph U.S. Geological Survey (public domain). Bottom, from left: lignite, about 4.5\, cm across, brown-black and dull; bituminous coal, with flattened plant fragments; anthracite, black and glassy. Photographs James St. John (CC BY 2.0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/fig-cec58647eee1.svg)

![Top left: carbon content of the ranks of coal. Data: U.S. Energy Information Administration. Top right: a bed of lignite, the black band under pale sands, exposed in a mine in Romania; photograph U.S. Geological Survey (public domain). Bottom, from left: lignite, about 4.5\, cm across, brown-black and dull; bituminous coal, with flattened plant fragments; anthracite, black and glassy. Photographs James St. John (CC BY 2.0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/img-0f05b6b6e2b6.jpg)

![Top left: carbon content of the ranks of coal. Data: U.S. Energy Information Administration. Top right: a bed of lignite, the black band under pale sands, exposed in a mine in Romania; photograph U.S. Geological Survey (public domain). Bottom, from left: lignite, about 4.5\, cm across, brown-black and dull; bituminous coal, with flattened plant fragments; anthracite, black and glassy. Photographs James St. John (CC BY 2.0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/img-6dd27b13aa1f.jpg)

![Top left: carbon content of the ranks of coal. Data: U.S. Energy Information Administration. Top right: a bed of lignite, the black band under pale sands, exposed in a mine in Romania; photograph U.S. Geological Survey (public domain). Bottom, from left: lignite, about 4.5\, cm across, brown-black and dull; bituminous coal, with flattened plant fragments; anthracite, black and glassy. Photographs James St. John (CC BY 2.0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-fossil-fuel-geology/img-33be0d755b75.jpg)

*Top left: carbon content of the [ranks](#def-g11-fossil-fuel-geology-coal) of [coal](#def-g11-fossil-fuel-geology-coal). Data: U.S. Energy Information Administration. Top right: a bed of lignite, the black band under pale sands, exposed in a mine in Romania; photograph U.S. Geological Survey (public domain). Bottom, from left: lignite, about $4.5\,\mathrm{cm}$ across, brown-black and dull; bituminous [coal](#def-g11-fossil-fuel-geology-coal), with flattened plant fragments; anthracite, black and glassy. Photographs James St. John (CC BY 2.0).*

Peat loses most of its volume on the way. In a [coal](#def-g11-fossil-fuel-geology-coal) bed of central Utah, comparing the peat cut away by an ancient river channel with the [coal](#def-g11-fossil-fuel-geology-coal) missing beside it gives a ratio of thicknesses of about $11{:}1$: every metre of that bituminous [coal](#def-g11-fossil-fuel-geology-coal) was once some eleven metres of peat.

## 18.6 Exercises

**Exercise 18.1 ★.**

Put in order the steps that lead from plankton to oil in a [trap](#def-g11-fossil-fuel-geology-trap): [migration](#def-g11-fossil-fuel-geology-migration); [maturation](#def-g11-fossil-fuel-geology-oil-window) in the [oil window](#def-g11-fossil-fuel-geology-oil-window); burial under younger sediments; dead plankton settles on a sea floor poor in oxygen; oil gathers under a [seal](#def-g11-fossil-fuel-geology-reservoir); the organic matter becomes [kerogen](#def-g11-fossil-fuel-geology-source-rock) in a mud.

**Solution of Exercise 18.1.**

Dead plankton settles on a sea floor poor in oxygen; the organic matter becomes [kerogen](#def-g11-fossil-fuel-geology-source-rock) in a mud; burial under younger sediments; [maturation](#def-g11-fossil-fuel-geology-oil-window) in the [oil window](#def-g11-fossil-fuel-geology-oil-window); [migration](#def-g11-fossil-fuel-geology-migration); oil gathers under a [seal](#def-g11-fossil-fuel-geology-reservoir).

**Exercise 18.2 ★.**

For each rock, say whether it is more likely a [source rock](#def-g11-fossil-fuel-geology-source-rock), a [reservoir rock](#def-g11-fossil-fuel-geology-reservoir) or a [seal](#def-g11-fossil-fuel-geology-reservoir): (1) a dark shale rich in organic matter; (2) a well-sorted sandstone with connected pores; (3) a layer of salt; (4) a mudstone with no organic matter; (5) a limestone full of open cavities.

**Solution of Exercise 18.2.**

(1) [Source rock](#def-g11-fossil-fuel-geology-source-rock). (2) [Reservoir rock](#def-g11-fossil-fuel-geology-reservoir). (3) [Seal](#def-g11-fossil-fuel-geology-reservoir). (4) [Seal](#def-g11-fossil-fuel-geology-reservoir). (5) [Reservoir rock](#def-g11-fossil-fuel-geology-reservoir).

**Exercise 18.3 ★.**

In a basin, the surface is at $10{}^{\circ}\mathrm{C}$ and the [geothermal gradient](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) is $25{}^{\circ}\mathrm{C}/\mathrm{km}$. Between which depths does the [oil window](#def-g11-fossil-fuel-geology-oil-window) lie? How thick is it?

**Solution of Exercise 18.3.**

Top: $(60 - 10)/25 = 2.0\,\mathrm{km}$; base: $(120 - 10)/25 = 4.4\,\mathrm{km}$. The window is $60/25 = 2.4\,\mathrm{km}$ thick.

**Exercise 18.4 ★.**

(1) Put lignite, anthracite, bituminous [coal](#def-g11-fossil-fuel-geology-coal) and peat in order of rising [rank](#def-g11-fossil-fuel-geology-coal). (2) Which has the highest carbon content? (3) Which formed at the greatest depth and temperature?

**Solution of Exercise 18.4.**

(1) Peat, lignite, bituminous [coal](#def-g11-fossil-fuel-geology-coal), anthracite. (2) Anthracite, 86 to $97\,\%$ carbon. (3) Anthracite, beyond about $5\,\mathrm{km}$ and $150{}^{\circ}\mathrm{C}$.

**Exercise 18.5 ★.**

(1) In a [trap](#def-g11-fossil-fuel-geology-trap), why does gas lie above oil, and oil above water? (2) Why must the rock above a reservoir be impermeable? Name two rocks that make good [seals](#def-g11-fossil-fuel-geology-reservoir).

**Solution of Exercise 18.5.**

(1) The fluids sort themselves by density: gas is the lightest, then oil, then water. (2) Oil and gas rise because they are lighter than water; only a rock they cannot pass through stops them. Mudstone (or shale) and salt.

**Exercise 18.6 ★★.**

Two basins have the same surface temperature, $15{}^{\circ}\mathrm{C}$. Basin A has a gradient of $20{}^{\circ}\mathrm{C}/\mathrm{km}$, basin B of $40{}^{\circ}\mathrm{C}/\mathrm{km}$. (1) Compute the depths of the top and the base of the [oil window](#def-g11-fossil-fuel-geology-oil-window) in each. (2) A [source rock](#def-g11-fossil-fuel-geology-source-rock) lies $2\,\mathrm{km}$ deep in both basins. What does it make in each? (3) In which basin must a well go deeper to find oil formed in place?

**Solution of Exercise 18.6.**

(1) A: $(60 - 15)/20 = 2.25\,\mathrm{km}$ and $(120 - 15)/20 = 5.25\,\mathrm{km}$. B: $45/40 \approx 1.13\,\mathrm{km}$ and $105/40 \approx 2.63\,\mathrm{km}$. (2) At $2\,\mathrm{km}$: in A, $15 + 40 = 55{}^{\circ}\mathrm{C}$, not yet mature, no oil; in B, $15 + 80 = 95{}^{\circ}\mathrm{C}$, in the [oil window](#def-g11-fossil-fuel-geology-oil-window), it makes oil. (3) In basin A: oil forms there only below $2.25\,\mathrm{km}$.

**Exercise 18.7 ★★.**

Take the ratio of $11{:}1$ measured in central Utah between the thickness of a peat and that of the [coal](#def-g11-fossil-fuel-geology-coal) it became. (1) What thickness of peat made a [coal](#def-g11-fossil-fuel-geology-coal) seam $3\,\mathrm{m}$ thick? (2) Suppose the peat piled up at $1\,\mathrm{mm}$ a year. How long did it take to accumulate? (3) Why must the swamp have been sinking all that time?

**Solution of Exercise 18.7.**

(1) $11 \times 3 = 33\,\mathrm{m}$ of peat. (2) $33\,000\,\mathrm{mm}$ at $1\,\mathrm{mm}$ a year: about $33\,000$ years. (3) The peat stays only where it is under stagnant water, out of reach of oxygen; as it piles up, the ground must sink at the same pace to keep its surface waterlogged.

**Exercise 18.8 ★★.**

Use the burial-history diagram of the Merrow Basin (surface $20{}^{\circ}\mathrm{C}$, gradient $30{}^{\circ}\mathrm{C}/\mathrm{km}$). The [source rock](#def-g11-fossil-fuel-geology-source-rock) was $1.2\,\mathrm{km}$ deep 120 million years ago, $2.3\,\mathrm{km}$ deep 95 million years ago and $4.0\,\mathrm{km}$ deep 20 million years ago. (1) Compute its temperature at each date. (2) At each date, was it immature, in the [oil window](#def-g11-fossil-fuel-geology-oil-window) or in the [gas window](#def-g11-fossil-fuel-geology-oil-window)?

**Solution of Exercise 18.8.**

(1) $20 + 30 \times 1.2 = 56{}^{\circ}\mathrm{C}$; $20 + 30 \times 2.3 = 89{}^{\circ}\mathrm{C}$; $20 + 30 \times 4.0 = 140{}^{\circ}\mathrm{C}$. (2) 120 million years ago, immature; 95 million years ago, in the [oil window](#def-g11-fossil-fuel-geology-oil-window); 20 million years ago, in the [gas window](#def-g11-fossil-fuel-geology-oil-window).

**Exercise 18.9 ★★.**

The top of a reservoir sandstone forms an arch whose crest is $2400\,\mathrm{m}$ deep; its deepest closed contour is at $2580\,\mathrm{m}$. A well at the crest finds gas to $2450\,\mathrm{m}$, then oil. (1) What is the greatest height of the column the [trap](#def-g11-fossil-fuel-geology-trap) can hold? (2) If the [trap](#def-g11-fossil-fuel-geology-trap) is full, how thick is the oil? (3) The oil and gas that keep arriving after the [trap](#def-g11-fossil-fuel-geology-trap) is full: where do they go?

**Solution of Exercise 18.9.**

(1) $2580 - 2400 = 180\,\mathrm{m}$. (2) Gas fills $2450 - 2400 = 50\,\mathrm{m}$, oil $2580 - 2450 = 130\,\mathrm{m}$. (3) They spill out at the spill point, below the deepest closed contour, and migrate on upwards, to another [trap](#def-g11-fossil-fuel-geology-trap) or to the surface.

**Exercise 18.10 ★★.**

When no measurement of the [kerogen](#def-g11-fossil-fuel-geology-source-rock) is available, petroleum geologists have taken a [source rock](#def-g11-fossil-fuel-geology-source-rock) buried under more than $3\,\mathrm{km}$ of rocks as mature. With a surface at $20{}^{\circ}\mathrm{C}$: (1) Which gradient puts $3\,\mathrm{km}$ at the top of the [oil window](#def-g11-fossil-fuel-geology-oil-window)? at its base? (2) With the gradients of 25 to $34{}^{\circ}\mathrm{C}/\mathrm{km}$ measured in drill holes, where does $3\,\mathrm{km}$ fall? Is the rule reasonable?

**Solution of Exercise 18.10.**

(1) Top of the window at $3\,\mathrm{km}$: $(60 - 20)/3 \approx
13{}^{\circ}\mathrm{C}/\mathrm{km}$; base: $(120 - 20)/3 \approx
33{}^{\circ}\mathrm{C}/\mathrm{km}$. (2) At $3\,\mathrm{km}$ the temperature is $20 + 75 = 95{}^{\circ}\mathrm{C}$ to $20 + 102 = 122{}^{\circ}\mathrm{C}$: in the [oil window](#def-g11-fossil-fuel-geology-oil-window), or at its base for the steepest gradients. The rule is reasonable: in most basins a [source rock](#def-g11-fossil-fuel-geology-source-rock) under $3\,\mathrm{km}$ of rocks has started to make oil.

**Exercise 18.11 ★★.**

Three [coal](#def-g11-fossil-fuel-geology-coal) samples hold $30\,\%$, $70\,\%$ and $92\,\%$ of carbon. (1) Give the [rank](#def-g11-fossil-fuel-geology-coal) of each. (2) Using the depths and temperatures given in the course, which was buried deepest? (3) The lignite has a dull, brown-black look; how does the anthracite look?

**Solution of Exercise 18.11.**

(1) $30\,\%$: lignite; $70\,\%$: bituminous [coal](#def-g11-fossil-fuel-geology-coal); $92\,\%$: anthracite. (2) The anthracite, beyond about $5\,\mathrm{km}$ and $150{}^{\circ}\mathrm{C}$. (3) Black and glassy.

**Exercise 18.12 ★★★.**

Between 120 and 95 million years ago, $1100\,\mathrm{m}$ of mud piled up in the Merrow Basin. (1) Compute the rate of burial in metres per million years. (2) With a gradient of $30{}^{\circ}\mathrm{C}/\mathrm{km}$, by how many degrees did the [source rock](#def-g11-fossil-fuel-geology-source-rock) warm per million years? (3) Had this rate gone on, how long would the [source rock](#def-g11-fossil-fuel-geology-source-rock) have taken to cross the whole [oil window](#def-g11-fossil-fuel-geology-oil-window)? (4) The model gives about 66 million years. Why longer?

**Solution of Exercise 18.12.**

(1) $1100/25 = 44\,\mathrm{m}$ per million years. (2) $0.044 \times 30
\approx 1.3{}^{\circ}\mathrm{C}$ per million years. (3) The window spans $60{}^{\circ}\mathrm{C}$: $60/1.32 \approx 45$ million years. (4) Burial slowed down after 95 million years ago (about 24, then 23 and $20\,\mathrm{m}$ per million years): the [source rock](#def-g11-fossil-fuel-geology-source-rock) warmed more slowly.

**Exercise 18.13 ★★★.**

In the Merrow Basin, the [source rock](#def-g11-fossil-fuel-geology-source-rock) was in the [oil window](#def-g11-fossil-fuel-geology-oil-window) from about 117 to about 51 million years ago, and the anticline that [traps](#def-g11-fossil-fuel-geology-trap) the oil formed 80 million years ago. (1) Assume the [source rock](#def-g11-fossil-fuel-geology-source-rock) made oil at a steady rate while in the window. What fraction of its oil formed after the [trap](#def-g11-fossil-fuel-geology-trap) existed? (2) What became of the rest? (3) Had the anticline formed 40 million years ago, what could it hold?

**Solution of Exercise 18.13.**

(1) $(80 - 51)/(117 - 51) = 29/66 \approx 0.44$: about $44\,\%$. (2) It migrated upwards before any [trap](#def-g11-fossil-fuel-geology-trap) existed above: it escaped to the surface, where it was oxidised, or gathered in some other [trap](#def-g11-fossil-fuel-geology-trap). (3) By 40 million years ago the [source rock](#def-g11-fossil-fuel-geology-source-rock) had left the [oil window](#def-g11-fossil-fuel-geology-oil-window): it made only gas, and the anticline could hold only gas from it.

**Exercise 18.14 ★★★.**

Take the burial history of the Merrow Basin ([Exercise 18.8](#exo-g11-fossil-fuel-geology-8): $0.4\,\mathrm{km}$ at 150 million years ago, $1.2\,\mathrm{km}$ at 120, $2.3\,\mathrm{km}$ at 95, $3.0\,\mathrm{km}$ at 66, depth growing steadily in between), but a hotter basin: gradient $40{}^{\circ}\mathrm{C}/\mathrm{km}$, surface $20{}^{\circ}\mathrm{C}$. (1) Compute the depths of the top and the base of the [oil window](#def-g11-fossil-fuel-geology-oil-window). (2) Compute the ages at which the [source rock](#def-g11-fossil-fuel-geology-source-rock) entered and left the window. (3) The anticline still forms 80 million years ago. Would it hold oil? Gas?

**Solution of Exercise 18.14.**

(1) Top: $(60 - 20)/40 = 1.0\,\mathrm{km}$; base: $(120 - 20)/40 =
2.5\,\mathrm{km}$. (2) $1.0\,\mathrm{km}$ lies between $0.4\,\mathrm{km}$ (150 million years ago) and $1.2\,\mathrm{km}$ (120): $150 - 30 \times 0.6/0.8 = 127.5$ million years ago. $2.5\,\mathrm{km}$ lies between $2.3\,\mathrm{km}$ (95) and $3.0\,\mathrm{km}$ (66): $95 - 29 \times 0.2/0.7 \approx 86.7$ million years ago. (3) No oil: the [source rock](#def-g11-fossil-fuel-geology-source-rock) left the [oil window](#def-g11-fossil-fuel-geology-oil-window) before the anticline formed, and its oil had migrated away. It could hold gas made after 80 million years ago.

**Exercise 18.15 ★★★.**

A seam of anthracite of the Carboniferous now crops out at the surface. The Carboniferous ended about 299 million years ago. (1) Using the depths given in the course, how deep was the seam buried at least? (2) With a surface at $15{}^{\circ}\mathrm{C}$ and a gradient of $30{}^{\circ}\mathrm{C}/\mathrm{km}$, what temperature did it reach at that depth? Is it consistent with the temperature given for anthracite? (3) What has happened to the rocks that lay above it? Give a lower bound of the average rate, in millimetres per year, assuming it all happened after the Carboniferous.

**Solution of Exercise 18.15.**

(1) At least $5\,\mathrm{km}$. (2) $15 + 30 \times 5 = 165{}^{\circ}\mathrm{C}$, above $150{}^{\circ}\mathrm{C}$: consistent. (3) At least $5\,\mathrm{km}$ of rock has been uplifted and eroded. $5 \times 10^{6}\,\mathrm{mm}$ in at most $299$ million years gives at least $5 \times 10^6/2.99 \times 10^8
\approx 0.017\,\mathrm{mm}$ a year on average.

## 18.7 Problem: When Did the Source Rock Make Oil?

**Problem 18.1.**

Weekend problem — dating the oil of an invented basin

A well drilled through the crest of an anticline in the Merrow Basin, an invented basin, crosses the layers below. Dated by their fossils, they record how the basin filled. From the oldest:

| **interval (millions of years ago)** | **thickness** | **rocks** |
| --- | --- | --- |
| 165 to 150 | $400\,\mathrm{m}$ | black shale rich in organic matter, then mudstone |
| 150 to 120 | $800\,\mathrm{m}$ | porous sandstone |
| 120 to 95 | $1100\,\mathrm{m}$ | mudstone |
| 95 to 66 | $700\,\mathrm{m}$ | chalk |
| 66 to 35 | $700\,\mathrm{m}$ | sands and clays |
| 35 to 20 | $300\,\mathrm{m}$ | sands |
| 20 to today | $-500\,\mathrm{m}$ | uplift: the top $500\,\mathrm{m}$ eroded |

The surface is at $20{}^{\circ}\mathrm{C}$ and the [geothermal gradient](https://one-course.com/books/earth/2/en/chapter/12-lithosphere-and-asthenosphere#def-g11-lithosphere-asthenosphere-geotherm) is $30{}^{\circ}\mathrm{C}/\mathrm{km}$, both taken as constant. The layers keep their thickness once laid down (compaction is ignored). The anticline formed 80 million years ago. Follow the black shale at the base, laid down 165 million years ago.

**Part I — The rocks.**

1. Which layer is the [source rock](#def-g11-fossil-fuel-geology-source-rock) , which the reservoir, which the [seal](#def-g11-fossil-fuel-geology-reservoir) ?
2. What made the black shale rich in organic matter?
3. The Jurassic lasted from about $201.4$ to $143.1$ million years ago. In which period was the [source rock](#def-g11-fossil-fuel-geology-source-rock) laid down? The sandstone?
4. What is [kerogen](#def-g11-fossil-fuel-geology-source-rock) ? Why does the black shale make no oil at the surface?
5. What thickness of rocks lies on top of the [source rock](#def-g11-fossil-fuel-geology-source-rock) today?

**Part II — Burial against time.**

6. Compute the depth of the [source rock](#def-g11-fossil-fuel-geology-source-rock) 150, 120, 95, 66, 35 and 20 million years ago, and today.
7. Compute the rate of burial, in metres per million years, during each of the first six intervals.
8. When was the [source rock](#def-g11-fossil-fuel-geology-source-rock) deepest, and how deep?
9. Why did its depth decrease over the last 20 million years?
10. How deep is the base of the sandstone today?

**Part III — Temperature against time.**

11. Compute the temperature of the [source rock](#def-g11-fossil-fuel-geology-source-rock) at each date of question 6.
12. Between which depths does the [oil window](#def-g11-fossil-fuel-geology-oil-window) lie in this basin?
13. Between which two dates of question 6 did the [source rock](#def-g11-fossil-fuel-geology-source-rock) pass $60{}^{\circ}\mathrm{C}$ ?
14. Assuming its depth grew steadily in that interval, compute the age at which it reached $60{}^{\circ}\mathrm{C}$ .
15. In the same way, compute when it reached $120{}^{\circ}\mathrm{C}$ .

**Part IV — Into the window, and the trap.**

16. What is the highest temperature it reached? What does it make today?
17. What became of the oil formed before the anticline existed?
18. Assuming oil formed at a steady rate while the [source rock](#def-g11-fossil-fuel-geology-source-rock) was in the window, what fraction of it formed after the anticline existed?
19. Which fluids would the well expect at the crest of the anticline, and in which order from the top?
20. Conclude: when did the [source rock](#def-g11-fossil-fuel-geology-source-rock) enter the [oil window](#def-g11-fossil-fuel-geology-oil-window) ?

**Solution of Problem 18.1.**

**1.** [Source rock](#def-g11-fossil-fuel-geology-source-rock): the black shale; reservoir: the porous sandstone; [seal](#def-g11-fossil-fuel-geology-reservoir): the mudstone above it.

**2.** Plankton that settled on a sea floor poor in oxygen escaped decay and was buried in the mud.

**3.** The [source rock](#def-g11-fossil-fuel-geology-source-rock) in the Jurassic (165 lies between 201.4 and 143.1). The sandstone began in the Jurassic and was mostly laid down in the Early Cretaceous.

**4.** The organic matter scattered in a sedimentary rock, insoluble in organic solvents. It makes oil only once heated to about $60{}^{\circ}\mathrm{C}$, which takes burial.

**5.** $400 + 800 + 1100 + 700 + 700 + 300 - 500 = 3500\,\mathrm{m}$.

**6.** $0.4\,\mathrm{km}$, $1.2\,\mathrm{km}$, $2.3\,\mathrm{km}$, $3.0\,\mathrm{km}$, $3.7\,\mathrm{km}$, $4.0\,\mathrm{km}$; today $3.5\,\mathrm{km}$.

**7.** $400/15 \approx 26.7$; $800/30 \approx 26.7$; $1100/25 = 44$; $700/29 \approx 24.1$; $700/31 \approx 22.6$; $300/15 = 20$ ($\mathrm{m}$ per million years).

**8.** 20 million years ago, at $4.0\,\mathrm{km}$.

**9.** The basin was uplifted and $500\,\mathrm{m}$ of rock eroded from its top.

**10.** The sandstone’s base was laid down 150 million years ago, on top of $400\,\mathrm{m}$ of shale and mudstone: $3.5 - 0.4 = 3.1\,\mathrm{km}$.

**11.** $T = 20 + 30 z$: $32{}^{\circ}\mathrm{C}$, $56$, $89$, $110$, $131$, $140{}^{\circ}\mathrm{C}$; today $125{}^{\circ}\mathrm{C}$.

**12.** $40/30 \approx 1.33\,\mathrm{km}$ and $100/30 \approx
3.33\,\mathrm{km}$.

**13.** Between 120 million years ago ($56{}^{\circ}\mathrm{C}$) and 95 million years ago ($89{}^{\circ}\mathrm{C}$).

**14.** The temperature rose by 33 degrees in 25 million years; it took $25 \times 4/33 \approx 3.0$ million years to rise by the 4 degrees from $56$ to $60{}^{\circ}\mathrm{C}$: about $120 - 3.0 = 117$ million years ago.

**15.** Between 66 million years ago ($110{}^{\circ}\mathrm{C}$) and 35 ($131{}^{\circ}\mathrm{C}$): $66 - 31 \times 10/21 \approx 51.2$ million years ago.

**16.** $140{}^{\circ}\mathrm{C}$, 20 million years ago. At $125{}^{\circ}\mathrm{C}$ today it is in the [gas window](#def-g11-fossil-fuel-geology-oil-window): it makes mostly methane.

**17.** With no [trap](#def-g11-fossil-fuel-geology-trap) above it, it migrated up and escaped, to the surface or to other [traps](#def-g11-fossil-fuel-geology-trap).

**18.** $(80 - 51.2)/(117.0 - 51.2) \approx 0.44$.

**19.** Gas at the top, then oil, then water.

**20.** The [source rock](#def-g11-fossil-fuel-geology-source-rock) entered the [oil window](#def-g11-fossil-fuel-geology-oil-window) about **117 million years ago**.