Coal and Petroleum

Reach for a plastic ruler, a nylon backpack strap, the petrol that runs a scooter, or the coal that once powered a steam engine, and you are touching the buried remains of forests and sea creatures that died hundreds of millions of years ago. Coal, petroleum and natural gas — the three great fossil fuels — are the slow, crushed, cooked-down legacy of living things: dense swampy forests for coal, tiny sea organisms for petroleum and natural gas. Both stories run on the same two ingredients, time and pressure, and neither can be rushed or repeated in a school laboratory no matter how hot the flame or how heavy the weight. This chapter tells that story end to end. It starts by sorting resources into two kinds: those nature keeps replenishing forever, like sunlight and air, and those that exist in a fixed, shrinking stock, like coal, petroleum, minerals and forests. It then follows coal from a buried, waterlogged forest through carbonisation to the hard black rock we mine, and shows what industry pulls out of it — coke, coal tar, coal gas. It follows petroleum from the drowned bodies of marine organisms through burial and pressure to a smelly, unrefined liquid, and then through refining into the family of fuels and materials in your daily life: LPG, petrol, kerosene, diesel, lubricating oil, paraffin wax, bitumen — even the plastics, detergents and synthetic fibres called petrochemicals. Natural gas, stored under pressure as CNG, gets its own short story as the cleanest-burning and easiest-to-pipe of the three. The chapter's real destination is the sober arithmetic of scarcity. These fuels took upward of 300 million years to form; the reserves we are burning through will last, at current rates, only a few more centuries — a geological instant. Every worked example, diagram and quiz in this article circles back to that one number, because understanding why fossil fuels are exhaustible is the entire point of learning how they were made.

Key takeaways

Natural resources split into two kinds. Inexhaustible resources (sunlight, air) cannot be used up by humans; exhaustible resources (coal, petroleum, natural gas, forests, minerals) exist in a fixed, limited stock.
Coal, petroleum and natural gas are fossil fuels — formed from the buried, compressed, heated remains of once-living organisms over millions of years.
Coal formed by carbonisation: about 300 million years ago, dense wetland forests got buried under soil; rising pressure and temperature slowly converted the dead plant matter (mostly carbon-based) into coal.
Processing coal gives three useful products: coke (a tough, porous, nearly pure form of carbon, used in steel-making), coal tar (a black mixture of about 200 substances, source of dyes, drugs, naphthalene balls and more), and coal gas (a fuel gas, once used to light streets).
Petroleum formed from marine organisms: dead sea organisms sank, got buried under sand and clay, and in the absence of air, under high temperature and pressure, turned into petroleum and natural gas over millions of years.
Petroleum is refined by fractional distillation in a refinery, separating the crude mixture into petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen — each with its own use.
Petrochemicals — detergents, synthetic fibres (polyester, nylon, acrylic), plastics, fertilisers — are manufactured from petroleum and natural gas, not mined directly.
Natural gas is stored as CNG (compressed natural gas): easy to transport by pipeline, cleaner-burning than petrol or diesel, and used for both vehicles and piped domestic supply.
Fossil fuels are exhaustible and non-renewable on a human timescale: they took millions of years to form but known reserves may last only a few hundred years, so burning them causes pollution and global warming and demands conservation (PCRA tips: steady speed, engine off at halts, correct tyre pressure, regular servicing).
Key idea: coal and petroleum are fossil fuels formed over millions of years by the burial, pressure and heating of dead plant and marine matter, and because that process cannot be redone on any human timescale, refining them into fuels and petrochemicals must be balanced against the fact that the stock is exhaustible.

The core insight

Coal and petroleum are fossil fuels formed over millions of years from buried, pressurised, heated remains of dead plants and marine organisms, and because they are exhaustible resources, refining them into coke, coal tar, coal gas, LPG, petrol, diesel, kerosene, lubricating oil, paraffin wax, bitumen and petrochemicals must be weighed against pollution, global warming and the need for conservation.

What this chapter covers

This article follows the chapter's own path. First, resources in general — why some are inexhaustible and some exhaustible. Second, coal — how carbonisation buries and cooks dead forests into coal, and the three products (coke, coal tar, coal gas) obtained by processing it. Third, petroleum — how dead marine organisms became a mixture trapped between layers of rock, why oil and gas float above water there, and how a refinery separates the raw mixture into named fractions and petrochemicals. Fourth, natural gas and CNG — clean-burning and easy to pipe. Finally, the chapter's real payoff: why all three are exhaustible, and what everyday conservation looks like.

The core ideas

Resources: inexhaustible versus exhaustible

Everything we use is either found ready-made in nature or manufactured by people, but the natural ones split further by a single test: can human use ever run them out? An inexhaustible natural resource is present in such vast quantity, or is continually renewed, that ordinary human activity cannot deplete it — sunlight and air are the standard examples. An exhaustible natural resource exists in a limited amount; using it faster than nature replaces it steadily shrinks the stock. Forests, wildlife, minerals, coal, petroleum and natural gas all belong here. Coal and petroleum sit at the sharp end of this category, because the burial depth, pressure and geological time needed to make them cannot be recreated to top up the supply.

Coal: from buried forest to fossil fuel

About 300 million years ago, low-lying wetland regions carried dense forests. Flooding buried these forests under layers of soil and sediment. As more material piled on top, the buried plant matter was compressed, and as it sank deeper its temperature climbed. Under this sustained high pressure and high temperature, the dead vegetation was slowly transformed, losing volatile components and concentrating carbon, until it became the hard, black, stone-like rock we call coal. This slow transformation of dead vegetation into carbon-rich coal is called carbonisation:

\text{dead vegetation} \xrightarrow[\text{high pressure, high temperature, millions of years}]{\text{burial under soil}} \text{coal (carbonisation)}

where: the arrow's conditions — burial, sustained high pressure, sustained high temperature, and geological time — are all necessary together; removing any one (shallow burial, a short time span, ordinary room pressure) does not produce coal. Because it comes from vegetation, coal is also called a fossil fuel. When heated in air, coal burns and produces mainly carbon dioxide gas, which is why it has long served as a fuel for cooking, for raising steam in old railway engines, for generating electricity in thermal power plants and as an industrial fuel. Coal is also a starting material: industry processes it to extract three further products.

Coke is a tough, porous, black solid — almost pure carbon. It is used to manufacture steel and to extract several metals from their ores.
Coal tar is a thick, black liquid with a sharp, unpleasant smell — a mixture of roughly 200 substances, whose products become starting materials for synthetic dyes, drugs, explosives, perfumes, plastics, paints, photographic chemicals, roofing materials, and naphthalene (mothball) balls. Bitumen, a petroleum product, has largely replaced coal tar for surfacing roads.
Coal gas is obtained while processing coal into coke. It first lit the streets of London in 1810 and New York around 1820; today it fuels industries near coal-processing plants.

Figure

Cross section of buried forest layers compressing over millions of years into a coal seam, labelled with soil, sediment layers, rising pressure and temperature, and the resulting coal seam

Petroleum: from sea life to a trapped mixture

Petroleum's origin is a parallel story set under the sea instead of under a swamp. Marine organisms died and their remains settled on the sea bed, gradually covered by layers of sand and clay. Over millions of years, cut off from air and under high temperature and pressure as they sank deeper under accumulating sediment, these remains were transformed into petroleum and natural gas. The word petroleum comes from petra (rock) and oleum (oil), because it is found trapped between layers of rock deep underground. A geological trap for petroleum looks like a sandwich: an impervious (non-porous) rock layer sits on top, sealing in gas, then oil, then water beneath, inside a porous reservoir rock. Oil and gas float above the water layer because both are lighter than water and do not mix with it — the same reason oil floats on water in a glass at home. Wells are drilled down through the impervious rock into the reservoir to bring the trapped oil and gas to the surface.

Figure

Cross section of a petroleum and natural gas deposit showing a well drilled through impervious rock into a reservoir rock containing gas above oil above water, with the layers labelled top to bottom

The world's first oil well was drilled in Pennsylvania, USA, in 1859; eight years later, in 1867, oil was struck at Makum in Assam. In India, petroleum is found in Assam, Gujarat, Mumbai High, and the river basins of the Godavari and Krishna. Crude petroleum as pumped out is a dark, oily, foul-smelling liquid — a mixture of substances from light gases to heavy waxes. Separating that mixture into useful parts is called refining, carried out in a petroleum refinery using fractional distillation: the crude oil is heated, and its components boil off and are collected separately according to how volatile each is.

\text{crude petroleum (mixture)} \xrightarrow{\text{fractional distillation in a refinery}} \text{petroleum gas} + \text{petrol} + \text{kerosene} + \text{diesel} + \text{lubricating oil} + \text{paraffin wax} + \text{bitumen}

where: each term on the right is a fraction — a group of substances collected together because they boil in a similar temperature range — not a single pure chemical. Lighter fractions (petroleum gas, petrol) boil off first and rise highest in the distillation tower; heavier fractions (lubricating oil, paraffin wax, bitumen) boil last and are drawn off lower down.

Constituent of petroleum	Typical use
Petroleum gas (liquefied as LPG)	Fuel for homes and industry
Petrol	Motor fuel (light vehicles), aviation fuel, dry-cleaning solvent
Kerosene	Fuel for stoves, lamps, jet aircraft
Diesel	Fuel for heavy motor vehicles, electric generators
Lubricating oil	Lubrication of machine parts
Paraffin wax	Ointments, candles, vaseline
Bitumen	Paints, road surfacing

Beyond these fractions, petroleum and natural gas are also starting materials for a family of manufactured substances called petrochemicals: detergents, synthetic fibres (polyester, nylon, acrylic), polythene and other plastics. Hydrogen gas from natural gas is used to manufacture fertilisers such as urea. Because of this commercial value, petroleum is often called "black gold."

Natural gas and CNG

Natural gas is an important fossil fuel in its own right, prized for being easy to transport through pipelines. For transport and storage it is compressed under high pressure into CNG (compressed natural gas), used for power generation and, increasingly, as a transport fuel, because it burns more cleanly and pollutes less than petrol or diesel. It can also be piped directly into homes and factories for burning — networks already exist in Vadodara (Gujarat) and parts of Delhi. In India, natural gas has been found in Tripura, Rajasthan, Maharashtra, and the Krishna-Godavari delta. Coal, petroleum and natural gas cannot be manufactured in a laboratory from dead organisms: their formation is too slow, and the exact combination of burial depth, pressure and time cannot be recreated on any practical timescale.

Derivation AWhy fossil fuels are exhaustible: comparing formation time and reserve life

The goal here is to make precise, using nothing beyond simple ratios, exactly why coal and petroleum are classed as exhaustible resources rather than simply "resources we should use carefully." The chapter states two numbers: fossil fuels took on the order of 300 million years to form, and known reserves are expected to last only a few hundred years at present rates of use. Comparing these two numbers, rather than quoting them separately, is what makes the scarcity concrete and testable rather than a vague warning.

Step 1. Write down the formation timescale. Coal formation, from buried forest to hard coal, took roughly $3\times10^{8}$ years (about 300 million years) under continuous burial, pressure and heat. Petroleum formation is of the same enormous order of magnitude.

Step 2. Write down the depletion timescale. The chapter states that known reserves, at current consumption rates, will last only "a few hundred years" — take this as roughly $3\times10^{2}$ years as a representative order of magnitude, a rough estimate that depends on future consumption and new discoveries.

Step 3. Compare the two timescales as a ratio. Dividing formation time by depletion time gives $\dfrac{3\times10^{8}\ \text{years}}{3\times10^{2}\ \text{years}} = 10^{6}$ , one million. It takes roughly one million times longer to make a fossil fuel reserve than it takes humanity to use it up — the mathematical heart of "exhaustible."

Step 4. State the consequence. Because replacement is not a live option on any human timescale, the only lever available is the rate of use. This is why the chapter closes with practical conservation advice (from the Petroleum Conservation Research Association, PCRA): drive at a steady, moderate speed; switch off the engine at halts; keep tyres at correct pressure; service the vehicle regularly — tips that only slow how fast the fixed stock is spent. Order-of-magnitude check: $3\times10^{8}\ /\ 3\times10^{2} = 1\times10^{6}$ , confirming reserves are consumed roughly a million times faster than they can be replenished.

Go deeper · Derivation B · 1 minWhy oil and gas sit above water in a petroleum trap

Derivation BWhy oil and gas sit above water in a petroleum trap

The goal is to explain, using the everyday idea of density and the fact that oil does not mix with water, exactly why a petroleum reservoir is always layered in the same order — gas on top, then oil, then water at the bottom — rather than in some jumbled arrangement. This is also exactly why wells must be drilled to the right depth to strike oil rather than only water or only gas.

Step 1. State the two relevant facts. First, petroleum (oil) is lighter than water — a given volume of oil has less mass than the same volume of water, so oil has lower density. Second, oil and water are immiscible: they do not dissolve into one another, unlike salt dissolving in water.

Step 2. Apply what happens when two liquids of different density do not mix. Left to settle under gravity, the denser liquid sinks and the less dense liquid rises to sit on top, because gravity pulls the denser material down more strongly per unit volume. This is why a layer of cooking oil floats visibly above water in a glass left standing.

Step 3. Extend the reasoning to natural gas. Gases are far less dense than either oil or water, so by the same argument gas rises above both liquids, forming the topmost layer beneath the sealing rock.

Step 4. Assemble the full trap, top to bottom. Directly under an impervious cap rock sits the gas layer, then the oil layer, then the water layer, all held within a porous reservoir rock. A well drilled through the cap rock meets gas first, then oil at greater depth, then water deeper still — exactly the ordering in the chapter's petroleum-deposit diagram. Consistency check: gas, then oil, then water, top to bottom, matches the standard NCERT figure and follows purely from the density ranking gas less than oil less than water.

Worked examples

Board level - Naming the coal product from its description

A tough, porous, black substance, almost pure carbon, is used in the manufacture of steel and in the extraction of several metals. Identify it and state where it comes from.

Step 1. List the clues: tough, porous, black solid; almost pure carbon; used in steel-making and metal extraction.

Step 2. Match against the three products of processing coal: coke fits every clue; coal tar is a black liquid, not a solid; coal gas is a gas, not a solid.

The substance is coke, obtained by processing coal.

Board level - Ranking petroleum fractions by volatility

During fractional distillation of crude petroleum, four fractions are collected: petroleum gas, kerosene, diesel, and bitumen. State which is collected first (lightest, most volatile) and which is collected last (heaviest, least volatile).

Step 1. Recall that in fractional distillation, the lightest, most volatile substances boil off first and are collected near the top of the tower, while the heaviest substances stay liquid longest and are drawn off lower down.

Step 2. Rank the four fractions from lightest to heaviest: petroleum gas is a gas at room temperature (lightest); kerosene and diesel are liquid fuels of intermediate weight, with diesel heavier than kerosene; bitumen is a thick, near-solid residue (heaviest).

Petroleum gas is collected first; bitumen is collected last.

Higher-order thinking (HOTS / olympiad-style) - Comparing formation-to-depletion ratios for two fuels

Coal took about $3.0\times10^{8}$ years to form and its reserves are estimated to last about $250$ years at current consumption. A hypothetical fast-forming mineral took only $1.0\times10^{5}$ years to form but its reserves also last about $250$ years. Compute the formation-to-depletion ratio for each, and say which resource has the weaker case for being called non-renewable.

Step 1. Compute coal's ratio: $\dfrac{3.0\times10^{8}\ \text{years}}{250\ \text{years}} = 1.2\times10^{6}$ .

Step 2. Compute the hypothetical mineral's ratio: $\dfrac{1.0\times10^{5}\ \text{years}}{250\ \text{years}} = 4.0\times10^{2} = 400$ .

Step 3. Compare the two ratios: coal's ratio ( $1.2\times10^6$ ) is roughly $3000$ times larger than the hypothetical mineral's ratio ( $400$ ). A large ratio means formation is vastly slower than depletion — an overwhelmingly one-way, non-renewable situation.

Step 4. State the conclusion. Even the hypothetical mineral, with formation "only" $400$ times slower than its depletion, would still be classed as exhaustible on any human timescale — there is no threshold ratio at which a resource becomes "renewable." But of the two, its non-renewable case rests on a far smaller mismatch, so coal has the overwhelmingly stronger case for being called non-renewable, since its formation-to-depletion ratio (about 1.2 million) is roughly 3000 times steeper than the hypothetical mineral's (400). Timing note: this two-part ratio computation with a comparison sentence should take about 4 to 5 minutes in an exam — budget one minute per ratio calculation and the rest for the comparative reasoning.

Common traps

A cluster of specific mix-ups recurs with this chapter, and each is worth naming precisely.

Confusing coal tar with coal gas. Coal tar is a thick black liquid mixture; coal gas is a gas used as fuel. Both come from processing coal, but they are not interchangeable in an answer.
Calling coke a fuel only. Coke is chiefly valued as a nearly pure form of carbon for steel-making and metal extraction, not primarily as a household fuel.
Saying petroleum "is" petrol. Petroleum is the raw, unrefined mixture pumped from underground; petrol is just one fraction obtained by refining it, alongside diesel, kerosene, LPG, lubricating oil, paraffin wax and bitumen.
Thinking refining "makes" new substances. Refining does not chemically create petrol or diesel from nothing — it separates substances already present, mixed together, in crude petroleum, based on how readily each one boils.
Mixing up why oil floats on water with "oil is thinner." The correct reason is that oil is less dense than water and does not mix with it — not that it is somehow a "thinner" liquid.
Forgetting natural gas is a fossil fuel too, and treating CNG and petrochemicals as the same thing. Natural gas forms by the same marine process as petroleum and is equally exhaustible; CNG is compressed natural gas used directly as fuel, while petrochemicals (detergents, fibres, plastics) are separate substances made from petroleum or natural gas.

You should be able to…

State the difference between inexhaustible and exhaustible natural resources, with two examples of each.
Explain, in your own words, why coal, petroleum and natural gas are together called fossil fuels.
Describe, in sequence, how a buried wetland forest becomes coal, and name that process (carbonisation).
Name the three products of processing coal, with one property and one use for each.
Describe how petroleum forms from marine organisms, and explain why gas, oil and water settle into separate layers.
Name the process used to separate crude petroleum, and list at least five named fractions with their uses.
Explain what petrochemicals are and give two examples.
Explain what CNG is and why it is considered a cleaner fuel.
Explain, using the mismatch between formation time and reserve life, why fossil fuels are exhaustible.
List at least three PCRA conservation measures for driving.

Ask the tutor: if a diagram in your textbook shows a petroleum trap without labels, ask the tutor to walk you through identifying the gas, oil and water layers from the density argument alone.

Why cannot coal and petroleum be made quickly in a factory if we already know how they form

We know the ingredients — dead organic matter, burial, high pressure, high temperature — but we cannot supply the one ingredient a factory can never provide: geological time measured in hundreds of millions of years. Coal took about 300 million years of continuous burial under growing layers of soil, with pressure and temperature rising gradually as it sank deeper. Even a very powerful industrial press and furnace applying enormous pressure and heat for months or years cannot substitute for that slow, uninterrupted process, because the transformation happens at a pace fixed by nature, not by how much pressure or heat is applied at once. This is why the chapter calls fossil-fuel formation something that "cannot be created in the laboratory."

If oil and gas are lighter than water, why do we need to drill wells instead of the oil just floating up to the surface on its own

Oil and gas do rise above the water layer within the trap, but they are sealed in by a layer of impervious rock sitting directly above the reservoir rock. Impervious means the rock has essentially no pores for fluid to pass through, so oil and gas stay trapped underneath it, much as a lid on a bottle of oil-and-water stops the oil escaping even though it floats to the top inside. A well is simply a drilled channel punched down through that impervious cap into the reservoir rock, giving the trapped oil and gas the pathway to the surface the surrounding solid rock does not provide.

What exactly is the difference between petroleum and petrochemicals

Petroleum is the natural raw material — the dark, unpleasant-smelling mixture pumped from underground, before anything is done to it. Refining it by fractional distillation gives direct fractions: petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen. Petrochemicals are a further step removed: manufactured substances such as detergents, synthetic fibres (polyester, nylon, acrylic), plastics and fertiliser ingredients, built using petroleum or natural gas as the chemical starting material. In short, petroleum fractions are separated out of crude oil; petrochemicals are newly manufactured from it.

Since CNG pollutes less than petrol, why does not everyone just switch to CNG vehicles

CNG genuinely does burn more cleanly than petrol or diesel, which is why the chapter highlights it as a cleaner fuel, and cities such as Vadodara and parts of Delhi already run piped CNG networks. But CNG is still natural gas — an exhaustible fossil fuel formed the same slow way as petroleum, so switching to it reduces pollution without solving the deeper scarcity problem. There are also practical limits: CNG needs pressurised storage tanks in vehicles, and pipeline or refuelling infrastructure has to be built out city by city, which is why the switch has been gradual rather than immediate everywhere.