Why Is There Oil in Some Places but Not Others?

You met Nepal’s minerals in Grade 8, and where each is found: building sand and groundwater in the Terai, iron, copper, lead and zinc in the hills, rock salt in Mustang Grade 8: Minerals in Nepal. Grade 8 told you where. This topic is about why there.

Look at the whole world and the unevenness is striking:

• About half of the world’s known oil lies under a small part of it, the Middle East (end of 2020).
• About a quarter of the world’s copper is mined in one country, Chile (in 2024).
• Some valleys sit on rock full of water; others, a few kilometers away, have almost none.

Here is a surprising fact. Almost every rock has a little copper in it, a little gold, a little iron. Ordinary rock in the Earth’s crust has, on average, about 28 grams of copper in every tonne. But no one could make a living from that: to get 1 kg of copper, you would have to dig up and crush about 36 tonnes of rock. A copper ore is rock in which something has gathered the copper together, a few hundred times richer than ordinary rock (Ores, in Grade 10).

Metal	In ordinary rock (grams per tonne)	In a typical ore (grams per tonne)	How many times richer
aluminium	82,000	about 250,000 (bauxite)	about 3
iron	39,000	about 600,000	about 15
copper	28	about 6,000	about 200
gold	0.0015	about 1.5	about 1,000

So the real question is: what gathers a resource together in one place? Nature has a few ways. Heat can cook buried mud into oil. Oil, being light, can float up through rock and collect under a lid. Hot water can dissolve metals from a great volume of rock and drop them in one crack. A river can sort heavy grains from light ones. Rain can soak into gravel and stay there. Each leaves its resource only where it happened.

Oil and natural gas are fossil fuels: you know from Grade 9 that they formed from living things buried millions of years ago Grade 9: Fossil fuels. But living things die everywhere. Why did only a few places get oil? Because it needs a recipe, and every step must happen.

Step 1: a lot of life, buried without air. Most oil began as plankton: tiny plants and animals, most too small to see, that float near the surface of the sea in their billions. When they die they sink. In most seas, the oxygen in the water lets bacteria and animals eat them up on the way down or on the sea floor. But in some seas the bottom water has little or no oxygen. There the remains pile up, mixed with fine mud, and are buried before they rot. The mud becomes a dark rock rich in the remains of life, the source rock, usually a black shale.

Step 2: buried deeper, and heated. As more mud settles on top over millions of years, the source rock sinks deeper. The deeper rock is, the hotter: on average about 25 °C hotter for every kilometer down. At about 60 °C the remains slowly begin to break up into oil. Between about 60 and 120 °C, over millions of years, they make oil; this range is the oil window. Hotter still, the big molecules break into small ones, and the rock gives natural gas, mostly methane. Hotter than about 200 °C, nothing more is made: the rock has been cooked dry.

Oil made in a source rock doesn’t stay there. The rock is squeezed by the weight above it, and oil is lighter than water. You have seen what that means: drops of oil on a bowl of soup rise and float on top. Underground, the tiny spaces in the rocks are full of water, so the oil, and the gas even more, slowly creep upwards, squeezing through any rock with spaces in it.

Most of it rises all the way to the surface and is lost. Over millions of years, bacteria eat it, or it dries into tar. Oil and gas are left for us only where something stopped them on the way. That needs two more things.

A reservoir rock, with spaces to hold the oil. Sandstone is made of sand grains stuck together, and between the grains there are tiny gaps, called pores. Up to a third of a sandstone can be pores. Some limestones are full of holes too. An oil field is not an underground lake: it is solid rock with oil in its pores, like water in a sponge.

A seal and a trap. Above the reservoir there must be a layer that oil can’t get through, a cap rock, such as shale (its pores are far too small) or salt. And the layers must be shaped so that the oil, rising under the lid, has nowhere to go. The commonest shape is an arch, an anticline: the layers bent upwards, like an upside-down bowl. Oil creeping up under the arch gathers at its top, as air gathers under an upturned cup held in a bucket of water. Such a place is an oil trap.

So an oil field needs five things, in the right order: a source rock, the right burial and heat, a reservoir rock, a seal, and a trap that formed before the oil came up. Miss any one and there is no oil. That is why oil is found in so few places, and why most wells drilled to look for it come up dry.

Why Saudi Arabia and not Nepal?

Much of the oil of Saudi Arabia formed from plankton buried about 150 million years ago, in warm, shallow seas at the edge of an ocean called the Tethys. Above the source rocks lie thick limestones full of pores, then a thick seal of anhydrite (a mineral like gypsum), all gently bent into huge, broad arches. Since then the rocks have been left almost undisturbed. Every step of the recipe happened, on a huge scale, and nothing broke the traps afterwards.

The same ocean is in Nepal too. The black shaligram stones of the Kali Gandaki hold ammonites that lived in the Tethys at about the same time. But Nepal’s rocks have had a rougher history. As India has pushed into Asia over the last 50 million years, they have been folded, broken by great faults, stacked on top of each other and heated. A break through a cap rock lets oil escape, as it does at Muktinath, and great heat turns any oil into gas. Only a few deep wells have been drilled in Nepal. Shell’s, near Biratnagar in the late 1980s, went down 3,520 m and came up dry; the Dailekh well found gas, not oil.

Most metal ores were made by hot water. Deep underground, near a body of molten rock (magma), water that has soaked down through cracks is heated to a few hundred degrees. Hot water under pressure can dissolve small amounts of metals out of the rock it passes through. The hot water rises, through kilometers of rock, along cracks. As it rises it cools, and the pressure drops, and it can no longer hold the metals. They come out of the water as solid minerals and line the cracks: a vein. A trickle of water carrying a few grams of metal per tonne, flowing for thousands of years, can fill a crack with ore that is hundreds of times richer than the rock it came from.

Where is there magma near the surface for long enough? Mostly where one of the great plates of the Earth’s outer shell slides down under another, as you read in Do Continents Really Move?. The sinking plate carries water down with it, and that water makes rock above it melt. The magma rises and feeds a chain of volcanoes along the edge of the continent. Under the volcanoes, magma cools slowly, and its hot water makes ore.

That is why the world’s biggest copper mines are in a line along the Andes mountains of Chile and Peru: under them, the floor of the Pacific Ocean (the Nazca Plate) is sinking under South America, as it has for well over a hundred million years. The ore bodies are old magma chambers whose roofs have since been worn away by rain and rivers, bringing the ore close enough to the surface to dig.

Sorted by a river

Ore in a mountain doesn’t stay there forever. Rain and frost break the rock, and rivers carry the pieces away. Most minerals break down or are ground into sand and mud. But gold doesn’t rust or dissolve in water, and it is very heavy for its size: gold’s density is 19.3 g/cm³, about seven times that of quartz sand (2.65 g/cm³).

A river carries sand while it flows fast. Where it slows, it drops what it carries, and the heaviest grains first. So gold specks wash out of the mountains and gather where the river slows: on the inside of bends, behind big boulders, in cracks in the river bed. Such a deposit of heavy grains, sorted out by water, is a placer. The gold in the sand of the Sunkoshi, the Kali Gandaki and the Rapti (Grade 8’s table), and of the Karnali, came from veins somewhere upstream, broken down and carried by the river.

Two other kinds of process make ores where the rock sits still:

• Weathering in a hot, wet climate. Rain soaking through the ground for millions of years can wash almost everything out of a rock except the things that hardly dissolve. That is how bauxite, the ore of aluminium, forms (Ores of aluminium, Grade 10): the aluminium is left behind, concentrated.
• Evaporation. Where a shallow sea or salt lake dries up, its salts are left behind in layers: rock salt, gypsum and anhydrite. Most of the world’s rock salt formed that way, and so did the anhydrite that seals Saudi Arabia’s oil.

When rain falls, some runs off into streams, some evaporates, and some soaks into the ground (The paths water takes, in Where Does the Rain Come From?). It doesn’t gather in underground rivers or lakes, except in caves. It fills the pores between grains of sand and gravel, and cracks in rock, just as oil fills the pores of sandstone. This water is groundwater.

Dig a hole in the ground and, at some depth, it starts to fill with water. Below that level, every pore is full of water; above it, the pores hold air and a little water. That level is the water table. A well is a hole dug or drilled below the water table.

How much water a rock or soil can hold, and how easily the water can flow through it, are two different things:

• The fraction of the volume that is pores is the rock’s porosity. Sand and gravel are about 25–40% pores. Clay can be 40–70% pores, but they are so tiny that water hardly moves through them.
• How easily water flows through is its permeability. Gravel and sand are very permeable: water flows through them, and a well in them fills quickly. Clay and shale are almost impermeable.

A layer of permeable rock or sediment full of water that a well can draw from is an aquifer. A layer of clay above it keeps it apart from the water table above, like a lid; then the water in the aquifer is under pressure, and may even rise up a well by itself.

Groundwater in Nepal

Why is there groundwater a few meters down in most of the Terai, but water is scarce in other places close by? Because of how the Terai was built. For millions of years, the rivers coming out of the Himalaya have dumped what they carry where they leave the hills and slow down: coarse boulders and gravel first, right at the foot of the Chure hills, then sand, then silt and clay farther south.

• The belt of boulders and gravel at the foot of the hills, the Bhabar, is so permeable that streams sink into it, and the water table lies deep. Villages there can be short of water although the ground below holds a great deal.
• Where the gravel meets finer sediment, water comes out in a line of springs.
• Farther south, in the Terai proper, layers of sand full of water lie between layers of clay. The water table is near the surface, and a hand pump a few meters deep gives water. A well drilled through a clay layer into the sand below sometimes overflows by itself: an artesian well, because the water has come from the higher Bhabar and is under pressure.

The Kathmandu Valley is different. Long ago it held a lake, and on its bed settled thick layers of fine black clay, the kalimati (“black soil”) that gives a part of Kathmandu its name. The clay is almost impermeable. So the deep aquifers, sand and gravel under the clay, are refilled only around the north of the valley, where sand and gravel reach the surface and rain can soak in. Everywhere else, rain on the clay can’t get down to them.

For centuries the valley’s stone spouts gave plenty of water from shallow aquifers, kept full by ponds and canals. Today hundreds of deep wells pump from the deep aquifers (more than a thousand, by one count in the late 2010s), and studies agree that the water is taken out several times faster than rain refills it, though their numbers differ. The water table has fallen, ponds have been built over, and many spouts have dried. Of 573 spouts on record in the 2019 survey, 94 were lost and fewer than half of the rest still gave water.

Soil is a resource too: all our food grows in it. It is made from rock broken down by rain, frost, roots and living things (weathering, the first step of the rock cycle), mixed with the remains of plants. So soil, like the others, is found where the Earth’s processes made it:

• where rock has been weathering in place for a long time, on gentle land where the soil isn’t washed away;
• where rivers drop what they wear from the mountains, as on the Terai, whose deep, fine soil is the mountains’ worn-down rock, and around the rivers of the Kathmandu Valley.

On steep slopes, soil is thin, because rain and landslides carry it away almost as fast as it forms. And it forms slowly: it can take up to 1,000 years to make 1 cm of topsoil (FAO, 2015). A heavy monsoon on a bare slope can wash off in one season what took centuries to make.

You learned in Grade 7 that some resources are renewable, made again as fast as we use them (sunlight, wind, river water), and others non-renewable. Now you can see why oil, gas, coal and ores are non-renewable. Oil is still forming today, in a few seas, but it takes millions of years: the oil we pump formed over tens of millions. Ore veins took thousands of years to fill, and millions more for the rock above them to wear away. We use them up in a few decades.

Groundwater is in between. It is renewed by rain, so it is renewable if it is pumped no faster than it is refilled. Pump faster, and you are mining it like ore: the water table falls, shallow wells and springs dry up, and in a clay-filled valley the ground itself can slowly sink as the drying clay squeezes together. Some aquifers, under deserts, hold water that soaked in thousands of years ago, when the climate was wetter, and get almost no rain now. Pumped out, it won’t return for thousands of years.

So people change where resources are, too. A mined-out ore body, an emptied oil field and a dry spout all used to be resources. And new ones appear on the map when new ways of finding or getting them are invented, like the Dailekh well.

• Where Does Plastic Come From? What happens to oil after it comes out of the ground: how its chains are taken apart and built into plastic.
• How Does One Rock Become Another? The rock cycle: how sandstone, shale and limestone, the rocks of oil fields, are made, and the heat that drives the cycle.
• What Happens When There Are More of Us, Each Using More? More people, each using more oil, metal and water: what that does to the Earth’s resources.