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The massive oil reserves of the Persian Gulf can be traced back to these mountains
"This is a gigantic petroleum system that works like a factory," says a professor.
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The closure of the Strait of Hormuz is being felt all over the world. That is hardly surprising. Combined, the countries around the Persian Gulf extract more oil than anywhere else in the world.
"People think there's a lot of oil in Norway, but there is actually little," says Dag Arild Karlsen.
He is a professor of geology at the University of Oslo.
So why is there so much petroleum in this area of the Middle East?
To answer that, we have to go underground and many millions of years back in time. The geological history of the Persian Gulf is completely different from that of the North Sea.
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"It's the result of completely different formation processes," says the professor.
Two plates crash into each other
In the North Sea, two continental plates in the Earth's crust moved away from each other. The ground split apart 300-145 million years ago.
Around the Persian Gulf, the opposite happened, Karlsen explains.
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About 25 million years ago, the Arabian plate began moving towards the Eurasian plate. The collision pushed up the Zagros Mountains, the vast mountain range along the west coast of Iran. These mountains are still rising by a couple of centimetres every year.
The pressure created by the mountains causes increasing amounts of oil to mature underground. But that is only part of the story.
The collision also formed an underground transport network for the oil. At the same time, it created enormous traps that collect and hold it.
"This is a gigantic petroleum system that has been operating like a factory for tens of millions of years and continues to do so today," says Karlsen.
Oil and gas are trapped in enormous reservoirs
The Earth's crust also changed shape in several places.
"As the Zagros Mountains are forced downwards, they create a ripple-like movement through the crust," says Karlsen.
These ripples stretch outwards into the Persian Gulf and continue across Saudi Arabia.
Oil and gas move through layers of porous rock within this folded crust. When they collect beneath one of these folds, they become trapped.
Geologists refer to these structures as anticlines.
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"There are gigantic anticlines of formidable size here," Karlsen says.
Like an ever-growing vault
Unlike the situation in the North Sea, these underground traps continue to grow over time, Karlsen explains.
He compares them to Scrooge McDuck's money vault.
"Imagine Scrooge having a sealed, durable vault that constantly grows larger while more and more money pours in. That is basically what exists there," says Karlsen.
In other words, the folds in the Earth's crust are gradually compressed, creating more space beneath the arch.
"Nature is not democratic"
Karlsen also points out that oil and gas continuously leak out of the ground.
As a result, large amounts are lost, especially in the North Sea.
The geological traps beneath the Persian Gulf have several advantages in this regard.
"There are very few faults and fractures in these traps," says Karlsen.
The traps also have an effective ‘lid.’ This is often made of salt, which becomes flexible under high pressure. As a result, these so-called cap rocks can, to some extent, heal themselves, Karlsen explains.
All of this creates highly favourable conditions for oil extraction. Lucky for the nations surrounding the Persian Gulf, you might say.
Karlsen notes that the geology responsible for petroleum and mineral deposits clearly influences the global balance of power.
"Nature is not democratic. Either you have it, or you don't," says the professor.
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Translated by Alette Bjordal Gjellesvik
Read the Norwegian version of this article on forskning.no
References:
2025 Statistical Review of World Energy, Energy Institute, 2025.
Montgomery, S.L. Why the Persian Gulf has more oil and gas than anywhere else on Earth, The Conversation, 2026.
Wikipedia article about anticlines
Wikipedia article about the Tethys Ocean
Wikipedia article about the Zagros fold and thrust belt
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