A quirk of geology explains Iran’s oil – and why it’s stuck in the Persian Gulf

The same reason there’s so much oil in the Middle East is why it’s all stuck there now

by Stephanie Pappas Edited by Andrea Thompson

One-fifth of the world’s oil and liquefied natural gas shipments typically pass through the narrow Strait of Hormuz on their way out of the Persian Gulf. But the strait was effectively closed soon after the US and Israel launched attacks on Iran on February 28, sending oil and gas prices soaring and raising concerns of an impending energy crisis.

This is a geopolitical dilemma but also a geological one. The reason for such a tight exit from the Gulf also explains why the region has such rich reserves of oil and gas: the continental collision took millions of years to form.

Iran sits on the line where the Arabian tectonic plate, which hosts Saudi Arabia and the Persian Gulf, subducts into the Eurasian plate. This continent-to-continent crash has uprooted the Earth and created the Zagros, a long range of mountains in Iran that puts pressure on the Arabian Plate and bends it like a tilted ruler. The flexure creates a low point in the Earth’s crust called a foreland basin, which traps vast amounts of hydrocarbons. This basin also collects water, forming the long, narrow Persian Gulf.

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“It’s a combination of geological facts that lead to these huge oil and gas reserves in the Middle East on both sides of the Persian Gulf,” says Mark Allen, professor of earth sciences at Durham University in England.

Hundreds of millions of years ago, the northern edge of what is now the Arabian Plate was a “passive margin,” acting as a boundary between continental and oceanic crust that is tectonically quiet, says Edwin Nissen, a professor of earth and ocean sciences at the University of Victoria in British Columbia. America’s eastern seaboard is a modern example of this system.

Nissen says that over the eons, this tranquil shore has seen sea levels rise and fall, and as a result, it has acquired layer after layer of organic-rich shale, porous sandstone, fractured limestone, salt and hard capstones. Deeply buried organic matter was transformed into oil and natural gas under immense pressure and heat. The sandstone and limestone provided the cracks and fractures where these hydrocarbons could sit, and the caprock held everything in place.

According to the 2024 Review, today this geological region contains an estimated 12 percent of the world’s oil reserves. Results in Earth Sciences.

Those kilometer-deep layers were still present when the Arabian Plate, driven by an opening in the southwestern part of the Red Sea, began moving northeast and colliding with Eurasia about 30 million years ago. Like the hoods of two cars in a traffic accident, the continents shrunk together, becoming simultaneously small and flexible. The Arabian and Eurasian plates continue to move toward each other at a rate of about 20 millimeters per year, sometimes causing deadly earthquakes.

The collision created the Zagros fold-and-thrust belt, which is a “geologist’s dream,” says Allen. The belt comprises a 1,600 kilometer long mountain range, stretching from eastern Türkiye to the end of the Strait of Hormuz in the Persian Gulf. Although processes such as glaciation and erosion largely shape the outlines of most mountains, the Zagros Mountains show literal layers of continental collision into tall, unbroken peaks. The mountains themselves are too deformed to hold hydrocarbons. But nearby, where the topography is more subtle, similar underground folds trap oil and gas over vast areas. “Zagros has everything for oil and gas,” Nissen says.

The weight of the mountains pushed down on the crust created the Persian Gulf Basin. Because the Zagros Mountains compress the crust into a narrow and shallow area, the gulf is only 110 m deep and a maximum of 340 km wide. In the Strait of Hormuz, the Musandam Peninsula, which includes northern Oman and parts of the northern United Arab Emirates, narrows the gulf to only about 55 km.

The strait is also the result of the collision of continents: much of Oman is composed of the Semel Ophiolite, a large chunk of oceanic crust that was pushed onto land when the ancient ocean closed between the Arabian and Eurasian plates. According to Renas Koshnaev, a research associate at the Georg August University of Gottingen in Germany who studies the region, the strait is narrower than the rest of the gulf because of the hard rock of the Musandam Peninsula, which extends perpendicular to the Zagros Mountains. When the collision between the Arabian and Eurasian plates forced these two features together, the peninsula forced the mountain front and thus the gulf to bend like a kink in a hose.

“Ultimately the strait is there because of geology, but the effect on humans at this present time is that you’ve got a sea barrier,” says Allen. “The tankers don’t have much room to sit and are sitting very close to the Iranian coast.”

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