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

The reason the Middle East has so much oil is the same reason everything is stuck there now

By Stephanie Pappas edited by Andrea Thompson

A fifth of the world’s oil and liquid natural gas shipments usually pass through the narrow Strait of Hormuz on their way out of the Persian Gulf. But the strait was effectively closed shortly after the US and Israel began attacks on Iran on February 28, sending oil and gas prices soaring and sparking fears of a looming energy crisis.

It is a geopolitical predicament, but also a geological one. The reason for such a tight exit from the Gulf also explains why the region has such rich oil and gas deposits in the first place: a continental collision millions of years in the making.

Iran sits on the line where the Arabian tectonic plate, which hosts Saudi Arabia and the Persian Gulf, crunches into the Eurasian plate. This continent-to-continent crash has ripped up the Earth to form the Zagros, a long range of mountains in Iran that presses down on the Arabian plate, bending it like a bent ruler. The bending creates a low point in the Earth’s crust called a foreland basin, which traps huge amounts of hydrocarbons. This basin also collects water, creating 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 geosciences 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, professor of Earth and Ocean Sciences at the University of Victoria in British Columbia. The Eastern Seaboard in the USA is a modern example of this scheme.

Over eons, this quiet margin saw sea level rise and fall, and as a result it built up layer after layer of organic-rich shale, porous sandstone, fractured limestone, salt and hard capstone, says Nissen. The organic material, dug deep, converted into oil and natural gas under enormous pressure and heat. Sandstone and limestone provided cracks and fissures where these hydrocarbons could sit, and caprock held everything in place.

Today, this geological region contains an estimated 12 percent of the world’s oil reserves, according to a 2024 review in Results in geosciences.

These kilometer-deep layers were still present when the Arabian Plate, driven by the opening of the Red Sea on its southwestern side, began to shoot northeast and crash into Eurasia about 30 million years ago. Like the hoods of two cars in a traffic accident, the continents snapped together, shortening and bending at the same time. The Arabian and Eurasian plates continue to move toward each other at about 20 millimeters a year, sometimes triggering deadly earthquakes.

The collision created the Zagros fold-and-thrust belt, which is a “geologist’s dream,” says Allen. The belt consists of a 1,600 kilometer long mountain chain that stretches from eastern Turkey all the way to the Strait of Hormuz at the end of the Persian Gulf. Although processes such as glaciation and erosion largely shape the profile of most mountains, the Zagros Mountains trace the literal folds of the continental collision in long, unbroken ridges. The mountains themselves are too deformed to hold hydrocarbons. But nearby, where the topography is more subtle, similar underground folding traps oil and gas in giant fields. “Zagros has everything for oil and gas,” says Nissen.

The weight of the mountains pushing down on the earth’s crust created the Persian Gulf basin. Because the Zagros Mountains depress the earth’s crust in a narrow and shallow area, the Gulf is only 110 meters deep and 340 km wide at its widest point. At the Strait of Hormuz, the Musandam Peninsula, which includes parts of northern Oman and the northern United Arab Emirates, further narrows the Gulf to only about 55 km across.

The strait is also a result of the collision between continents: Much of Oman is made of the Semail Ophiolite, a large part of oceanic crust that was pushed onto land when the ancient sea between the Arabian and Eurasian plates closed. According to Renas Koshnaw, a researcher at Georg August University in Göttingen, Germany, who studies the region, the strait is narrower than the rest of the gulf because of the rigid rock of the Musandam Peninsula, which juts out 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 snake.

The strait is “ultimately there because of the geology, but the impact on people at this current time is that you have a marine bottleneck,” Allen says. – The tankers do not have much room to sit in, and they sit very close to the Iranian coast.

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