As the U.S. draws down the Strategic Petroleum Reserve to a level not seen since the Reagan era, the physical story behind the policy turns on geology—Jurassic-era salt that can seal oil for decades, and the engineering that keeps those salt caverns stable.
By the time the war with Iran began, the Trump administration was already reaching for a deep lever—pulling the U.S. emergency oil reserve down to near its lowest level since the Reagan era.
The pace has been brisk: so far. the administration has drawn out 66 million barrels and counting from the Strategic Petroleum Reserve. a national buffer of crude stored in colossal underground salt caverns in Texas and Louisiana. The rationale is political and economic at once—keeping exports flowing. reducing domestic gas prices. and responding to a world in which a fifth of oil supply remains trapped behind the Strait of Hormuz.
The questions, however, are immediate and physical. How far can the reserve be depleted before markets react? When will it be replenished? And what does it even mean to “store oil” inside salt—down at geological depth, where rock behaves nothing like kitchen seasoning?
The reserve’s answer begins long before modern markets—about 160 million years earlier, during the late Jurassic. At that time, a rift opened between what is now the Gulf Coast and Mexico’s Yucatán Peninsula. As the land masses slowly drifted apart, a basin formed between them. For a time. that basin was isolated from the world’s oceans. so rainwater runoff from upland had nowhere to go. It evaporated. leaving behind dissolved minerals—chief among them halite. the mineral form of sodium chloride. better known as table salt. Through untold cycles of evaporation. the basin became. as geology consultant Mark Rowan puts it. “a big hole filled with salt.”.
Today, that ancient salt sits thousands of feet underground. At that depth and pressure, salt behaves differently from most rocks. Most rocks are porous and permeable, packed with interconnected spaces that let liquids—like oil—seep through. Salt, by contrast, is impermeable. It flows and deforms under pressure, more like plastic than rigid rock. That movement lets it self-heal when tiny cracks begin to form.
“It’s a fantastic seal,” Rowan says. “If you want to store something and not have it leak out into the surrounding ground or earth, then salt is a fantastic place.”
The sealing property works in both directions. Shangyou Nie. a former strategy adviser at Shell and an editor of the American Association of Petroleum Geologists’ Well Read newsletter. emphasizes that salt’s ability to deform “also prevents other things from getting into the caves. ” so the oil will not be contaminated.
To turn this geology into a storage system, the government drilled into the salt at various locations. Fresh water was injected to dissolve the salt, and the brine was pumped out. The result was roughly 60 massive caverns with a combined capacity of 714 million barrels of oil. The largest cavern, the article notes, could easily fit Chicago’s 110-story Willis Tower.
Engineers also framed the approach as a cost advantage. According to the Department of Energy, the method is up to 10 times cheaper than aboveground tanks and 20 times cheaper than hard rock mines.
When officials decide it’s time to withdraw, the process flips. Workers take advantage of physics: oil is lighter than water. They inject fresh water into the bottom of the caverns. pushing the oil upward until it can be distributed via pipeline to refineries across the country. Most of it doesn’t have to travel far—Texas and Louisiana account for roughly half the nation’s refining capacity.
Security is part of the promise, but it comes with a warning rooted in fundamentals. Salt caverns are among the most secure ways to store large quantities of oil, but they aren’t perfectly impermeable. “Nothing is,” Rowan says. “If you have enough pressure, and there’s a weak point, then you’ve got a problem.”.
Researchers monitor pressure daily to ensure the caverns remain structurally sound, and earthquakes are rare in the Gulf Coast region. Still, there’s a cautionary precedent. The German government stored radioactive waste in salt caverns for decades before groundwater finally penetrated, rendering the site unstable.
The policy debate now presses on a different risk: economics. Nie says he isn’t concerned about the market implications of depleting the Strategic Petroleum Reserve. Companies purchasing the oil are required to replenish it. and the Trump administration is coordinating its withdrawals with 31 fellow member countries of the International Energy Agency—aimed at calming global oil markets.
In Nie’s view, releasing oil at a time when prices are lower is simply good timing. “You should buy when the oil price is low, to fill it up,” he says, “and you should sell or release when the oil price is high.”
Taken together, the story reads like a bargain struck between ancient geology and urgent policy—one that depends on salt staying sealed, engineers continuing to monitor pressure daily, and governments deciding when to draw down and when to rebuild the buffer.
Strategic Petroleum Reserve SPR salt caverns Texas Louisiana oil storage geology halite Strategic Petroleum Reserve drawdown International Energy Agency Strait of Hormuz refining capacity