U.S. Gulf Coast heavy crude prices surged on Friday as the ongoing conflict in Iran prompted several Middle Eastern producers to cut output, while buyers rushed to secure U.S. barrels.

Mars sour crude, the flagship grade produced in the U.S. Gulf of Mexico and widely used by refiners around the world, traded at an $11 premium to U.S. benchmark West Texas Intermediate (WTI) on Friday, brokers said. That marked the highest premium since April 2020 and a $4 increase from Thursday. Just a week ago, the premium stood at $1.50.

Other heavy grades, including Heavy Louisiana Sweet and West Texas Sour, also saw price increases.

Global benchmark crude prices have risen sharply since last week’s initial attacks, with Brent crude settling at $92.69 per barrel on Friday—its highest level since October 2023.

The effective closure of the Strait of Hormuz has forced several countries, including Iraq, to reduce production. The strait is a crucial route for medium and heavy sour crude from the Persian Gulf, and these flows are now largely cut off. Further production cuts announced in Kuwait on Friday also contributed to the rise in Mars crude prices, according to traders.

“Refiners that rely on these grades need alternatives to replace the lost barrels, so U.S. Gulf sour heavies and medium crudes are natural substitutes and are being aggressively bid up,” said Matt Smith, lead Americas oil analyst at Kpler. He noted that buyers, particularly in Asia, are scrambling to secure more of these medium and heavy barrels.

Tim Snyder, chief economist at Matador Economics, said the price surge is also influenced by seasonal factors. “This time of year marks the shift from winter into the driving season, when demand typically rises across all crude grades,” he explained. “Ultimately, the supply disruption caused by the war is driving prices higher. In the short term, we will continue to see these grades rise until the Strait of Hormuz reopens.”