(Bloomberg) -- Total U.S. oil production has plunged by nearly third as an unprecedented cold blast freezes well operations across the central U.S., according to traders and industry executives with direct knowledge of the operations.

Crude output has now fallen by about 3.5 million barrels a day or more nationwide, they said, asking not to be identified because the information isn’t public. Before the cold snap, the U.S. was pumping about 11 million barrels a day, according to last government data. Production in the Texas’s Permian Basin alone -- America’s biggest oil field -- has plummeted by as much as 65%.

Operations in Texas have stumbled because temperatures are low enough to freeze oil and gas liquids at the well head and in pipelines that are laid on the ground, as opposed to under the surface as practiced in more northerly oil regions. The big question now is how quickly temperatures return to normal.

The huge scale of the disruption has helped oil prices to rise to their highest so far this year. It’s also threatening to starve oil refineries, although such is the chaos in Texas right now that many of the largest plants have already had to shut down.

Among the companies hit by outages is Occidental Petroleum Corp., the second-largest producer of oil in the Permian, which has issued a force majeure notice to suppliers, and Chevron Corp., which has shut in compression and production at wells in Culberson County in West Texas due to cold weather.

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