U.S. oil output has plunged by about one-third — a record — as the unprecedented cold blast freezes well operations through the Midwest. The drag amounts to about 3.5 million barrels a day, according to traders and executives. Occidental Petroleum issued a force majeure in the Permian Basin, while Citgo did the same for its Corpus Christi, Texas, refinery. Chevron shut compression and production at wells in West Texas.