Shell Oil is the most recent of the big refiners to take such action as the refining industry struggles worldwide.

The moves will make Chevron closer in size to other big refiners like Exxon, and the company said it would no longer be the No. 1 American refiner.

The biggest refiners, like Exxon Mobil, now make reformulated gasoline by "blending around" the patents, which slows the refining process, crimps supplies of reformulated gasoline and drives up prices, they contend.

The Royal Dutch/Shell Group sold its 2.1 percent stake in China Petroleum and Chemical, China's biggest refiner, on Wednesday, raising $742 million.

Still, Exxon remains a far bigger refiner than Valero when considering its plants around the world.

Kuwait was one of the world's big refiners of jet fuel.

Tosco, based in Stamford, Conn., has become one of the nation's biggest independent refiners by buying up refineries that others no longer find profitable.

With its acquisition of Ultramar, Valero will become the second- biggest refiner in the country, trailing only Exxon Mobil.

A South Korean court blocked the latest move by a foreign investor to oust the head of Korea's biggest refiner, the SK Corporation.

Virtually all of the big refiners were hurt by the Oct. 1 shift to oxygenated fuels in 39 major metropolitan areas in the United States.