Where are all the major oil companies located in the U.S.?

Like BP, Shell and the other ones.
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Shell is located in Houston Texas, and BP is located in London I think.
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Quite a few are in Houston Texas. Exxon is in Irving Texas. Occidental and Chevron are in California. Conoco Phillips is in Houston.
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http://en.wikipedia.org/wiki/List_of_pet...
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all i know is Shell is in Texas