Oil headed for its biggest weekly gain in three weeks. : (Bloomberg) -- Oil headed for its biggest weekly gain in three weeks as simmering Middle East tensions, shrinking U.S. crude inventories and a storm in the Gulf of Mexico all conspired to threaten the supply outlook. Futures rose 0.7% in New York Friday and are up more than 5% for the week. Tropical Storm Barry, which could hit the Louisiana coast on Saturday, has already curbed about half of the energy output in the Gulf.