OPEC helps savvy oil traders sell millions of barrels

"OPEC's whole idea in bringing production down has been to draw oil out of storage and lower import levels," said Roger Diwan, a partner at PFC Energy in Washington.

U.S.-bound oil exports from the Gulf, OPEC's cradle, have recently fallen by about a third, according to George Los, an analyst with tanker broker C.R. Weber.

U.S. imports from OPEC kingpin Saudi Arabia, the largest exporter, have plunged to their lowest level in 21 years, according to government data. At 967,000 bpd in March, they were less than half of historic highs.

A major U.S. crude buyer told Reuters that Saudi exports to the United States don't appear to have risen since March. U.S. oil imports from West African and Latin America, home to other OPEC nations, also haven't risen noticeably, the buyer said.

OPEC may be boosting shipments to East Asia, but that hasn't interfered with sales from offshore, since most tankers storing crude are floating near the United States and Europe.

OPEC hasn't officially discussed ending output cuts, but members could open the spigots more and make it harder for traders to unleash more oil from tankers.

"With oil at $70 and a narrow contango, OPEC now has a much bigger incentive to cheat," Schork said.

Oil trading profits are hard to track. But analysts say the crude contango probably helped several firms book billions in collective profits since late 2008.

Koch, Shell and Vitol -- which held the largest floating stocks -- declined comment on their storage positions or trading gains. BP has said crude storage plays helped it gain $500 million in trading profits during the first quarter alone.

(Additional reporting by Luke Pachymuthu in Dubai; Editing by Walter Bagley)