A sharp drop the number of oil barrels Saudi Arabia loaded onto tankers in April appears to deliver what the market has been awaiting: a big decline not just in crude output, but exports.

Data from transportation tracking firm ClipperData indicate that the number of barrels the kingdom put on tankers for shipment around the world fell by more than 670,000 per day in April from October, the reference level for OPEC's deal to cut output. That would mark the sharpest decline this year and a reversal of March's increase.

Members of the OPEC have achieved historically high compliance with a deal to cut their combined production by 1.2 million barrels a day. But the oil cartel hasn't really taken that crude off the market because exports remain high, said Matt Smith, director of commodity research at ClipperData.

OPEC's combined water-borne shipments of nearly 24 million barrels a day in March were up slightly from October levels, before OPEC hammered out the deal to cut output. They jumped to about 24.3 million barrels a day in January, the month the deal was implemented.

According to Smith, consistently high exports have prevented the output cuts from achieving their goal: shrinking huge stockpiles of crude oil around the world.

Crude in storage at U.S. facilities has fallen in recent weeks, but stockpiles remain near all-time highs. In its latest monthly report, the International Energy Agency estimated inventories in developed countries were 336 million barrels a day above the five-year average in February.

Those elevated inventory levels have weighed on sentiment in recent weeks, contributing to a sell-off in the oil market that shaved about $8 off the price of benchmark U.S. crude in the last three weeks.

Prices are now below $50 for both international benchmark and U.S. crude less than three weeks before OPEC meets to weigh extending the cuts for another six months.

Iranian export loadings appear to be dropping in April, as well, but this appears to be a Saudi story for the moment. Saudi Arabia's cut to outward shipments gives them leverage heading into this month's OPEC meeting, according to Smith.

"It's all about this meeting on May 25, and so they'll be going into that meeting knowing that they have cut exports by far more than they said they would. [They'll] go in there and bang the table and try to get everyone else in line," he said.

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