If the comings and goings of oil tankers from the Persian Gulf are any guide, then OPEC’s Middle East powerhouses did their bit to curb a global crude glut last month. Saudi Arabia shouldered the burden.

The region’s six OPEC members shipped less than 18 million barrels a day of crude in December, the lowest combined outflow since August, according to vessel-tracking data compiled by Bloomberg. Saudi Arabia made by far the biggest contribution, reducing daily shipments by more than half a million barrels from the previous month. Its observed deliveries to China plunged to the lowest level of the year.

Here’s what the tracking showed in millions of barrels of oil a day:

Country December November Difference
Saudi Arabia 6.61 7.18 -0.57
Southern Iraq 3.46 3.51 -0.05
U.A.E. 2.50 2.37 0.13
Iran 2.41 2.41 Unchanged
Kuwait 2.00 2.09 -0.09
Qatar 0.96 0.80 0.16
Total 17.95 18.37 -0.42

“We’ve definitely seen lower Saudi exports to Asia” recently, said Amrita Sen, chief oil market analyst at Energy Aspects Ltd. in London. De-stocking of crude inventories and year-end book-squaring in China in December helped account for lower import volumes there, she said.

Saudi Arabia and fellow Middle East producers—Iraq, the United Arab Emirates, Iran, Kuwait and Qatar—are vital to the health of crude markets because they pump about 75 percent of output by the Organization of Petroleum Exporting Countries, which itself is spearheading an international effort to trim oil supplies through the end of the year. Brent futures surged almost 2 percent on Wednesday, amid signs U.S. inventories will continue a recent slump.

Deriving cargo flows from tanker monitoring can have wide variations with official data because of what’s counted and when. Just two supertankers that load at the end of a month can create a swing of 260,000 barrels a day if tanker-tracking and government data count them in different periods. There’s also a lag between when cargoes leave countries and arrive at their destinations.

U.S. inventories slid by 4.99 million barrels last week, the American Petroleum Institute was said to report Wednesday. If confirmed in Energy Information Administration data later on Thursday, that would continue a trend that’s seen stockpiles drop by more than 100 million barrels since March. The nation’s producers are increasingly directing their output to overseas markets, where OPEC’s actions have helped to push prices higher.

Stockpiles have fallen to 432 million barrels from 536 million at the end of March. To get down to the five-year average for industrialized countries, one of OPEC’s yardsticks of market re-balancing, U.S. stockpiles would need to fall a further 41 million barrels, according to Bloomberg calculations.

“If we continue to draw stocks in the next six months, that will probably be an over-tightening,” said Torbjorn Kjus, chief oil analyst at DNB Bank ASA.