[Editor’s note: This article was updated at 11:02 a.m. CT Dec. 18.]

Oil prices tumbled on Dec. 18 after reports of swelling inventories and forecasts that record U.S. and Russian output will hit a market that may see weaker demand if global growth deteriorates as many expect.

U.S. crude oil fell $2.78, or 5.5% to trade at $47.10 per barrel by 11:44 a.m. EST (16:44 GMT). The session low was $46.97, its weakest since September 2017.

Global benchmark Brent lost $2.54, or 4.2% to trade at $57.07 per barrel. During the session, Brent hit a 14-month low of $56.86.

World stock markets inched higher as investors looked ahead to whether the U.S. Federal Reserve will be able to raise interest rates much further. Equity markets have had steep declines over the last two months.

Investor confidence is deteriorating, with more fund managers expecting global growth to weaken over the next 12 months, the worst outlook in a decade, Bank of America Merrill Lynch’s December investor survey showed.

“There was a flood of supply side news yesterday which, in combination with the demand destruction that the stock market slide implied, got us below $50 [per barrel for U.S. crude], and that gave us a strong sell signal,” said Bob Yawger, director of futures with Mizuho in New York.

Britain’s largest oil field restarted, increasing supply, the U.S. government said output from shale would top 8 million barrels per day (MMbbl/d) this year, and data suggested U.S. crude inventories would rise this week.

Both oil benchmarks have slid more than 30% since early October due to swelling global inventories. Volumes were low on Dec. 18 heading into the holiday season, and ahead of expiry for the front-month U.S. crude futures contract.

OPEC and other oil producers agreed this month to curb production by 1.2 MMbbl/d, equivalent to more than 1% of global demand, in an attempt to drain tanks and boost prices.

But the cuts will not happen until next month and production has been at or near record highs in the United States, Russia and Saudi Arabia.

Russian oil output hit a record 11.42 MMbbl/d this month, an industry source told Reuters.

Oil production from seven major U.S. shale basins is by the year-end expected to surpass 8 MMbbl/d for the first time, the U.S. Energy Information Administration said.

Inventories at the U.S. storage hub of Cushing, Oklahoma, delivery point for the oil futures contract, rose more than 1 MMbbl from Dec. 11 to Dec. 14, traders said, citing data from market intelligence firm Genscape.

Britain’s Buzzard oil field restarted after repairs on pipework, a spokesman for operator Nexen said on Monday. Buzzard produces about 150,000 bbl/d and is the largest contributor to the Forties pipeline which brings oil to shore from more than 50 fields.

The United States has surpassed Russia and Saudi Arabia as the world’s biggest oil producer, with total crude output climbing to a record 11.7 MMbbl/d.