By Dan Molinski and Sarah McFarlane 

Oil prices fell to two-week lows Thursday because of a big increase in U.S. oil inventories, rising crude production from OPEC and broader-market moves away from riskier assets including crude oil.

Light, sweet crude for November delivery was 2.2% lower at $71.56 a barrel on the New York Mercantile Exchange. Brent crude, the global benchmark, was 2.7% lower at $80.85 a barrel.

The Energy Information Administration on Thursday reported a huge, six-million-barrel increase in U.S. stockpiles of crude oil during the week that ended Oct. 5. That was four times the 1.5-million-barrel rise analysts expected, and put total U.S. crude oil inventories at 410 million barrels, the highest in nearly two months.

"Energy stocks showed a buildup of six million barrels -- more than expected -- and triggered more selling," said Alfonso Esparza, senior analyst at foreign-exchange trading group Oanda. "The buildup confirms the data from [industry group] API, who yesterday reported a rise of 9.7 million barrels."

Also Thursday, the Organization of the Petroleum Exporting Countries issued its closely watched monthly oil market report, which showed OPEC and its ally Russia increased crude oil production in September.

OPEC output was 32.76 million barrels a day last month, up 132,000-barrels-a-day from August, while Russia increased production by 150,000 barrels a day to reach a new post-Soviet record of 11.54 million barrels a day, the OPEC report said.

"The monthly oil market report from OPEC showed its members are making up the shortfall from Iranian crude exports," Mr. Esparza said.

Looming U.S. sanctions have caused Iranian oil exports to decline more quickly than many had expected over recent months, pushing oil prices toward four-year highs late last month. That led President Trump on Sept. 20 to urge OPEC to increase output to lower prices. "The OPEC monopoly must get prices down now!" Mr. Trump said on Twitter.

Meantime, sharp declines in stocks on Wall Street were also pressuring oil prices lower Thursday.

"The de-risking that we saw in equity markets swept other markets with it, including oil," said Harry Tchilinguirian, global head of commodity markets strategy at BNP Paribas.

Prices have also declined this week after the International Monetary Fund cut its forecasts for global economic growth for both this year and next, citing trade protectionism and instability in emerging markets. A slowdown in economic growth would likely crimp oil demand, which analysts say is already at risk from rising oil prices.

On a fundamental basis, however, many say the overall bullish picture for oil remains intact, with further losses of production from Iran and Venezuela expected. Plus, any economic slowdown is unlikely to occur so quickly as to adversely impact oil demand in the near term, Mr. Tchilinguirian said.

Helping limit losses were ongoing revisions to the volume of Iranian oil expected to be lost from the global market when U.S. sanctions on Iran are reinstated on Nov. 4.

Among refined products, gasoline futures for November delivery fell 3.6% to $1.9483 a gallon. Diesel futures fell 2.1% to $2.3431 a gallon.

--Christopher Alessi contributed to this article.

Write to Dan Molinski at Dan.Molinski@wsj.com and Sarah McFarlane at sarah.mcfarlane@wsj.com

(END) Dow Jones Newswires

October 11, 2018 12:50 ET (16:50 GMT)