Investors are piling into the United States Oil Fund on speculation prices may have bottomed near $40 a barrel.

The share count of the biggest exchange-traded fund that tracks oil prices climbed in the past 10 sessions to 241.4 million Thursday, the highest level since the fund’s inception in 2006. Shares outstanding increased by 27 percent since Nov. 5. Investors have poured in $717.8 million into the fund so far this month, after adding $283 million in October.

“People are putting money into the ETF because there is limited downside and a lot of upside,” says Carl Larry, head of oil and gas for Frost & Sullivan LP in Houston. “There are still a lot of uncertainties, but prices have likely hit the bottom.”

West Texas Intermediate crude, the U.S. benchmark, headed for a third weekly loss after falling below $40 this week for the first time since August on concern a global supply glut will deepen. Prices at lower than $40 are a "strong support level" as refineries are set to boost demand for crude oil, BNP Paribas SA said this week.

December WTI futures, which expire today, added 32 cents to $40.86 a barrel at 1:01 p.m. Friday on the New York Mercantile Exchange. Prices slumped 46 percent last year and are down 23 percent in 2015.

The U.S. Oil Fund held 74,979 contracts of January WTI futures as of Thursday, according to the fund. That’s about 14 percent of open interest of the January futures. Prices of the fund rose 3 cents to $12.99.

U.S. crude  production slipped 3,000 barrels a day to 9.18 million while refineries boosted their operation rate to 90.3 percent, the most since September, according to the Energy Information Administration. Inventories increased for an eighth week in the seven days ended Nov. 13, the report said.

“ETF investors continue to find the temptation to call a bottom in oil too strong to resist,” said Eric Balchunas, a Bloomberg Intelligence analyst. “A few have managed to time it perfectly before.”