CUSHING, Okla. – Since May, speculators have sold millions of barrels of crude oil, causing Cushing, Oklahoma’s inventory to drop nearly 35 percent, The Wall Street Journal reports. Other oil depots have seen storage levels fall as well, driving oil prices to a record of almost $84 a barrel in September.

On Friday, oil closed at $81.22 on the Nymex, an increase of 33 percent this year. For Cushing, the reasons for its drop in oil inventory include increased demand and the shift in oil-trading strategies by investment banks and other financial firms,

“Factors other than supply and demand are now impacting the price,” oil-and-gas trader Stephen Schork, who publishes the Schork Report on energy markets, told the newspaper. “We now have to factor in how the speculators are going to affect the market, because they have different priorities in managing their portfolios.”

With fears of recession looming, the drainage of U.S. storage tanks has some analysts predicting that the economy could be more vulnerable to oil-price hikes that would in turn jack up gasoline prices.

Shifting market conditions have economists worried that investors will run up oil prices even higher. While it’s usual for oil inventories to decline in the fall because of a temporary slower fuel use, this year’s drops have been more than normal.