As energy prices rebound, commodity traders have jumped on oil exchange traded funds to ride the rally from 13-year lows. However, long-term investors should understand that these ETFs track the futures market and come with specific risks.

For instance, many have turned to the U.S. Oil Fund (NYSEArca: USO), which tracks West Texas Intermediate crude oil futures, to play the turn in the energy market. USO is the largest and most popular oil-related ETF option on the market, with $3.6 billion in assets and 33.6 million  shares changing hands daily.

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However, oil traders should be aware that USO’s underlying portfolio includes front-month WTI future contracts and the oil futures market is currently in a state of contango. Consequently, USO could experience a negative roll yield when rolling a maturing futures contract for next month’s contract.

Contango occurs when the price on a futures contract is higher than the expected future spot price, which creates the upward sloping curve on future commodity prices over time. Essentially, the phenomenon reflects a current spot price that is lower than the futures price. For instance, WTI futures were trading around $46.44 per barrel Thursday for June 2016 delivery, but contracts with a later delivery are trading higher, with contracts for June 2017 delivery at $49.85 per barrel.

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The futures market may diverge from the spot price due to a number of factors, including storage costs, carry costs and seasonal patterns, among others. Consequently, oil ETF investors would be exposed to the intricacies of the futures market, which may not perfectly reflect the spot price return of the underlying energy commodity.

While this phenomena is normal in the futures market, contango can have a negative effect on ETFs. ETFs that hold futures contracts sell the contracts before they mature to avoid physical delivery and purchase a later-dated contract. In a contangoed market, the ETF loses money each time it rolls contracts to a costlier later-dated contract – the fund would technically sell low and buy high each time. Consequently, long-term investors may notice underperformance to the oil market since the ETF holds front-month contracts and would see a slight cost when rolling each front-month contract.

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