Both of the world's top crude oil benchmarks, North Sea Brent and U.S light crude, have been in contango for months, limiting any gains.
Oil funds are particularly attractive to many investors because they offer high gearing at a time when interest rates and fixed-income bond yields are at record lows.
One fund, VelocityShares 3X Long Crude ETN, has seen its net assets rocket to $698.4 million at the end of February from $1.6 million at the end of July last year.
The fund is leveraged three times and produces even bigger daily swings than the moves of up to 5 percent that the market has come to expect since prices hit six-year lows in January.
Carsten Fritsch, senior oil and commodities analyst at Commerzbank, says investors have flooded into oil derivatives over the last few months, helping to support prices.
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"It was money flowing into oil ETPs (exchange-traded products) with stocks at record highs and bond yields at record lows," he told Reuters Global Oil Forum, an online chatroom for oil market professionals.
Oil funds are also seeing interest from computer-driven algorithmic and high-frequency trading.
That may be contributing to volatility in oil prices, especially the U.S. crude benchmark that most oil ETFs seek to track. Since January, the Oil Volatility Index has risen to highs not seen since 2011, the year of the Arab Spring.
"Investors have focused very specifically on oil and gold, using exchange-traded products to get opportunistic exposure to the two markets which have been particularly volatile recently," Barclays analyst Suki Cooper said in a note to clients.