Sinopec leased the 3.2 million-barrel supertanker TI Europe, above, to hold crude in storage, and plans to pick up more cargoes in the coming days. Euronav

Big oil companies and traders are stashing millions of barrels of crude on massive tankers bobbing in the ocean, in a bid to profit from a quirk in oil markets.

Instead of moving crude from one port to another, a growing number of tankers are serving as floating warehouses for companies including Sinopec Ltd. and Vitol Group, according to people with knowledge of their operations. Other companies such as Mercuria Energy Group are using the tankers to haul crude to on-shore storage facilities, these people said.

In a rare split, crude is cheaper in the spot market than in the futures market, where bets are made on where prices will be in the months ahead. By buying physical stocks of oil and immediately selling futures, traders can lock in a profit.

The storage trade isn't without its pitfalls. If interest rates or storage costs rapidly increase, the costs of the trade could eclipse the money earned from the future sale. Also, some traders say the window to put on the trade could close if demand picks up while supplies wane. The risks were on display Thursday when a late move narrowed the price gap between contracts for near-month delivery and the following month to below the level needed for the strategy to be profitable.

The tankers, weighing as much as 550,000 tons and stretching up to 1,300 feet long, store the oil until the trade is unwound. So do land-based storage facilities, which are filling up as well.

The amount of oil tied up in the strategy has risen to between 25 million and 50 million barrels of crude from almost zero as of April, oil-market traders and analysts estimate, based on trading and ship-chartering data. That amount represents more than one to two days' worth of U.S. demand.

More than 70 million barrels were stored as part of the trade in April 2009, the last time spot prices stayed below futures prices for a sustained period, according to Energy Aspects, a London-based research and consulting firm.

The spike in oil being stored on the high seas has caught the attention of many investors, who say it is the hallmark of a global supply glut and signals that oil prices—already at two-year lows—are likely to keep falling.

"It shows that there's oversupply in the market due to weak demand," said Amrita Sen, an analyst with Energy Aspects.

In recent weeks, Mercuria, one of the world's largest commodity traders, chartered tankers to haul crude to storage facilities in Saldanha Bay, South Africa, traders and analysts said. Sinopec, the world's third-largest company by revenue, leased the 3.2 million-barrel supertanker TI Europe, anchored off the eastern coast of Malaysia on Wednesday, to hold crude in storage and is planning to pick up more cargoes in the coming days, traders said. Also last week, Vitol offered a cargo for sale directly from a tanker, rather than from a port, a sign that the trading firm had used the oil to conduct a storage trade, a London trader said.

Mercuria and Sinopec didn't respond to requests for comment. A Vitol spokeswoman said the firm doesn't comment on trading activities.

Brent crude oil, the benchmark for world prices, has slid 14% in the past three months amid rising production in places like the U.S., Libya, Iraq and West Africa—and the belief that supplies will continue to outstrip demand.

In July, the spot price of Brent contracts fell below the price of Brent for delivery in later months for a sustained period for the first time since early 2011. When this price pattern emerged, the gap between contracts for near-month delivery and the following month was five cents. On Thursday, the difference was as much as $2.04, surpassing the 70-cent gap analysts and traders say is necessary to make a profit on the trade, taking into account storage and capital costs. The gap shrank to 66 cents at Thursday's close.

While buying and storing crude is a trade available mostly to companies steeped in physical-oil markets, some investors have started trying to replicate it in financial markets, rather than making simple up-or-down bets on the direction of prices.

Michel Salden, co-manager of $600 million at Harcourt, an asset manager in Zurich, is wagering that gap between short- and long-term Brent prices will continue to widen as oil demand stays sluggish.

"In this environment, it's hard to play the directionality of the market," Mr. Salden said.

Some analysts say excess oil supplies already are reflected in the current price of front-month Brent, which settled down 1.3% on Thursday at $97.70 a barrel. Brent for delivery in the next month, December, settled down 1.2% at $98.36 a barrel.

Mr. Salden said he has profited from his positions but declined to disclose details.

Although it is a boon for physical traders, the higher price of Brent for delivery further out can punish many money managers who invest in commodities through passive index funds. Managers of these index funds sell futures contracts before they expire to avoid taking physical delivery of the commodity. To maintain steady exposure, they then buy the more-expensive contract for later delivery, which erodes returns.

"If it does persist, it will be a meaningful incremental drag on returns," said Nicholas Johnson, who oversees $25 billion in commodity investments at Pacific Investment Management Co., a division of Allianz AG. Pimco is placing bets on the price gap between Brent and U.S. crude, which, unlike Brent, is cheaper for delivery in several months.

Companies are seeking to take advantage of the unusual prices in oil market while they can.

"Crude-oil storage is once again happening in the Eastern Atlantic, South Africa and Asia," said Stephen Wolfe, senior analyst with commodity-trading firm Trafigura Beheer BV in Houston. "Regional surpluses arose in Asia, Africa and the North Sea at times over the last two months, placing pressure on prices and making storage profitable."

—Sarah Kent