Oil supply running ahead of demand hasn't just pressured prices, it's also filling up storage space, potentially pushing crude toward another leg down. "We're going to see pretty fast inventory builds over the next few weeks," Francisco Blanch, head of commodity research at Bank of America-Merrill Lynch, told CNBC Wednesday, noting that global supply is running around 1.4 million barrels a day above demand. "If you run out of space, prices tend to react a lot more violently to adjust that supply and demand imbalance and that's what we expect over the next few weeks," he said, forecasting both WTI and Brent will fall toward $30 a barrel. Prices settled at $50.99 and $61.97, respectively, on Wednesday. Read MoreOil may fall again … but buy energy stocks?

He cited fresh American Petroleum Institute (API) data which showed U.S. crude inventories climbed by a larger-than-expected 8.9 million barrels in the week ended Feb. 20, for a total of around 437 million barrels squirreled away. Around 50 million to 100 million barrels of crude oil may be gathering dust in floating storage by the end of the second quarter, compared with around 110 million barrels in April 2009, during the global financial crisis, he estimated.



The supply build isn't helped by an oil market that's in contango, or when the "spot" price is lower than the price of the future contract. That makes it more profitable for traders to stick their oil in storage to sell at a higher price later.

As much as 80 percent of the commercially available storage in the U.S. may already be utilized, Premasish Das, downstream analyst at IHS Energy Insight, told CNBC last week.

Oil tanks and pump jacks are seen in an oil field near Bakersfield, Calif. Lucy Nicholson | Reuters