October Brent crude oil futures fell below $100 a barrel Monday for the first time in 16 months, and some experts see continued declines ahead amid growing supply and sagging demand.

"There's as much as 30 million barrels sitting on floating storage, and it's got to go somewhere at some point. That has to resolve itself," Eric Lee, an energy analyst at Citigroup, told CNBC.

He sees Brent prices falling to $70 to $90 in the long run. That would represent a 10 to 30 percent drop from Wednesday morning's level of $98.94 for October Brent contracts on the ICE Futures Europe exchange.

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"Brent prices have been pretty stable around $110," Lee explained. "But below that calm surface, a lot has been happening — Iranian sanctions, Libya ... Russia, Ukraine, ongoing issues in Nigeria and now Iraq. . . . The question is does it stay at this level without a supply disruption."

Oil demand has slid, particularly in China, and production has particularly grown in the United States. Chinese oil imports fell 2.4 percent in August.

October West Texas Intermediate crude futures hit an almost-eight-month closing low of $92.66 on the Nymex Monday. The contract traded at $92.37 Wednesday morning.

The price could ultimately drop below $80, John Kilduff of Again Capital told CNBC.

He's not the only bear. "Demand fears will take some time to dissipate," Andrey Kryuchenkov, an analyst at VTB Capital in London, told Bloomberg.

"China's slowing imports . . . did not help lift these concerns. The Atlantic basin supply glut is still in place."

Meanwhile, "the price decline comes as Libya is trying to re-enter the market," National Oil Corp. of Libya spokesman Mohamed Elharari noted to Bloomberg.

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