Crude oil's march toward $100 a barrel seems unstoppable, as futures prices broke through $96 a barrel today.

Oil prices have been climbing at a rapid clip since August, boosted by tensions between the United States and Iran, border clashes between Iraq and Turkey, a weak dollar, and concerns over winter heating supplies.

Worries over supplies were stoked on Wednesday after the Energy Department reported that U.S. inventories declined by 3.9 million barrels of oil last week.

With the recent rise in prices, "we are stepping into an unknown area," Ken Hasegawa, a broker at Fimat Japan, told the BBC. "Nobody wants to sell, given the fear of a further rise."

Higher oil prices must be good news for ExxonMobil, the world's biggest publicly held oil company, right?

The surge in oil turns out to be a double-edged sword. The margins in the company's business in refining gasoline and chemicals have been severely pressured by the higher costs for crude oil.

Exxon reported today that it earned $9.4 billion, or $1.70 per share, in the third quarter, compared with $10.5 billion, or $1.77 per share, in the quarter a year earlier. Revenue rose 2.8 percent, to $102.3 billion.

During the quarter, as production for the summer driving season eased, retail gasoline prices in the United States fell even as crude futures rose. Profit at the company's refining and fuel-marketing business fell 27 percent from the quarter a year ago.