Exxon Mobil on Friday said it could write down billions of barrels of reserves, marking a change of course as the company faces federal investigations into how it values those reserves.

During the third quarter, The Wall Street Journal reported that the Securities and Exchange Commission and New York Attorney General Eric Schneiderman's office launched probes to determine if Exxon should have written down the value of its reserves based on drop in global oil prices that began about two years ago. Unlike peers such as BP and Chevron, Exxon has resisted reducing its assessment of how much its reserves may be worth.

In its third-quarter results, reported Friday, Exxon said it could write down as much as 4.6 billion barrels of oil equivalent if the average price of oil for the first nine months of 2016 holds through year end. Low oil prices decrease the value of a company's crude reserves that remain locked underground, but Exxon has argued that it is conservative in the way it books reserves to begin with, so write-downs have not been necessary.

Writing down assets essentially moves them off a company's balance sheet, reducing the potential value they could reap in the future, thereby potentially making the company itself less attractive to investors.

Exxon now says that 3.6 billion barrels of bitumen in its Kearl oil sands project in Canada, and about 1 billion barrels of oil equivalent in North American operations, may not qualify as proved reserves under SEC rules.

Prior to starting the Kearl oil sands project in 2013, Exxon said it "will produce one of the largest and highest quality oil sands deposits in Canada." But Canada's oil sands have been some of the hardest hit energy assets over the last two years, due to the high cost of extracting oil in that part of the world.

Jeff Woodbury, Exxon's vice president of investor relations, noted in a conference call on Friday that the company had conducted a comprehensive evaluation of its reserves in 2015 in light of the oil price downturn. He said cash flow at the time fully covered the costs of carrying those assets on Exxon's books.

However, Exxon "thought it was appropriate to signal the potential impact" of low prices on the assets on an SEC pricing basis. He did not elaborate on the specific mechanisms Exxon uses to evaluate the value of reserves.

"Rest assured we're in full compliance with the rules and standards of the SEC and the Financial Accounting Standard Bureau," he said.

The SEC is also seeking to determine whether Exxon adequately accounts for how climate change could affect its business, after an earlier investigation that Schneiderman launched into that matter.

