The company said it was "fully complying" with a request from the Securities and Exchange Commission (SEC) for accounting information.
Exxon is the only big US energy firm not to have taken a charge to reflect the slump in oil prices.
The SEC is also examining how Exxon accounts for the potential impact on its business of climate change policy.
It follows a similar investigation announced last year by a group of state attorneys general.
Led by New York state's top prosecutor, Eric Schneiderman, they are examining whether Exxon misled investors and the public on the impact of climate change on its business.
Responding to the news of the SEC probe, Exxon Mobil said in a statement: "The SEC is the appropriate entity to examine issues related to impairment, reserves and other communications important to investors.
"We are fully complying with the SEC request for information and are confident our financial reporting meets all legal and accounting requirements."
The investigation was first reported in the Wall Street Journal on Tuesday and the report prompted a 1.5% decline in Exxon Mobil shares.
The price of US crude oil has fallen from $100 a barrel in mid-2014 to around $44 a barrel. That's prompted many oil companies to reduce the value of their oil projects.
According to the WSJ report, Exxon feels that is unnecessary because it takes a conservative view when valuing new fields and wells.
US authorities are investigating the way Exxon Mobil has valued its assets following a plunge in the price of oil.