Exxon Mobil Corp. warned that low energy prices may wipe as much as one-fifth of its oil and natural gas reserves off the books. If depressed prices persist for the rest of the year, “certain quantities of crude oil, bitumen and natural gas will not qualify as proved reserves at year-end 2020,” the company said in a regulatory filing on Wednesday. A 20% hit would impact the equivalent of almost 4.5 billion barrels of crude, or enough to supply every refinery on the U.S. Gulf Coast for 18 months.

Exxon isn’t waiting until the traditional end-of-year period to reasses reserves. After slashing its drilling budget by $10 billion to cope with the virus-driven market collapse, the company on Wednesday said it removed about 1 billion barrels from its books. Most of that involved shale fields, according to the filing.