ExxonMobil Corp. (NYSE: XOM) has revised down its proved crude reserves by 3.3 billion barrels of oil equivalent (Bboe) as a result of low oil prices throughout 2016, a company filing showed Feb. 22.

The de-booking includes the entire 3.5 billion barrels (Bbbl) of bitumen reserves at the Kearl oil sands project in northern Alberta, operated by Imperial Oil Ltd. (NYSE MKT: IMO), a Calgary, Alberta-based company in which the U.S. oil major has a majority share.

It comes a day after ConocoPhillips Co. (NYSE: COP) de-booked more than1 Bbbl of its oil sands bitumen reserves, citing weak global crude prices.

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In total, ExxonMobil has 20 Bboe at year-end 2016, the Securities Exchange Commission (SEC) filing said. The reduction reflects the number of boe that were now deemed uneconomic due to lower crude prices.

In addition to the Kearl volumes, another 800 MMboe in North America failed to qualify as proved reserves.

However, the reductions were partly offset by ExxonMobil adding 1 Bbbl of new oil and gas reserves in the U.S., Kazakhstan, Papua New Guinea, Indonesia and Norway.

Under SEC rules, ExxonMobil and other U.S.-listed companies report reserves based on the average crude price on the first day of each calendar month during the year.

Benchmark crude prices in 2017 have so far been higher than in 2016, meaning some of the volumes could be rebooked as proved reserves if these levels hold.