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ExxonMobil has revised its oil and natural gas reserves down by over 3 billion barrels.

The company said on Thursday that its proved reserves stood at 20 billion oil-equivalent barrels at the end of 2016.

That reserve level marked a net reduction of 3.3 billion oil-equivalent barrels from 2015 levels.

The company said that the changes reflected new developments as well as revisions and extensions to existing fields resulting from drilling, studies, analysis of reservoir performance and application of the methodology prescribed by the U.S. Securities and Exchange Commission (SEC).

Exxon said that “very low prices” during 2016 caused certain quantities of liquids and natural gas to no longer qualified as proved reserves under SEC guidelines.

The reserve decline included the entire 3.5 billion barrels of bitumen at Kearl in Alberta, Canada.

Exxon said that an additional 800 million oil-equivalent barrels in North America did not qualify as proved reserves, “mainly due to the acceleration of the projected economic end-of-field life.”

The reductions were partially offset by oil and natural gas reserves totaling about 1 billion barrels of oil equivalent in the United States, Kazakhstan, Papua New Guinea, Indonesia and Norway.

Exxon said the gains replaced 65 percent of production and were the result of acquisitions, improved asset performance and a decision to fund an expansion of the Tengiz project in Kazakhstan.

Exxon added that the downward revisions are not expected to affect the operation of the underlying projects or to alter the company’s outlook for future production volumes.

Exxon said that factors that would result in the reserves being recognized as proved reserves at some point in the future are a recovery in average price levels, a further decline in costs as well as operating efficiencies.

Exxon added nearly 2.5 billion oil-equivalent barrels to its resource base in 2016 through by-the-bit exploration discoveries, undeveloped resource additions and strategic acquisitions.

The resource base includes proved reserves, plus other discovered resources that are expected to be ultimately recovered.

The company said that its  exploration success in 2016 included “significant” oil discoveries in Nigeria and Guyana.

Strategic unconventional resource additions were made in the Appalachian Basin in Pennsylvania, the Permian Basin in West Texas and Neuquén Province in Argentina.

Overall, the corporation’s resource base totaled more than 91 billion oil-equivalent barrels at year-end 2016 when field revisions, production and asset sales are taken into account.