Exxon Mobil Corp. warned that it may be forced to eliminate almost 20% of its future oil and gas prospects, yielding to the sharp decline in global energy prices.

Under investigation by the U.S. Securities and Exchange Commission and New York state over its accounting practices—and the impact of future climate change regulations on its business—Exxon on Friday disclosed that some 4.6 billion barrels of oil in its reserves, primarily in Canada, may be too expensive to tap.

Exxon...