Mexican state oil giant Pemex said on May 16 it failed to replenish its proven reserves in 2015 with enough new discoveries, after its reserve-replacement ratio hit just 18% in 2014.

"During 2015, we obtained 120 million barrels of oil equivalent of proved reserves from discoveries. However, this volume was not enough to compensate for the reductions in reserves resulting from revisions, delimitations and decreased development and production in 2015," Pemex said. "As a result, there was no replacement of proved reserves in 2015."

A reserve-replacement ratio of 100% would mean that an oil producer is replacing all the oil and gas it pumps with its reserves. Anything below 100% means the oil producer is unable to replenish proven reserves of its extracted hydrocarbons.