Mexico, one of the largest suppliers of oil to the United States, has a big problem: Its production of crude is falling fast. In 2008, the country’s production peaked at 3.2 million barrels a day, according to the U.S. Energy Information Administration. Last year, it didn’t even produce 3 million a day. The reason: aging oil fields and years of underinvestment. Industry experts say Mexico could revive production if it allowed more investment from international oil companies. But under current policy, EIA says Mexico will have to start importing oil by 2020. For the United States, the decline in Mexico’s oil industry means it will likely be buying more oil from Canada and Saudi Arabia, the No. 1 and No. 2 sources of U.S. oil exports. Mexico is now third.