The U.S. will rein in methane from its biggest human source—the oil and gas industry—but the new rules won't touch a wide swath of places already leaking the invisible, heat-trapping gas.

Strengthened Environmental Protection Agency standards aimed at new energy facilities, proposed Tuesday, will cut somewhere between 20 to 30 percent of the sector's methane emissions from 2012 levels over the next decade, estimates Janet McCabe, acting assistant administrator for the EPA's Office of Air and Radiation.