Daily output at Mexico's biggest oil field tumbled by half a million barrels last year, according to figures released Friday by the Mexican government. The ongoing decline at the Cantarell field could pressure prices on the global oil market, complicate U.S. efforts to diversify its oil imports away from the Middle East, and threaten Mexico's financial stability.

The virtual collapse at Cantarell -- the world's second-biggest oil field in terms of output at the start of last year -- is unfolding much faster than projections from Mexico's state-run oil giant Petroleos Mexicanos, or Pemex. Cantarell's daily output fell to 1.5 million barrels in December compared to 1.99 million barrels in January, according to figures from the Mexican Energy Ministry.

Mexico made up for some of the field's decline. Mexico's overall oil output fell to just below three million barrels a day in December, down from almost 3.4 million barrels at the start of the year. It marked Mexico's lowest rate of oil output since 2000.

Mexico's troubles at Cantarell mirror the larger problems in the global oil market. Many of the world's biggest fields are old and face decline, which can be sharp and sudden. Like other big producers, Mexico is struggling to make up the difference because new big fields are in harder-to-reach places like the deep waters of the Gulf of Mexico.

..."This is bad news for Mexico. The field is declining faster than even the government's pessimistic scenarios," says David Shields, an oil industry consultant in Mexico City who has been warning about Cantarell's collapse for the past two years.

...Mexico's growing economy is demanding more fuel each year, which is expected to translate to even lower oil exports.