Petroleos Mexicanos, the world’s fourth-largest oil producer, is expanding drilling after discovering a crude deposit in Tabasco state, estimating the area could hold as much as 1 billion barrels of reserves.

The light-crude deposit announced today in the on-shore Navegante I well may have as much as 500 million barrels, said a press official who asked not to be named because of company policy. The deposit has a depth of 6 kilometers (3.7 miles), the official said yesterday in a phone interview.

The well is Mexico’s most important solid-ground discovery in the past decade, President Felipe Calderon said yesterday in a speech in Veracruz state. It’s the company’s third major discovery announced in the past three months, following two deep-water deposits in the Gulf of Mexico.

“Pemex has steered itself toward investments in exploration, and it’s showing results,” Calderon said, according to a transcript distributed by the president’s office.

The state-run company, based in Mexico City, has invested almost $10 billion in exploration since 2009.

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