MEXICO CITY (Reuters) - Mexican state energy company Pemex is in talks with companies owned by billionaire investor Carlos Slim about an investment in the country's most important natural gas field, Ixachi, President Claudia Sheinbaum said on Tuesday.

Reuters revealed last week that officials at Pemex and Slim's companies were considering a so-called mixed contract for Ixachi in Veracruz state and a joint operating agreement for the deepwater Zama field in the Gulf of Mexico.

"We're discussing this with the companies belonging to Slim," Sheinbaum said during her regular press conference when asked about further participation.

Sheinbaum added that Grupo Carso, as one of Slim's companies is known, was already involved in some parts of the operation of Ixachi. It currently drills wells.

Mixed contracts, which sources told Reuters would likely be used for Ixachi, are a new form of partnership between Pemex and private companies.

These allow Pemex to work with private companies, complementing its own expertise and capital, while retaining ownership of resources.

Cash-strapped Pemex is struggling to meet the crude oil production target of 1.8 million barrels per day (bpd) as fields, especially in the Gulf of Mexico, are being depleted and new discoveries fail to compensate for it.

Ixachi has been celebrated as the most important find in more than a quarter of a century for Mexico, which seeks to become self-sufficient in energy.

Nearly two-thirds of the gas consumed locally is imported, mostly from its northern neighbor via pipelines.

RBN Energy, a market advisory service, wrote in a recent note that U.S. imports, mostly from the Permian and the Eagle Ford, have increased threefold over the past decade.

Mexico's own production has declined rapidly, it noted, adding that much of it is considered unusable because of the high nitrogen content.