(New throughout, adds more details on problem, possiblesolutions)

By Robert Campbell

MEXICO CITY (Reuters) - Mexico's state oil companyPemex may face a $115-billion pension shortfall by 2019 asbenefits owed to tens of thousands of retiring workers dwarffunds set aside, a Pemex document obtained by Reuters showed.

Nearly 45,000 workers will be eligible to retire over thenext decade, joining more than 80,000 people already receivingbenefits funded by Pemex's cash flow, according tothe document.

By 2019 that deficit, the shortfall between what Pemex owesretirees and how much it has set aside to do so, is expected toclimb to 1.507 trillion pesos. That is roughly $115 billion attoday's exchange rate, 17 percent more than the value ofPemex's overall assets last year.

Pemex was not immediately able to comment on the document.

The ballooning deficit underscores the shaky finances ofthe world's No. 7 oil producer and raises doubts about how longPemex can keep borrowing to sustain crude output.

Pemex's deteriorating finances pose a political problem forthe Mexican government, which relies on oil exports for over athird of revenues as it struggles to pull the economy out of adeep recession and battles a brutal war with drug cartels.

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For a graphic on Pemex's pension shortfall, please click:

http://link.reuters.com/rym98n

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Pemex's debt and liabilities already exceed the value ofits assets, and the pension shortfall, worth $44.1 billion in2009, is now the largest liability on its balance sheet.

Pemex has warned that some rating agencies are concerned byits finances, but the state company is still able to routinelysell debt needed to fund capital spending.

TOUGH DECISIONS

Mexico's oil production has leveled off after a longdecline, but output remains far below its 2004 peak. Pemex mustspend billions of dollars to maintain output at aging fieldsand find new deposits to compensate for natural depletion.

Chief Executive Juan Jose Suarez said this week that Pemexaims to lift capital spending to an annual $26.8 billion fromaround $20 billion in 2010. But those plans will depend oncontinued access to capital markets.

Pemex, one of the biggest employers in Mexico, for decadesfailed to set aside pension reserves and has relied mainly onincoming oil revenues to pay current retirees. Only since 1997has it begun to build up a pension fund, but it remains tinycompared to the company's obligations.

Over the last two years, Pemex has actually been takingmore out of that fund than it has paid in, nearly halving itsvalue to 4.1 billion pesos ($313 million) last year.

Last year, then-Chief Executive Jesus Reyes Herolesdescribed the pension conundrum as his "biggest headache".

A dire scenario is not inevitable, and Pemex managers areworking on plans to cut the company's costs and improvefinances, according to public summaries of board meetings.

But turning around such a large shortfall will requiredifficult decisions.

Retiree benefits could be slashed, but that would likelyprovoke a battle with the influential oil union.

Cutting Pemex's taxes would free up funds for its pensionfund, but that would require the politically unpalatable moveto raise taxes elsewhere ahead of presidential polls in 2012. (Editing by Missy Ryan and David Gregorio)