A new report says that social gaming giant Zynga will file documents with the SEC as early as tomorrow for an initial public stock offering that will seek to raise $1.5 to $2 billion for the company.

CNBC cites "people familiar with the matter" as saying the IPO will be led by investment giant Morgan Stanley, with Goldman Sachs also helping fund the move. Zynga declined to comment to Gamasutra about the report.

If the reports are accurate, the offering could value the FarmVille maker at $15 to $20 billion, depending on how much stock is issued. Previous valuations of the company, based on private investment, have been as high as $9 billion.

For comparison, a $20 billion valuation would put Zynga just above the current market cap for cereal maker Kellogg, and make the company the 175th largest listed on Wall Street.

Speculation about a Zynga IPO has persisted since at least last August, and this isn't the first time the move has been rumored to be imminent -- The Wall Street Journal's All Things D blog reported the offering was less than a week away over a month ago.

The company, which enjoys hundreds of millions of players for its social games, has attracted multiple rounds of private funding, including a reported $500 million earlier this year.

Zynga has used that money to expand rapidly through acquisitions, buying up game companies including New York's Area/Code, Texas' Newtoy Inc., Tokyo's Unoh Games and Beijing's XPD.

Fellow casual game makers Popcap and Rovio have also reportedly been aiming for public stock offerings in the near future.