Don't expect a thank-you note, but Citigroup is ever so grateful for that $50 tank of gas you just burned through on your way home from Thanksgiving with the folks.
 
The reason? Some of that oil money heading overseas is coming back to U.S. shores in the form of major equity investments in struggling corporations that desperately need it. But not everyone is happy about it.

The Abu Dhabi Investment Authority, which is the investment arm of the government of the oil-rich emirate, has reached a deal to invest $7.5 billion in Citigroup. It will become the besieged bank's largest shareholder with a 4.9 percent stake.

The Citi deal is the latest of a string of investments by international sovereign wealth funds, which have seen their coffers expand as the price of oil has taken its long, steady climb. Earlier this month, the chipmaker AMD got an infusion from another investment fund controlled by Abu Dhabi, and Dubai International Capital took a stake in Sony.

Many sovereign funds have been around for decades—the A.D.I.A. was established in 1976—but they've only recently reached the massive size required to make such sizable direct investments like the one in Citigroup. The Abu Dhabi fund has an estimated $875 billion in assets.
 
The International Monetary Fund estimates that all sovereign funds combined held $500 billion in 1990. Today, they total between $2 trillion and $3 trillion, and are expected to grow to $10 trillion by 2012. And it's not all oil money—thanks to China's rapid economic development and its growing currency reserves, the Chinese government has amassed a $200 billion war chest to spend. It has recently announced stakes in the Blackstone Group and Bear Stearns.

Not surprising, these deals come with their fair share of controversy. After all, no one wants the United Arab Emirates to take over Lockheed Martin. When a Dubai fund tried to take over the management contracts for six major American ports last year, the effort was blocked by Congress due to security concerns.

Since then, legislators have greeted international deals with skepticism, but little opposition. Senator Charles Schumer (D-NY), who spearheaded the Dubai ports resistance, gives a thumbs-up to the Citigroup deal. He told CNBC that the security threat level isn't the same, because he expects Citigroup will refrain from sharing sensitive information with any stakeholder.

Indeed, as with many of these kinds of investments, the terms of the Citigroup deal give no indication that Abu Dhabi wants anything more than a return from its long-term investment. The fund agreed to cap its stake at 4.9 percent, is restricted from selling its investment for two years, and will receive no board seat or management role.

But plenty of policymakers want to establish some kind of standards for these investment vehicles before they've garnered so much influence that these seemingly tame terms turn into something bigger.

Earlier this year, German chancellor Angela Merkel called for a European screening of sovereign-fund investments, which France also supports. The I.M.F. is examining whether or not it should establish an international set of standards. And at a Congressional hearing earlier this month, several major Democratic lawmakers called for increased U.S. regulation over foreign direct investments.

Interestingly, with all of the controversy surrounding their investments, the sovereign funds have failed to demonstrate much investment savvy to other Wall Street traders. Shares of Blackstone have plummeted since China took a stake and AMD shares have lost 21 percent since Abu Dhabi made its move just two weeks ago.

And Citigroup? Its shares were unchanged today, even as the Dow Jones soared nearly 200 points.