Many oil investors are also worried about a potential political reaction in the United States similar to the furor of last year when Dubai tried to acquire a company that operates American ports. European leaders, at the same time, worry that Russia is using its oil revenues to snatch up pipelines and other energy infrastructure in their region.

Such concerns seem to be driving investments to other parts of the world, many analysts say.

“The investments are diversifying outside the United States, though the U.S. still has the bulk of it,” said Diana Farrell, director of the McKinsey Global Institute, a research arm of the McKinsey consulting firm, which calculated in October that petrodollar investments reached $3.4 trillion to $3.8 trillion at the end of 2006.

“Europe is a prime target,” she added, “but at least 25 percent of foreign investments from the Persian Gulf are in Asia, the Middle East and North Africa.”

Though oil-producing countries have been looking at investments in the West since the 1970s, their strategies back then were largely confined to safe assets with a low return, like United States Treasury debt.

By 2001, with the collapse in oil prices, many of the oil exporters had depleted their dollar reserves, economists say.

But the boom in oil prices in the last five years has changed all that. It has persuaded oil producers to set up or expand “sovereign wealth funds” as vehicles to invest far more aggressively in the West, in their own economies and in emerging markets.

Other petrodollar investments are made through government-owned corporations, corporations and individuals like Prince Walid, who owns stakes not only in Citigroup but also News Corporation, Procter & Gamble, Hewlett-Packard, PepsiCo, Time Warner and Walt Disney.