Already traded in Hong Kong and New York, PetroChina shares were snapped up by hungry investors flush with cash from savings accounts.

With PetroChina's share price on the Shanghai bourse rising as high as 48.62 yuan, or $US6.52, it trebled in value on its opening day and surpassed the US energy supermajor Exxon Mobil as the world's most valuable company.

At its intra-day high in Shanghai trading, the company was valued at almost $US1.2 trillion, more than twice Exxon Mobil's market capitalisation on Friday of $US487 billion.

But there must be a question of whether it is overvalued, because at that level PetroChina was worth more than Australia's gross national product.

China is now has five of the world's 10 largest companies by market cap.

But the prices attained on Chinese exchanges have little relationship to company performance, or to markets in other parts of the world.

While PetroChina's market cap is now much greater than ExxonMobil’s, it is only about half as profitable as its rival. In the first half of 2007, PetroChina's net income was $US10.9 billion, compared to $US19.5 billion for Exxon Mobil, according to the New York Times.

The Times of London warned that PetroChina’s spectacluar performance on the Shanghai exchange was as much a warning of a developing Chinese market bubble as a reflection of investor demand.