HONG KONG (MarketWatch) -- PetroChina Co. shares more than doubled in their Shanghai debut Monday, giving the oil giant a $1 trillion market capitalization and easily surpassing Exxon Mobil as the world's largest company.

The shares ended at 43.96 yuan, up from their initial-public-offering price of 16.70 yuan.

The runup gave the oil giant a market capitalization of $1.005 trillion, more than twice that of Exxon Mobil XOM, -2.91% at $480.45 billion.

The rally in PetroChina's yuan-denominated, or Class A, shares, came despite a decline in the broader Shanghai Composite Index, which fell 2.5% to 5,634.45.

"The A-share listing is positive because it will open another avenue for the company to raise funds in the future," said Louis Wong, research director at Phillip Securities in Hong Kong.

"From the investors' point of view it's also positive because they have access to invest in the largest-market-cap oil company in the world."

PetroChina PTR, -1.37% (857) sold 4 billion yuan-denominated shares in Shanghai last week, raising 66.8 billion yuan ($8.94 billion) in China's largest-ever IPO.

"The increase in the A-share price will give PetroChina a very big market-capitalization boost," said Core Pacific-Yamaichi's China equities analyst Richard Lee.

PetroChina listed shares in Hong Kong and New York in 2000 at a time when the nation's stock markets were considered ill-equipped to handle a multibillion-dollar offering.

The 21.1 billion PetroChina shares listed in Hong Kong, known as H shares, represent a market value of $48.96 billion. PetroChina's Hong Kong shares fell 8.2% to HK$18 on Monday.

The Chinese government holds 157.92 billion PetroChina shares indirectly through the state-owned China National Petroleum Corp. PetroChina's 4-billion-share sale last week represents about 2.5% of the oil giant's total shares listed there.

About two-thirds of the shares on Shanghai market are held by state entities and classed as inactive.

Valuations for the non-traded shares are based on the higher Shanghai price, rather than the Hong Kong-listed shares, Lee cautioned.

He added PetroChina ranks among the few oil majors to claim a major discovery in recent years.

PetroChina said in March it had uncovered oil and gas in shallow water areas of Bohai Bay in northern China. It said the find was the most significant in China in more than a decade.

"PetroChina is one few companies that is continuing to find major new oil reserves in the world," Lee said.

PetroChina which had 20.5 billion barrels of oil and gas reserves in 2006, is adding new reserves at an average annual rate of 5%, according to Bloomberg. Exxon Mobil reportedly had 22.1 billion barrels of reserves in 2006.