BEIJING, July 10 -- Exxon Mobil Corp. has started work with China's third-biggest state oil company and a Saudi partner on a $3.5 billion project to expand a refinery in southern China.

The project comes amid efforts by energy companies to tap China's market for fuel to drive its booming economy, already one of the world's biggest oil consumers, along with the United States and Japan.

A groundbreaking ceremony was held Friday by Exxon Mobil and its partners, Sinopec of China and Aramco of Saudi Arabia, in the southern city of Quanzhou in Fujian province.

The project will triple the refinery's output to 85 million barrels of petroleum a year and equip it to process imported Saudi crude, the companies said.

The Chinese partner will own 50 percent of the venture and Exxon and the Saudi company will each hold 25 percent.

The refinery gives Exxon Mobil, the world's biggest oil company, and the Saudi company, also known as Aramco, a major new foothold in China's state-dominated oil industry.

Booming economic growth that has topped 9 percent in recent years has strained China's energy supplies and driven official efforts to secure new oil and gas sources abroad.

China's third-biggest oil company, CNOOC Ltd., is in the midst of a takeover battle with Chevron Corp. for California-based Unocal Corp.

CNOOC has offered $18.5 billion for the U.S. company, saying it wants access to its Asian oil and gas reserves to serve Chinese customers.

The refinery project in Fujian will include facilities to produce ethylene, polyethylene, polypropylene and other petrochemicals, the companies said.

Exxon Mobile, based in Irving, Tex., said it was developing projects in China, Qatar, Singapore and Venezuela to ensure supplies for the refinery.