The probe follows the investigation of four senior petroleum managers last week.

Wang Yongchun, deputy general manager of CNPC, was investigated for "grave discipline violations", the Central Commission for Discipline Inspection said on Aug 26.

The company announced on Thursday that it has decided to remove its four senior executives, including Wang, from their posts.

The other three executives are Li Hualin, deputy general manager of the CNPC, Ran Xinquan, vice-president of PetroChina and Wang Daofu, chief geologist of PetroChina.

The company vowed to support the crackdown on corruption and ensure normal production order, according to a notice issued on Thursday.

Caixin.com reported on Sunday that the investigation of the four petroleum officials was triggered by the audit on Jiang's departure from the company, citing a source close to some senior officials.

According to current rules, the chairman of State-owned enterprises must be audited when obtaining and leaving a post.

Jiang Ming'an, a law professor at Peking University, said State-owned monopoly giants like CNPC must receive enhanced supervision since such enterprises have tens of billions of yuan in assets, providing numerous chances for corruption.

"Government departments at different levels have already been required to disclose their information to the public, but there is no compulsory rule requiring State-owned companies to do so," he said.

Monopoly giants in the petroleum, telecommunications and banking industries have accumulated wealth through their monopoly advantages, he added.

Apart from Jiang Jiemin, many other ministerial-level officials have been investigated on allegations of corruption since the CPC held its 18th national congress last November.

Liu Tienan, former vice-chairman of the National Development and Reform Commission, was expelled from the Party and handed over to judicial authorities for further investigation, it was announced on Aug 8.

Bao Chang contributed to this story.