Three more senior executives of China National Petroleum Corp (CNPC), the nation's largest oil and gas firm and parent of listed PetroChina, are under investigation by Beijing authorities for alleged violations of discipline.
They include the head of its largest gas field in the north, Changqing.
CNPC vice general manager Li Hualin, vice-president and Changqing general manager Ran Xinquan, and PetroChina chief geologist Wang Daofu are being investigated, Xinhua reported, citing the State Council's State-owned Assets Supervision and Administration Commission.
The report did not elaborate on the nature of the allegations.
Speculation is rife that Zhou Yongkang, a former member of the party's supreme Politburo Standing Committee and a former CNPC general manager, may be a target of President Xi Jinping's anti-corruption drive, with a few of his former aides under investigation.
In a statement, PetroChina said Li, who was its board secretary and a vice-president besides his role in CNPC, Ran, an executive director and vice-president, and Wang, were under investigation by "relevant" authorities.
It said the three men resigned from PetroChina with immediate effect, "due to personal reasons".
A PetroChina spokesman said the investigations were into their personal conduct and not related to their work for the company.
"There has been no disagreement with the company and the board, and there is no other matter that should be brought to the attention of the shareholders of the company", PetroChina said, adding its operations were not affected. Citing the Central Commission for Discipline Inspection, the party's top graft-buster, Xinhua reported on Monday that CNPC deputy general manager Wang Yongchun was being investigated for "gross violation of party discipline".
He is also head of PetroChina's Daqing Oilfield, which runs China's largest oil field.
A PetroChina spokesman said Wang Yongchun's position as head of the Daqing Oilfield would be taken up by CNPC vice general manager Liu Hongbin.
Ran's position as Changqing's general manager would be assumed by Zhao Zhenzhang, a vice-president of PetroChina who is also the general manager of its main exploration and production subsidiary.
Wang Daofu's chief geologist post would be assumed by Zhao Wenzhi, party secretary of the exploration and production unit.
Li's position as chairman of PetroChina's listed gas distribution subsidiary Kunlun Energy, would be taken by Wen Qingshan, CNPC's chief accountant, after a board meeting passes a resolution at a later date, the spokesman said.
Analysts said that while the scandal would damage investment sentiment it was unlikely to affect PetroChina's development given that it was a large state firm whose direction was not affected by individual managers. However, investors would be more concerned about Kunlun's prospects without Li at the helm because he held senior positions in PetroChina and Kunlun, and Kunlun had benefited from asset injections from PetroChina at cheap valuations.