Former top Communist Zhou Yongkang, who rose through China's state oil industry to become the country's internal security chief, amassed so much power that he brought about his own downfall, according to analysts.

Zhou -- the highest ranking former official to be investigated in an anti-corruption campaign for several decades -- was sentenced on Thursday to life in prison on charges of bribery, abuse of power and leaking state secrets.

Arrested and expelled from the Communist Party in December, Zhou's undoing comes on the back of President Xi Jinping's much-publicised anti-corruption drive, but experts say it was driven more by internal politics within the factionalised ruling party.

Zhou was born in the eastern industrial city of Wuxi in 1942, reportedly the son of an eel farmer.

He got his start in the 1970s as a technician for the Liaohe Oil Exploration Bureau in the northeastern province of Liaoning, home to China's third-largest oil field.

By 1996, he had worked his way up to head giant state-owned oil producer China National Petroleum Corporation (CNPC) and went on to become the Communist Party chief in the southwestern province of Sichuan.

There he established a reputation as a hardliner, including against the banned Falun Gong spiritual movement, whose supporters say he "paved a road to his own promotion" by persecuting members.

Zhou is a central figure in what some analysts have termed the "oil faction" within the Communist Party, a network of influential politicians who have ties with China's powerful and lucrative petroleum industry -- and is sometimes described as "China's Dick Cheney".

- Too powerful -

In 2002 he ascended to the upper echelons of Chinese leadership, with a slot in the ruling party's 25-member Politburo and the role of minister of public security.

Five years later he stepped up to the elite Politburo Standing Committee (PSC), China's most powerful body, and head of the party's Central Politics and Law Commission (CPLC), responsible for all of China's internal security, including its police, courts, jails and domestic surveillance.

His tenure was marked by the brutal use of force in response to civic unrest, as he oversaw the quelling of riots in Tibet in 2008 and in the restive far-western region of Xinjiang -- the homeland of China's mainly Muslim Uighur minority -- in 2009.

According to a Chinese finance ministry report, in 2013 the official budget overseen by the CPLC exceeded the national defence budget for the fourth year in a row, with a staggering 769 billion yuan (now $124 billion) spent on domestic security compared with 760 billion yuan (now $124 billion) in military expenditure.

"Maintenance of stability is something very, very vague, and there's a lot of room for corruption," said Joseph Cheng, a professor of political science at the City University of Hong Kong.

"And since there's a lot of room for corruption -- a lot of leeway for spending -- you have a lot of resources to build up your network of ties. That is why he has become so powerful."

That build-up of power and resources -- and a network of proteges and allies eager to establish themselves at the top of the party -- was part of what triggered Zhou's political demise, Cheng added.

He had been "in charge of a growing and expanding public security machinery" and was "seen to be too powerful to be comfortable to the leadership, especially Xi Jinping," he said.

- 'Conspiratorial cabal' -

Zhou retired in 2012 as part of a once-a-decade leadership handover, but senior Chinese politicians normally remain significant players even after officially stepping down.

The charges against Zhou make him the highest-ranking official to be prosecuted since the infamous Gang of Four -- a faction that included the widow of founding leader Mao Zedong.

Xi came to power promising to target all levels from high-ranking "tigers" to low-level "flies" but according to experts Zhou's fate was probably sealed by his alliance with Bo Xilai, the Communist Party star who in 2013 was sentenced to life in prison for graft.

"That is widespread innuendo, and I think it's true -- Bo Xilai had formed a kind of anti-party, conspiratorial cabal, a faction within the power establishment, and Zhou Yongkang was one of the closest comrades of Bo Xilai," said Willy Lam, a politics specialist at the Chinese University of Hong Kong.

"So, that was what did him in."

Four of Zhou's top allies were expelled from the Communist Party in October, among at least 13 officials connected to him who are under investigation, according to announcements in official Chinese media.

They include five current and former top officials in Sichuan; four CNPC officials, including its head and vice president; a vice minister of public security; and three others believed to be right-hand men of Zhou.

Some overseas reports say more than 20 Zhou proteges are currently in detention.

But the first sign the top leadership believed Zhou had become too powerful for their own good came long before rumours of an investigation began swirling.

When Zhou stepped down the PSC was cut back from nine members to seven -- with no slot for the country's security chief any more.