A Chinese state-owned oil company is in talks with Nigeria to buy large stakes in some of the world's richest oil blocks in a deal that would eclipse Beijing's previous efforts to secure crude overseas.

The bids could pitch the Chinese into competition with western oil groups, including Shell, Chevron, Total and ExxonMobil, which partially or wholly control and operate the 23 blocks under discussion. Sixteen licences are up for renewal.

CNOOC, one of China's three energy majors, is seeking to acquire 6bn barrels of oil, equivalent to one in every six barrels of the proven reserves in Nigeria, sub-Saharan Africa's biggest crude producer and a major supplier to the US.

Details of the talks are revealed in a letter from the office of Nigeria's president, Umaru Yar'Adua, to Sunrise, CNOOC's representative, a copy of which was obtained by the Financial Times. The overall value of the Chinese offer is not disclosed, although some details suggest a figure of about $30bn. Some oil sector executives claim the total on the table is $50bn.

A spokesman for Mr Yar'Adua said: “Negotiations are ongoing not only with Sunrise/CNOOC but also with all other stakeholders in the industry. The federal government has not taken any final position on the issue.”

The letter, dated August 13, said an initial offer was “unacceptable” but added: “Your interest in all the listed blocks will be considered if your revised offer is favourable.”

Details of how the Nigerian government would allocate equity in the blocks to CNOOC have yet to emerge and it is unclear whether this would involve forcing western groups to relinquish stakes.

“There are serious legal implications. You don't want to go to court but if it gets to this then you have little choice,” an oil industry insider said.

China's push to gain a significant foothold in Nigeria underlines the scale of its long-term ambitions to secure access to energy resources across the globe. Much of its investment has been for exploration, in contrast with the Nigerian blocks which are already producing or slated to start pumping soon.

Tanimu Yakubu, the Nigerian president's economic adviser, said China might not secure “anything close” to 6bn barrels from the negotiations, adding: “We want to retain our traditional friends.”

However, Mr Yakubu told the Financial Times the Chinese “are really offering multiples of what existing producers are pledging [for licences]... we love to see this kind of competition”.

Basil Omiyi, Shell's country chair in Nigeria, said: “The blocks referred to are under active exploration, development and production, mostly by the majority government-owned joint venture operated by Shell.” CNOOC declined to comment.