An oil field off the Nigerian coast with substantial reserves – and potential risks – has become the latest foreign prize scooped up as part of China Petrochemical Corp.'s African expansion campaign.

The state-owned oil gorilla better known as Sinopec announced in mid-November that its wholly owned subsidiary Sinopec International Petroleum Exploration and Production Corp. had agreed to buy a 20 percent stake in an offshore block from French oil major Total SA.

The transaction valued at US$ 2.46 billion gives Sinopec access to about 100 million barrels of proven reserves in the OML138 block in the waters of the Niger River Delta basin, the company said.

So far, Total is the operator of undersea oil at the site, and it's unclear whether Sinopec will fully take over the operation or sign a contract to have the French company continue pumping.

Three other multinational oil giants – Chevron Corp., ExxonMobil and Nexen Inc. – control the remaining 80 percent of oil rights in the block covering 906 square kilometers. Another Chinese state-owned oil company, China National Offshore Oil Corp., recently won Canadian government approval to buy Nexen.

Since Total launched OML138 pumping operations in February, production has averaged 24,000 barrels a day. It's expected to rise to 26,000 barrels.

The deal came three years after Sinopec's global expansion campaign reached West Africa via a US$ 7.2 billion deal for the Swiss firm Addax Petroleum's oil reserves in Nigeria, Gabon and Cameroon, as well as Iraq. The Nigerian buyout included one onshore and six offshore blocks.

Moreover, Sinopec has been pumping oil from fields in Angola since 2007.

Details of the Sinopec-Total agreement have yet to be finalized, a source said. An issue in the talks, which began in August and primarily centered on price, is that Sinopec has yet to prove on the international stage its ability to operate ocean drilling rigs and successfully replace Total.

Total's plan to scale down in Nigeria matched Sinopec's efforts to scale up. Indeed, the Chinese company has been working hard in recent years to expand overseas and catch up with its more globally established, state-run rival China National Petroleum Corp. (CNPC), said Wang Zhen, director of the China Energy Strategic Research Institute at the China University of Petroleum.

According to Wu Mouyuan, an analyst at the CNPC Economics Technology Research Institute, Sinopec spent more than US$ 10 billion for overseas acquisitions between 2010 and 2011. The size of the buying spree is "quite unusual in the industry," he said.

Sinopec signed five of the 11 overseas acquisitions announced last year by Chinese oil companies. In January, the company said it would pay U.S.-based Devon Energy Corp. US$ 2.2 billion for stakes in five shale gas blocks in the United States. And in July, it bought a 49 percent stake in Canadian Talisman Energy Inc.'s British subsidiary for US$ 1.5 billion.

Total's decision to sell the OML138 stake is part of a wider plan to spin off up to US$ 20 billion worth of holdings by 2014.