By V. Phani Kumar

Shares of Chinese energy majors Cnooc Ltd. and China Petroleum and Chemical Corp. (Sinopec) gained in Hong Kong trading Monday, helped by news that their joint venture will acquire a 20% stake in an Angolan oil block from a subsidiary of U.S.-based Marathon Oil Corp.

The two Chinese state-owned firms will together pay Marathon (MRO) $1.3 billion to acquire the stake in the so-called Angola Block 32, which has 12 previously announced discoveries.

The oil field "is a significant resource base with estimated recoverable light crude oil reserves of 1.5 billion barrels," Goldman Sachs analysts wrote in a report. "The $1.3 billion consideration compares with our valuation of $1.4 billion to $1.65 billion and Marathon's publicly disclosed offer of $1.8 billion to $2 billion."

They said the acquisition would build on Cnooc's "growing deepwater exposure" and values the recoverable reserves at $4.30 a barrel. Cnooc's share of the proven reserves from the field, at 150 million barrels, compares with its 2008 proven reserves of 2.5 billion barrels.

The project is however expected to "only meaningfully contribute in 2015" to Cnooc's earnings, they said.

Shares of Cnooc (CEO) climbed 1.8% in Hong Kong mid-morning trade. Shares of Sinopec (SNP) gained 1.7% in Hong Kong and 1.4% in Shanghai.

In wider market action, the Hang Seng Index jumped 2.3% to 19,229.25, while the Shanghai Composite rose 1% to 3,219.92. South Korea's Kospi added 2.2%, Australia's S&P/ASX 200 advanced 1.3%, Singapore's Straits Times Index rose 0.5% and Taiwan's Taiex climbed 1.1%.

The stake sale is subject to government and regulatory approvals, with Marathon's existing partners in the block also having a right of first refusal.

Total Exploration and Production Angola is the operator of Block 32 with a 30% interest; Angola's Sonangol P&P holds a 20% stake; Esso Exportation and Production Angola has 15%; and Petrogal has a 5% interest.

Marathon's (MRO) own stake will reduce to 10% following the stake sale. The deal is expected to close by the end of this year.