CNOOC and Sinopec have agreed to buy a stake in an Angolan oil block from U.S. oil major Marathon Oil, as the Chinese state-owned oil companies continue to buy up overseas energy assets.

The companies said in statements on Friday that the Chinese companies would form a 50-50 venture and pay $1.3 billion for a 20 percent stake in the highly prospective block, which has already yielded 12 discoveries.

The deal price is a comedown for Marathon, which tried to sell the stake for $2 billion in 2008, sources involved in the process said at the time.

Bidders then included the CNOOC-Sinopec consortium, India's ONGC and Brazil's Petrobras, the sources said.

In recent years, Chinese oil companies have been pacing the globe in a government-backed campaign to secure energy assets to fuel the fast-growing Asian powerhouse's economy.

Angola, which in recent years has become a major international oil producer, and an OPEC member, has been a particular focus.

Marathon will retain a 10 percent working interest in the block, the company said.

The companies said they expected to close the transaction by the end of 2009.

Shares in Marathon closed at $30.14 Thursday on the New York Stock Exchange.

The block is operated by French oil major Total, which owns a 30 percent stake, while Texas-based Exxon Mobil Corp holds 15 percent, Portugal's Galp owns 5 percent and Angola's state-owned oil company Sonangol owns 20 percent.