Colombia's state-controlled oil company Ecopetrol (ECOPETROL.BO) agreed to buy a 50% stake in an offshore Brazilian oil block from the U.S. company Anadarko Petroleum Corp. (APC).

The two companies will explore the offshore block C-M-202, located in waters between 30 to 100 meters deep in Brazil's key Campos basin, Ecopetrol said Tuesday in a statement.

The concession is already in its exploratory phase, Ecopetrol added.

Anadarko had acquired 100% of the block at the sixth round auction of exploration and production licenses in 2004.

The Campos Basin is responsible for more than 85% of Brazil's crude oil output.

With this agreement Ecopetrol keeps expanding in Brazil where it already has exploration and production contracts with Brazilian state-controlled Petroleo Brasileiro (PRB), mining company Companhia Vale do Rio Doce (RIO) and Portuguese oil company Petrogal, Ecopetrol said.

Ecopetrol didn't say how much it paid for the stake. Officials at Ecopetrol were not immediately available for comment.

Shares of Ecopetrol were up 0.2% to 2,125 Colombian pesos ($0.89) as of 12:57 p.m. EDT, while the IGBC stock index was up 0.3%

-By Diana Delgado, Dow Jones Newswires; 57-1-610 70 44 Ext. 1132; diana.delgado@dowjones.com

(Jeff Fick in Brazil contributed to this article)