Brazil’s Petrobras put up for sale a 50% stake in its legacy deepwater Marlim oil cluster, the company said Nov. 16 in a filing.

In production since the 1980s in the Atlantic Ocean’s Campos Basin, the giant Marlim cluster has four fields—Marlim, Voador, Marlim Leste and Marlim Sul—producing 217,000 barrels of oil per day (bbl/d), or almost 10% of the company’s total output.

The sale, in its initial stage, is part of Petrobras’ plans to sell non-core assets to cut debt and focus investments in the world’s biggest deepwater discoveries this century, in the so-called presalt area.

The newer presalt reserves, found beneath a thick layer of salt at the seabed of Brazil’s waters, had a fast ramp-up in the past decade and are responsible for more than 70% of Petrobras’ production.

Once Brazil’s largest oil field with more than 500,000 bbl/d, Marlim has seen declining production in the past decade. Today, Marlim Sul and Marlim are, respectively, Brazil’s sixth and eighth largest oil fields in Brazil. Marlim Sul has the largest number of producing wells in Brazil, 67.

The four fields, which also produce 3.6 million cubic meters of natural gas, are located between 90 and 150 km off the coast and up to 2,500 m below the ocean floor.

Petrobras shares rose more than 4% in Sao Paulo following the announcement.