Carlyle Group has bought Royal Dutch Shell Plc's (NYSE: RDS.A) onshore assets in Gabon for $587 million as the world's largest private-equity fund expands in the global oil and gas sector.

For Shell, the deal marks a further step in a $30 billion asset disposal program to help cut debt after its $54 billion acquisition of BG Group last year. The Anglo-Dutch oil company has sold assets for more than $15 billion since 2016.

Shell's Gabon assets will be incorporated into Carlyle-backed Assala Energy, which is led by former Tullow Oil executive David Roux and will focus on energy opportunities in sub-Saharan Africa, Carlyle said in a statement March 24.

The assets operated by Shell produce about 60,000 barrels of oil equivalent per day (Mboe/d), of which 40 Mboe/d go to the company. Under the deal, which is expected to close in the summer, Assala Energy will assume a debt of $285 million.

For Shell, the transaction will result in an impairment charge of $53 million after tax which will be taken in the first quarter of 2017, it said in a separate statement. About 430 local Shell employees will become part of Assala Energy.

The capital for the investment will come from Carlyle International Energy Partners (CIEP), a $2.5 billion fund that invests in global oil and gas E&P, and the $698 million Carlyle Sub-Saharan Africa Fund (SSA).