It has a crescent shape and it goes through Guinea , Mali , Niger , on the border with Benin and then through Nigeria . Finally , it reaches the sea at a large delta , called the Niger Delta of the Oil Rivers . It runs in a crescent through Mali , Niger , on the border with Benin and then through Nigeria , discharging through a massive delta , known as the Niger Delta or the Oil Rivers , into the Gulf of Guinea in the Atlantic Ocean .