Tullow Oil made a loss after tax of $309m for the first half of the year, after what the company described as "continued challenging market conditions".
Revenue for the period of $788m was up by 46% on the corresponding period in 2016, while gross profit was 66% higher.
The post-tax loss was largely down to an impairment charge for property, plant and equipment of $642m.
Net debt reduced by around $1 billion since the year-end to $3.8 billion at the half year following the generation of free cash flow and a $750m rights issue in April.
The company’s facility headroom and free cash now stands at $1.2 billion.
The independent oil and gas exploration group said its oil production in West Africa is in line with guidance averaging 81,400bpd.
Tullow CEO Paul Mcdade said: "Despite continued challenging market conditions, Tullow performed well in the first half of 2017 delivering strong revenues and organic free cash flow.
"Combined with the Rights Issue completed in April, this has allowed us to retain operational and financial flexibility and reduce our debt during the first half by around $1 billion.
"Since taking over as CEO, I have appointed a new and highly experienced Executive team who are focused on returning Tullow to growth through financial discipline, efficient use of capital and by delivering on the potential of our diverse portfolio of low-cost production, development and exploration assets."
