BHP Billiton writes down $7.2bn of shale assets

The impairment charge adds to BHP's recent woes following a fatal dam collapse in Brazil and tumbling commodity prices.

Plummeting iron ore, coal, copper and other commodity prices led to a slump in earnings last year.

BHP's latest move means it has written down nearly two-thirds of its investment in US shale.

Oil prices have slumped by 70% since June 2014 to just above $30 a barrel, putting oil companies of all sizes and a number of oil exporting countries under significant financial pressure.

"Oil and gas markets have been significantly weaker than the industry expected," BHP chief executive Andrew Mackenzie said in a statement.

"Although we expect prices to improve from their current lows, we have reduced our oil price assumptions for the short to medium term. Our long-term price assumptions continue to reflect the market's attractive supply and demand fundamentals."

The company has cut operating costs and capital spending at its US onshore operations since the collapse in oil prices, reducing the number of its shale oil rigs from 26 a year ago to five now.

Oil companies large and small have been writing down the value of shale assets over the past 20 months since oil prices started crashing, and some investors expect further write-downs from BHP.

Some have also called into question the miner's policy of holding or raising its dividend at every result, and there is speculation BHP might have to cut its payouts to shareholders.

Ratings agencies Moody's and Standard & Poor's have both warned that the company's dividend policy poses a risk to its credit rating.