Mining giant BHP Billiton Friday said it expects to book a US$7.2 billion pre-tax writedown against the value of its onshore assets in the United States as tumbling oil prices hit shale gas.

The hefty impairment charge equates to US$4.9 billion after tax with the Anglo-Australian company blaming "significant volatility and much weaker prices" in the oil and gas industry.

It will also reduce the number of working onshore US rigs from seven to five in the March 2016 quarter.