BHP Billiton investors are bracing for further write-downs on the resources giant's stricken shale oil and gas business, comparing its decision to invest $US20 billion in the industry in the United States at the top of the market to Rio Tinto's disastrous Alcan acquisition.

BHP took a $US7.2 billion ($10.3 billion) write-down on Friday, its second impairment in six months. The dramatic collapse in oil and gas prices has rendered most of the business unprofitable.