Today, BHP Billiton (ASX: BHP) announced a US$10 billion after tax write down on its onshore United States (US) oil and gas assets resulting from an oil price fall of more than 30% over the last three months. The company said it expects to take an impairment charge of US$7.2 billion (A$10.3 billion) in its half year results, its second writedown on its US oil and gas assets in six months. BHP Chief Executive Andrew Mackenzie said oil and gas prices were behind the writedown as markets have been “significantly weaker than the industry expected”. BHP shares closed down 8.0% for the week. We do not own BHP in the investment portfolio.