- BHP Billiton's [LON:BLT] shares were down in late trading after it announcing that it expects to take a hit of approximately USD4.9bn post-tax (or about USD7.2bn pre-tax) against the carrying value of its onshore US assets.

This charge will be recognised as an exceptional item in the financial results for the half year ended 31 December. The impairment follows the bi-annual review of the company's asset values and reflects changes to price assumptions, discount rates and development plans which have more than offset substantial productivity improvements.

The impairment will reduce onshore US net operating assets to approximately USD16 billion.

The group said the oil and gas industry had experienced significant volatility and much weaker prices. It says the US gas price remains low as industry-wide productivity improvements have resulted in higher than expected supply at lower cost.

BHP Billiton has previously suspended development of its dry gas acreage. The company has now also reduced its medium and long-term gas price assumptions.

It adds: "In addition, the oil price has fallen by more than 30 per cent over the last three months following the disruption of OPEC and stronger than anticipated non-OPEC production. 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 increased volatility in prices has also increased the discount rates applied by BHP Billiton, which has a significant flow through impact on the Company's assessment of its Onshore US asset value. The group will reduce the number of operated rigs in its Onshore US business from seven to five in the March 2016 quarter. This will comprise three rigs in the Black Hawk and two rigs in the Permian. Beyond this, investment and development plans for the remainder of the 2016 financial year are under review, with a focus on preserving cash flow. The oil and gas industry has recently experienced significant volatility and much weaker prices.

Chief executive Andrew Mackenzie said "Oil and gas markets have been significantly weaker than the industry expected. We responded quickly by dramatically cutting our operating and capital costs, and reducing the number of operated rigs in the Onshore US business from 26 a year ago to five by the end of the current quarter. "While we have made significant progress, the dramatic fall in prices has led to the disappointing write down announced today. However, we remain confident in the long-term outlook and the quality of our acreage. We are well positioned to respond to a recovery."

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Highland Gold Mining [LON:HGM] has appointed Denis Alexandrov as chief executive of its Moscow-based management company, Russdragmet LLC, from 18 January.

Alexandrov will be responsible for overseeing the company's operations in the Russian Federation and Kyrgyzstan. Outgoing CEO Valery Oyf will remain with the company as non-executive director.

Executive chairman Eugene Shvidler said: "On behalf of the board, I would like to welcome Denis Alexandrov back to the company. His strong natural resources industry management experience and financial background will be instrumental in helping to navigate the challenging period ahead for Highland Gold." The board also put on record its thanks to Oyf for his stewardship over the past eight years, during which the company witnessed substantial growth, brought the Novoshirokinskoye and Belaya Gora mines online, reduced costs, and improved efficiency.

The board is pleased Oyf remains with the group as a director, ensuring continuity and his continued valuable input and insight. Oyf also remains a key shareholder in Highland Gold.

* * *

Alexander Mining's [LON:AXM] MetaLeach subsidiary has been granted a patent in Peru for a method for extracting zinc from aqueous ammoniacal solutions.

The patent, number 7520, has a standard term of 20 years from the effective date of 22 December 2009 (the date of original filing of the PCT Application from which the Peru patent is derived).

Chief executive Martin Rosser said: "The technology which is the subject of this patent provides for the economic recovery of zinc from aqueous ammoniacal zinc solutions, such as those produced in our proprietary AmmLeach® processing technology. We are pleased with the grant of the patent in Peru as the country has high potential for the use of our AmmLeach processing technology for zinc extraction."

The patent, which has also been granted in the USA, Canada, Australia and Mexico, describes a method for extracting zinc from an aqueous ammoniacal zinc solution containing impurities.

* * *

The sector's biggest faller was Beowulf Mining [LON:BEM] - down by more than 21.5% in late trading. The biggest riser was Altona Energy [LON:ANR] - down by more than 7.6%.

At 4:13pm:

[LON:ANR] Altona Energy PLC share price was +0.05p at 0.7p

[LON:AQP] Aquarius Platinum Ltd share price was -0.12p at 11.88p

[LON:AXM] Alexander Mining PLC share price was 0p at 0.18p

[LON:BEM] Beowulf Mining PLC share price was -1.2p at 4.95p

[LON:BKY] Berkeley Resources Ltd share price was 0p at 25p

[LON:BLT] BHP Billiton PLC share price was -44.65p at 612.35p

[LON:CEY] Centamin PLC share price was +2.4p at 65.9p

[LON:CHL] Churchill Mining PLC share price was 0p at 19.5p

[LON:CZA] Coal of Africa Ltd share price was 0p at 2.38p

[LON:FDI] Firestone Diamonds PLC share price was -0.5p at 17.38p

[LON:FRES] Fresnillo PLC share price was -12p at 690.5p

[LON:GEMD] Gem Diamonds Ltd share price was -2.75p at 107.25p

[LON:HGM] Highland Gold Mining Ltd share price was -0.37p at 54.38p

[LON:HOC] Hochschild Mining PLC share price was -0.75p at 41.25p

[LON:KMR] Kenmare Resources PLC share price was -0.02p at 0.57p

[LON:VED] Vedanta Resources PLC share price was -17.65p at 223.35p

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