Shares in Woodside Petroleum have slumped this afternoon, after the company lowered its production guidance by more than 5 per cent. The downgrade comes on the day Woodside mourns the death of the man considered its founder, Geoff Donaldson. Woodside reported today that LNG production for 2013 would be no more than 89 million barrels of oil equivalent, after previously suggesting production could be as high as 94. The slump is mostly due to an unplanned shutdown at the company's flagship Pluto LNG asset off the WA coast. Problems there have been compounded by refurbishment over-runs at the company's Vincent asset, which is also off the north-west of WA. Woodside shares had been steady around $35.30 for most of the day, but release of the news saw the stock plummet as low as $34.05 within minutes. Shares finished the day down 3.4 per cent to $34.62. The bad news has virtually eroded the gains made by Woodside shares late last week when the company continued its plan to internationalise its portfolio by buying into two offshore permits on the west coast of Ireland. Mr Donaldson was foundation investor in the company and was chairman for 28 of its first 30 years, and Woodside today announced that he had passed away late last week. Woodside was named after a small town in eastern Victoria where it was exploring at the time, but Mr Donaldson was instrumental in the company changing its focus to Western Australia, where it remains based today. Woodside chief executive Peter Coleman said Mr Donaldson was a pioneer of the Australian oil and gas industry and would always be remembered as a crucial player in the formation of the company.