Oil giant Shell is to start a sale of up to $30bn (£18bn) of assets next year after weak refining margins and oil theft in Nigeria caused a sharp fall in profits, it can be disclosed.

Assets on the chopping block include a $7bn stake in Woodside Petroleum, Australia’s second largest oil and gas producer; oil assets in the Niger Delta worth $2bn; and other assets totalling $20bn, according to oil and gas analysts from JP Morgan Cazenove.

Last month, Peter Voser, Shell’s chief executive, said: “We are entering into a divestment phase like we had a few years ago.”

According to Fred Lucas, of JP Morgan Cazenove, Shell’s guidance for capital spending implies at least $15bn in asset sales during the next two years. “Yet without too much thought to portfolio stress, we can isolate almost double that figure of potentially non-core assets in Shell’s global portfolio,” he added.

The $30bn disposal plan could be unveiled as early as next month and would be the largest in the Anglo-Dutch oil group’s history, representing 17pc of the FTSE 100 company’s $178bn net assets.

“We are facing headwinds from weak industry refining margins, and the security situation in Nigeria, which continue to erode the near-term outlook,” said Mr Voser when he reported in October that third-quarter pre-tax profits had slumped by 31pc to £2.56bn. Results took a $300m hit due to the “deteriorating security situation” in Nigeria and a blockade there.

But Shell has also come under pressure from investors this year, as capital investment increased while the shares underperformed.

The shares fell the most in two years on October 31 when Shell said net spending had risen this year because of acquisitions and higher costs at projects. Mr Voser said that the five-year $130bn investment plan will peak this year.

Mr Voser, a Swiss national, is to step down at the end of the year and will be succeeded in January by Shell’s Dutch refining and marketing chief, Ben van Beurden.

The company came under fire from major investors over the weekend for allegedly not treating British investors on a par with Dutch shareholders. Shell cancelled a London event last year that provided a live TV link-up to its annual meeting in the Hague, angering British-based investors.

Shell said more clarity on the asset disposals would be given in full-year results on January 30.