Investment companies backed by some of the world’s biggest private equity groups have expressed interest in North Sea assets being sold by .

Neptune, funded by Carlyle Group and CVC Capital Partners, and Siccar Point Energy, whose owners include Blackstone, are among potential bidders.

Both companies are looking at a package of North Sea assets offered by as part of its $30bn disposal programme, according to people involved in the process.

under the leadership of Sam Laidlaw, former chief executive of Centrica, the UK energy supplier, with a target to invest up to $5bn in the North Sea, north Africa and Southeast Asia.

Siccar Point, backed by Blackstone and Blue Water Energy, a specialist private equity company, is led by another Centrica veteran, Jonathan Roger, with the former chief executive of BG Group, Chris Finlayson, as chairman.

Shell has a target to raise $30bn from by the end of 2018 as it battles to curb rising debts after its £35bn takeover of BG Group. Simon Henry, Shell’s chief financial officer, wants to make “significant progress” on deals worth $6bn-$8bn this year.

One person involved in the process cautioned that talks over the North Sea assets were at an early stage and the likelihood of a deal would become clearer in the next two months.

Shell declined to comment.

Any disposals would add to the retreat by large oil groups from the North Sea as the ageing basin struggles with high costs and falling output. Shell’s acquisition of BG brought together two of the biggest operators in UK waters but the primary motivation lay in BG’s strength in high-growth regions such as and Tanzania.

Shell insists it will not abandon the North Sea, where it has 33 platforms and interests in 65 fields. Further multibillion-dollar investment is planned in two big developments — Clair and Schiehallion — west of the Shetland Islands. However, Shell is looking to sell a range of older assets, as well as stakes in newer fields, in what would amount to a significant downsizing, according to people involved in the process.

North Sea deals have slowed to a trickle since the collapse in oil prices two years ago depressed valuations and drained capital from the sector. But this month of an 8.9 per cent stake in the Mariner oilfield east of Shetland from JX Nippon of Japan has raised hopes that private equity cash could bring urgently needed investment.

Neptune, Siccar Point and their backers declined to comment on the Shell assets but made clear their appetite for North Sea deals.

Mustafa Siddiqui, managing director at Blackstone, said: “We have a market where there are not many strategic buyers; so a company like Siccar Point, with the capabilities and the capital, is in a very good position to acquire assets that require further investment.”

Greig Aitken, an analyst at Wood Mackenzie, the energy consultancy, reported signs of deal activity picking up as continued oil price weakness led sellers to accept that valuations were not returning to previous highs. “We’ve seen some movement on the side of the sellers,” he said.

People close to Neptune and Siccar Point said the Shell assets were just one among a range of investment options and there was no guarantee they would pursue their interest. Liabilities related to the future cost of decommissioning depleted oil and gasfields — the sums can run into billions of dollars — are among potential sticking points.

Asset sales are crucial to Shell’s efforts to safeguard its prized dividend and streamline operations after the BG deal. The group’s debt-to-equity ratio has more than doubled in the past year to 28.1 per cent — on the brink of the 30 per cent “upper limit” declared by Mr Henry.

“Gearing has gone up to the point where they do not want it to go much higher and disposals are one of the few levers available to bring it down,” said Mr Aitken.

Shell has so far agreed or completed $3.1bn of deals this year, mostly involving downstream pipeline and refining assets less exposed to the deflationary impact of low oil prices. A further sizeable sum is expected from , the Saudi state-owned oil company, as part of the break-up of their Motiva US refining joint venture.

However, the sale of upstream production and exploration assets will provide a bigger test. Shell faces a balancing act between maximising proceeds and pressure to get deals done. Upstream assets in New Zealand, Thailand and Gabon are among others on the block, as well as a 13 per cent stake in Woodside Petroleum of Australia, valued at about $2.4bn at its current stock price.

Mr Henry knows the first big upstream transaction will set a benchmark. Addressing investors last month, he signalled Shell would rather let its gearing ratio temporarily drift above 30 per cent than lower the $30bn disposal target. “We are not planning for asset sales at giveaway prices,” he said.