Firms that had used unrealistically low cost estimates for extracting oil from ageing fields have suddenly been bailed out of trouble by the price rise, which makes extraction viable, and thus reserves proved, even at a higher cost.

ECONOMIST: Oil companies

The Saudis are not alone in their desperation to stem the declining output of big and ageing oil fields.

ECONOMIST: The oil-services industry

The survey found the problem of an ageing oil and gas sector workforce had been effectively tackled, with a sizeable proportion of employees now aged under 25.

BBC: 'Skills boost needed' for oil and gas sector

Norway is salting away a big chunk of its oil income in a State Petroleum Fund to help pay the pensions of its ageing population when oil and gas production tails off.

ECONOMIST: Remodelling Scandinavia

Messrs Paterson and Gordon also raise concerns that the transfer from Whitehall to Scotland of regulation, licensing, inspection and safety is a major undertaking, particularly in a mature region for oil and gas, with ageing equipment, hard-to-reach reserves and a looming boom in decommissioning.

BBC: Scottish independence: energy's alternating currents