"It's Scotland's oil" is one of the most highly charged slogans in Scottish politics. First used by the Scottish National Party in 1974, the notion that Scotland "owns" up to 90% of the North Sea's reserves remains one of the strongest sources of grievance for nationalists.
And in the next two years, as Alex Salmond leads the country into a referendum on independence, it is likely to become one of the central arguments for nationalists: they believe it will help decide the fate of the UK.
Ever since it became clear that North Sea oil fields would generate immense riches, the SNP has insisted that that wealth has been squandered by successive governments at Westminster. They point out that Norway, a country with a similar population to Scotland at just under 5 million, has saved much of its oil income: surplus revenue is ploughed into the government pension fund, which is now Europe's largest owner of shares and is worth about 3.3 trillion kroner (£360bn).
The SNP argues that if you extend a line east from where the Scotland-England border hits the coast north of Berwick, the division of the seabed would give Scotland control over nearly all North Sea oil and gas fields.
The Scottish government, which is drawing up a detailed case to support that 90% claim as its civil servants prepare for the referendum, asserts the North Sea will generate about £54bn in revenues over the next five years, while its "asset base" is valued at roughly £1tn, including remaining reserves.
Salmond claims that, combined with Scotland's significant offshore wind and marine energy resources, and other industries such as whisky, this would make Scotland the OECD's sixth wealthiest nation, leaving the rest of the UK trailing in 15th place.
But Salmond will have to answer anxieties about whether Scotland can offer the industry the same international muscle and relatively stable tax and regulatory regime the UK provides. Experts believe the oil companies and the engineering firms that rely heavily on North Sea contracts will be weighing up the potential negatives of independence. They would have to juggle two regulatory and tax regimes: the UK has expert health and safety and environmental agencies, largely controlled from Whitehall. Scotland would have to build its own from scratch.
Would an independent Scotland, notionally one of the smallest of the EU member states, be able to match the UK's political influence and power in Brussels or on a global stage?
There also remains the problem of the long-term oil exploration and drilling licences the UK government has issued to oil firms: many last for 30 years. How would those be transferred?
Think too of how Scotland might defend North Sea rigs: would an Edinburgh government be able to afford to maintain a navy capable of tackling a terrorist incident?
But the oil industry may also decide that Scottish governments would be more amenable on taxation: North Sea oil wealth is of far greater significance to the Scottish economy than it is to the UK's.
While the figures fluctuate year by year, oil and gas generates about 2% of the UK's GDP and tax, but the Scottish government's annual economic report estimates it would make up 12% of Scotland's overall revenues.
These delicate questions explain why UK ministers are being very polite to the Scottish government. Charles Hendry, the UK energy minister, said his government had not begun to look at the territorial claim. "We haven't even got into those discussions; at this stage that issue may never arise," he says. "The nature of what we'll be arguing in the runup to the referendum is that it's best to keep the structure we've got; that it's the best way of getting investment coming into Scotland; the best way of keeping people's electricity bills down in Scotland and therefore that we have the right policy in place.
"Our view at the moment is that this is a UK-wide industry, people have invested based on an assumption of that," he said. "We believe that is the best way forward, we have not got into detailed discussions and negotiations about how independence would affect that because the referendum hasn't even been established and we've got all of those issues to go through.
"But what we do think is that when people look at the broader issues – and I think energy is a very important part of that process – then the attractiveness of the UK overall and in Scotland is enhanced by being part of the United Kingdom; the security of supply is enhanced by looking at it on a broader UK-wide basis; we're more able to move to a low-carbon, sustainable and affordable UK energy policy if we do it on a broader basis."
Hendry argues that the difficulties of operating two different taxation and regulatory regimes in what is essentially the same region – the North Sea – will be for the Scottish government to set out. So far it has failed to do so.
"Primarily I think it's the Scottish government which needs to explain how those difficulties can be overcome. And we don't have their sense of how best this can be handled at this stage," he said. Asked how the UK government would deal with the huge political weight attached to North Sea oil by nationalists, Hendry also implies that the question has been superseded by the far more complex issue of producing and supplying energy as a whole, including renewables and nuclear power, in the UK and western Europe. Oil ownership is just part of a larger picture, to do with energy security and the energy market.
Salmond has set the renewables industry a target of being able to supply 100% of Scotland's electricity demand by 2020 when conditions are favourable, while maintaining a back-up of two nuclear stations and existing coal-fired plants. But Hendry points to problems: if Scotland is independent by 2014, it will not have UK subsidies for renewables to draw on; it will have to pay for the grid infrastructure to export and import power; and at the same time be competing against French, Irish and Norwegian suppliers.
A spokesman for John Swinney, the Scottish finance secretary, said: "Scotland has a £1tn North Sea asset base – at the same time as the UK's national debt has broken through £1tn for the first time – with some 40% of reserves remaining and over half of the value still to be extracted. With the oil price forecast to rise over 0 a barrel, these figures absolutely underline the need for Scotland to gain control over our own resources with independence, for the long-term benefit of the country."
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