The SNP stresses the economic importance of oil and gas but experts aren't sure how much is out there or how much it's worth.
The Scottish economy has grown on the back of whisky, tourism and financial services but by far the biggest potential money-spinner comes from resources still in the ground: oil.
The North Sea has always been seen as crucial, not only to employment and tax revenues, but to the whole viability of Scotland as a thriving and independent nation state.
It is not alone. Countries from Russia to Venezuela and as close as Norway have relied on oil and gas to create wealth to fund public spending programmes.
Greenland is desperately trying to encourage companies to drill in the Arctic. Why? Because it recognises it cannot rid itself of its political master, Denmark, without economic independence. And that cannot be built on seal skins.
Equally you have got to sell a lot of whisky to match the 866,000 barrels of oil a day – at more than $100 a barrel – produced from the UK North Sea in 2013, never mind the 36bn cubic metres of gas on top of that.
Most of that cash will go to the companies involved in the extraction but the tax take on these hydrocarbon riches goes at present to the Treasury in London to the tune of £4.7bn in 2013-14 and more than £6bn for the 12 months earlier.
It is assumed on both sides of the border that 91% of those tax revenues could be taken by Scotland on the basis of an internationally accepted median line drawn out between Scotland and England straight out into the sea.
Alex Salmond, a former oil economist and now first minister of Scotland and leader of the Scottish Nationalist party, has argued that a newly independent country could exploit the £54bn in tax taken from the North Sea in the six years up to 2016-17.
And he believes that there are 24bn barrels of oil equivalents (boe) of reserves – which includes gas – still lying under the seabed and waiting to be exploited.
Based on that number, Salmond has argued that there is £1.5tn worth of fuel to help create jobs, boost tax returns and add benefits for the people of Scotland.
Such figures have proved a major bone of contention. The Office for Budget Responsibility (OBR) has predicted recoverable reserves of 10bn boe and North Sea tax revenues of £61.6bn between now and 2040.
The Office for National Statistics put the worth of untapped reserves to the UK Treasury at closer to £120bn while Sir Ian Wood, a leading offshore oil industrialist in Aberdeen, put the figure at 15bn-16.5bn barrels.
Why the variation? A letter to explain the £61.6bn figure used by the OBR written by the chairman, Robert Chote, to the Scottish parliament explained
"Oil and gas receipts are one of the most volatile streams of revenue coming into the exchequer, which also makes them one of the most difficult to forecast. This reflects the number and the nature of the factors that determine these revenues: the levels of oil and gas production, global dollar prices, the sterling/dollar exchange rate, the scale of tax-deductible capital and operating expenditure, and the history of past profits and losses for each company in the industry that determine if and when they will pay tax on newly generated profits. Most of these individual determinants are difficult to predict in their own right, even over a very short time horizon."
Others go further, pointing out that to reach the 24bn figure the SNP is using "proven, probable and possible reserves ... as well as further exploration". The party has also admitted that its £1.5tn figure is dependent on oil prices staying above $100. Most industry experts would expect that but it cannot be guaranteed.
The reserve figure described by Salmond as "robust" is much more controversial. John Busby, an independent energy consultant, argues that the barrel of oil equivalent numbers used by the SNP and others are largely meaningless. "The only value is really attached to recoverable reserves – oil or gas that can definitely be got out economically. Those are the only numbers that are recognised [when assessing oil company values] by the securities and exchange commission in the US."
He points out that BP, in its respected annual statistical review of the global industry, puts the amount of recoverable reserves in Britain at 3bn barrels and says the fast rundown of oil and gas output suggests there is an energy and fiscal problem looming whoever owns the hydrocarbons.
Not discussed here but highlighted by the Guardian in the past is the fact that there are almost no truly indigenous oil or gas exploration and development companies in Scotland.
Does this matter? Some would say that the big – nominally – local companies such as BP and Shell have reduced their North Sea activities hugely over the last decade without any dramatic impact on the UK; oil money is highly mobile and the same companies that invest in the UK – or Scotland – are happy to work anywhere. And will.
If the tax regime is more attractive in a new up-and-coming area such as, say, Mexico – as opposed to a mature North Sea field where big finds are now deemed unlikely – then they are likely to switch investment there.
Small countries dependent on oil find it difficult to negotiate with massive international oil companies. Salmond must know an independent Scotland would have to give more lucrative tax treatment to Big Oil if he is going to encourage them to extract the 3bn barrels highlighted by BP or the 24bn that the first minister hopes for.
Oil & Gas UK, the industry lobby group, said last month: "We are clear that fundamental change is needed to change the way the industry is taxed and regulated, if the UK is to maximise economic recovery from the UK Continental Shelf (UKCS)."
The cost of operations has soared in recent years and drilling is currently at very low levels, with estimates that the North Sea could produce less than 800,000 barrels this year compared to the near 3m in 1999.
In the background lurks the row over whether or not any country should be drilling out reserves at all just as we need to move away from fossil fuels towards a low-carbon economy.
There is increasing talk by organisations with pension funds to divest from sectors which cause global warming in the light of a wider debate about "stranded assets" .
Salmond has rightly talked up the prospects of Scotland's windfarms and green economy but a much deeper prosperity based heavily on oil cannot be guaranteed by anyone. It depends on a range of factors; many not under his control.