By Hayley Millar
Business correspondent, BBC Scotland
The North Sea has almost as much oil left as has already been extracted, a BBC Scotland investigation has been told.
Experts believe between 25 and 30 billion barrels could still be recovered over the next 40 years.
Calculating oil reserves is not an exact science and this fact has made it difficult over the years to weigh up the true wealth of oil beneath the North Sea.
Oil producers have tended to play down their oil reserves. The markets do not cope well with shocks, so companies take the view that it is better to pleasantly surprise than disappoint them.
In 2004, Shell stunned shareholders when it revised its proven oil and gas reserves, slimming the figure by 20%. The revelation had an instant impact on the company's share price and has served as a lesson to the industry ever since.
The first minister and former oil economist Alex Salmond told me that there was another motivation for oil companies' reticence.
He said: "If oil companies said 'look we've got lots of reserves in the future', the immediate response of government would be to stick taxation up. So there was a kind of incentive for the big companies to underplay the significance of the province."
That is hardly surprising - since the late 1960s, the oil companies have paid £140bn in taxation to the Treasury.
We're bringing on four new fields this year. We're talking hundreds of millions of pounds
John Gallagher Shell
That is a pretty big disincentive. No wonder there is such divergence over how much oil is left in the North Sea.
Sir Bernard Ingham, who was press secretary in the Department of Energy when North Sea oil came ashore, admitted that government negotiations with the oil companies always boiled down to the same thing - taxation.
As he put it: "The whole political calculation is how much brass can you get out of North Sea oil without driving the oil companies away."
The one thing that I did not expect to find in my investigation was that the proven reserves on some of the region's oldest fields are in fact rising.
The Forties Field, one of the biggest and most iconic, is still producing oil 33 years after the first oil was pumped ashore.
Five years ago, BP sold it to the Texas-based oil exploration and production company Apache. Since then Apache has invested $2bn in the field's sub sea network, its platforms and in re-evaluating how much oil is in the field.
According to Jim House, CEO of Apache, flow rates from the Forties field have increased and the amount of recoverable oil has also increased.
He said: "Forties was definitely showing her age when Apache took it. At the time when it was sold, pre-developed reserved were in the region of 150 million barrels. We ended last year with 200 million barrels on our books."
Preview of the BBC Scotland investigation
The downside to operating off the coast of Scotland has always been the relatively higher cost and, as the region matures, that cost factor is becoming more fundamental for the oil majors.
Shell has also been selling off some of its core assets in the North Sea to newer and more aggressive companies like Fairfield Energy.
Through shooting new seismic surveys and drilling new wells these new players are extending the life of North Sea oil fields.
John Gallagher, from Shell in Aberdeen, said his company was still committed to the area.
He said: "We're bringing on four new fields this year. We're talking hundreds of millions of pounds.
"The plant rejuvenation work that we're doing is hundreds of millions of pounds and the work we're doing cross-border - linking up Norway and the UK, which is a growing trend in our industry - is again hundreds of millions of pounds."
The same high oil price that makes it harder for you and me to drive to the shops on a whim is making it easier for companies to take a gamble on previously undeveloped parts of the North Sea.
With a simple oil well costing $22m to drill, exploration companies are taking a huge gamble.
Canadian oil exploration company Oilexco last year drilled 39 out of 140 exploration and appraisal wells in the North Sea, despite rising costs.
Its chief executive officer, Arthur Millholland, said "In 2004 we were paying approximately $55,000 a day for a drilling rig.
"Today we're paying $350,000 a day. So even though the price of oil today is higher than it was in 2004, our costs of doing business here have increased just as dramatically."
It still seems to be worth their while staying in the North Sea, despite the high costs.
What I have seen in my investigation is an industry that is better placed to shoulder these costs. Smaller companies with lower overheads can go after smaller pockets of oil and still make a decent profit.
How long will North Sea oil last? The answer to that depends on many things, including market conditions, future government initiatives and constant technological improvements.
Technology alone is playing its part in extending the activity in the North Sea. John Forrest, Talisman Energy's general manager for the Flotta Catchment Area, said the company was now drilling wells that they were unable to drill five to 10 years ago.
He said: "We're bringing on fields that we've known were there for quite a long time but they just weren't economic or we didn't have the technologies. We recently brought on a field without drilling any more wells. We just had better technology."
The Talisman-owned Claymore platform is expected to keep pumping oil for another 30 years.
Mr Forrest said: "We foresee an economic life at that installation until the late 2030s and that's with the ideas we currently have. We think other innovations will come along that will almost certainly extend that."
The North Sea has many things in its favour. It has world-class geology which makes it attractive to those wanting to harvest its rich reservoirs.
Compared with many of the world's oil producing regions, it is governed by a relatively stable political and economic regime.
The market price of oil is a much more volatile factor, but at current prices the oil that still lies beneath the North Sea is becoming more and more valuable every day.
BBC Scotland's journalists are focusing on the high price of oil, and what this means for the industry in the North Sea, this week.
Hayley Miller will present a special hour-long documentary - Truth, Lies, Scotland and Oil - at 2240 BST on Wednesday on BBC One Scotland.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.