Up to 30 billion barrels may be left, almost as much as has already been extracted, according to Alex Kemp, Professor of Petroleum Economics at Aberdeen University.

He also said that up to 300 fields remain undeveloped.

"We've produced, since day one, 37-and-a-half billion barrels of oil equivalent," Prof Kemp told BBC Scotland's Good Morning Scotland.

"The remaining reserves on central estimates could be 20-22 billion barrels equivalent and on optimistic estimates could be over 30.

"So there still is a substantial amount left."

But the size of fields is smaller with a production average of less than 20 million barrels - compared with 500 million in the early 1970s.

Challenging conditions such as heavy oil, deep formations and high pressures also mean the price per barrel is "very, very high" to extract, Prof Kemp added.

"So although we could have 300 fields undeveloped there are good reasons why everybody's not rushing to develop them all," he added.

Professor Peter O'Dell, of the Erasmus University in the Netherlands, also tells a BBC Scotland documentary entitled Truth, Lies, Oil And Scotland that, on current demand, there is the equivalent of 44 years worth of oil in the North Sea.