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The future of North Sea oil is one of the key campaign battlegrounds ahead of the Scottish independence referendum.

Since the first licences were issued for the extraction of oil and gas from the North Sea in 1964, about 42 billion barrels of oil have been produced. It is estimated there could be up to 24 billion more in untapped reserves.

The industry employs 450,000 people across the UK and in 2012-13 the industry paid £6.5 billion in taxes to the UK government.

North Sea oil supplied 67% of the UK's oil demand in 2012 and 53% of the country's gas requirements and is a major boost to the country's economy.

If oil revenues are included in GDP figures, Scotland is shown to generate more per head of population than the UK as a whole.

Since a peak in 1999, production has steadily declined.

Maintenance work on ageing infrastructure and a spate of helicopter accidents have caused temporary halts to production in recent years.

A greater focus on health and safety following the Gulf of Mexico disaster in 2010 has also had an impact, as has an increase in taxation on North Sea production introduced in March 2011.

In 2013 the UK government commissioned Sir Ian Wood to carry out a review of the industry.

, released on 24 February 2014, makes a series of recommendations, including the setting up of a new independent regulator.

He also recommends more investment in infrastructure to improve efficiency, and more money to be spent on exploration and exploitation of untapped reserves.

This would put the UK in a much stronger position to exploit the estimated 24 billion barrels of oil remaining, the report says.

Prime Minister David Cameron believes the UK, with Scotland remaining part of the Union, would be best placed to fund future exploration and exploit the increasingly hard-to-reach oil and gas resources.

He also said a united UK would be better able to cope with fluctuations in oil prices.

But Scottish First Minister Alex Salmond insists an independent Scotland could withstand the volatility of the oil market.

He wants to set up a Norwegian-style sovereign wealth fund - setting aside a tenth of oil and gas revenue each year - to help offset some of the problems caused by the price fluctuations.

The Norwegian fund - or Government Pension Fund Global - was worth an estimated $785 billion (£471 bn) in September 2013, and is one of the largest in the world.

It was set up in 1990, initially to help cope with the rising costs of pensions for a population that was living longer, and also accommodate changes in oil prices.