John Manzoni, president and chief executive of Talisman Energy, told the Offshore Europe conference in Aberdeen that plans to extend the European carbon trading scheme to such fields would make them uneconomic. He estimated that this might force Talisman to abandon 50 million barrels-worth of reserves.

He said: “Older fields have higher costs with relatively low throughputs, even though they still have significant oil in place... If we have to buy carbon permits it will result in earlier abandonment.”

As part of a volley of calls for looser regulation at the conference, which previously drew veiled criticism from First Minister Alex Salmond, Manzoni said his company was already investing overseas rather than in UK territorial waters because he could get “better rates of return”.

Sir Ian Wood, chairman of John Wood Group, told an audience that the UK government could be missing out on a share of a £1 trillion prize unless it improved the regulatory regime. He warned tax revenues from the remaining 13 billion to 25 billion barrels of North Sea reserves would be unlikely to be extracted as things stood, comparing his industry with the “huge subsidies” received by the renewable sector.

On the basis that the lower-estimate 13 billion barrels would sell at an average $100 (£60) each, he said: “That’s $1300bn over 20 or 30 or 40 years. I am going to repeat that: $1300bn of economic activity to the UK over the next period. That’s something that has to be a priority [for the government] given the economic crisis.”

He was speaking despite the fact that Salmond had cautioned the industry that it should consider itself lucky to have received at least a mention in Chancellor Alastair Darling’s most recent Budget, considering the recent economic difficulties.

Although he said he would have gone further than Darling’s tax breaks announcement for smaller fields, he said: “We have to look at the context: unprecedented fiscal pressure... To get any recognition that the potential was there and therefore incentives should be offered was something of an achievement. ”

Manzoni also raised the point that fear of decommissioning liabilities were exacerbating the problems that smaller operators were experiencing around raising finance and putting them off buying Talisman fields that the group regarded as too small for its core interests. He welcomed the fact that the UK government was considering introducing a guarantee on decommissioning, but stressed: “These companies can’t afford the decommissioning liabilities.”

Greg Guidry, executive vice president of Shell’s upstream activities in Europe, raised similar concerns about smaller operators by saying that the UK government should improve the tax regime for brownfield sites -- industry speak for previously produced reservoirs -- since they amounted to around 60% of what was left in the North Sea and would become more dominant in future.

He also hinted that Shell might be forced to go the same way as Talisman and prioritise more investment elsewhere by saying that greater levels of discovery in regions like the Gulf of Mexico and the Middle East now meant that both regions were more attractive on a cost-per-barrel basis, which made current levels of investment in the UK “unsustainable”.

This was a less rosy picture than that painted by Salmond, who told delegates that the industry ought to be brimming with confidence given that hydrocarbon reserves ought to last for another generation. He also pointed out that capital markets were beginning to return to normal and that the industry would one day be matched in size by the renewable energy sector.

Speaking in the next session along with Manzoni, energy minister Lord Philip Hunt said the government was “very open to dialogue” with the industry with a view to maximising the exploitation of the country’s remaining oil reserves.