Politically driven state energy companies are hindering access to oil reserves, threatening worldwide energy security, top executives of some the of the globe's biggest oil majors said.

High oil prices and perceptions of soaring industry profits have prompted many producing countries to give national oil firms more power to extract richer fiscal terms and greater control over resources, pushing out international players, Rex Tillerson, chief executive of Exxon Mobil, said.

Such policies will lead to tighter oil supplies as Exxon and other majors stop deploying their technology, know-how and capital in some those countries, he said.

'If there's no room to share it then there is no role for us -- it's a simple as that,' he told business and political leaders at the Spruce Meadows equestrian facility on the outskirts of Calgary.

'I think that's something that national oil companies have to get their minds around and they've got to help their governments get their minds around.'

Exxon left its Venezuelan operations behind this year after it refused to accept more nationalistic terms set by President Hugo Chavez, which gave state-controlled PDVSA bigger stakes.

 'The big value in resources development is not in the upfront bonus money that you're paid, it's in the incremental recovery over the life of that resource that's of huge value to the host country,' he said.

State oil companies now control 75 percent of the world's proven oil reserves and major international firms like Exxon, Royal Dutch Shell and Total account for about 10 per cent, he said.

Meanwhile, the International Energy Agency has estimated that the oil industry will have to spend $20 trillion through 2030 to meet estimated demand.

Thierry Desmarest, chairman of France's Total, agreed that many countries in regions like the Middle East, former Soviet Union, Africa and South America have been putting more political obstacles in place, restricting access to new reserves.

Meanwhile, production in most of the rich countries in the Organisation for Economic Co-operation and Development is expected to keep declining.

Still, Total agreed to Venezuela's new terms, allowing it to maintain some heavy oil operations in the Opec country.

'We have made our choice because we think that Venezuela's oil potential is substantial. The country needs the world-class technology that my company and other companies provide,' Desmarest said. 'And we think that in the long term the convergence of interests should be recognised.'

The two other big issues for the industry as it struggles to meet rising demand in developed and developing countries are improving technology to get at hard-to-reach reserves and meeting stringent environmental regulations, oil bosses said.

'For every barrel of oil or cubic metre of gas that you consume today, to replace that barrel it costs more, so our industry gets a lot more more capital intensive per unit all the time,' said Jeroen van der Veer, chief executive of Royal Dutch Shell.

All three of the oil majors, along with numerous others, have invested billions of dollars to develop Canada's massive oil sands, access to which has not been hindered by politics.

However, developing those resources requires new technology to boost output and reduce environmental impact.Reuters