Rising nationalism, insufficient talent and scarce supplies are limiting investment opportunities for the world's major oil companies, leading them to increase share buybacks, said Jim Mulva, chief executive of ConocoPhillips.

Conoco, the US's third largest oil company, budgets $15bn a year on capital spending. "We, technically, could be spending more," Mr Mulva said in an interview. "But we have a number of issues or concerns."