The large underground deposits of shale gas in the US are the lowest-cost option for future oil output.
About 60 per cent of production that's cost-effective with crude at $60 per barrel is in US shale, compared to about 20 per cent accessible by deep well drilling, according to energy consultancy Wood Mackenzie.
It added that US shale regions will be more competitive in the coming years. Meanwhile, the North Sea or oil fields in the waters off west Africa will have to cut costs or endure reduced output.
"There are more opportunities to invest in the US, and that's where the investment will take place," Simon Flowers, chairman and chief analyst at Wood Mackenzie, told the Financial Times.
"If your investment options are in deep water, you've got quite a task on your hands. You might be asking: 'Should we be getting into tight [shale] oil?'"