Image caption Development of shale oil and gas is "extraordinary", says Lord Browne

The big growth in oil extracted from shale rock means the US will not need to import any crude within two decades, the former boss of BP has said.

Lord Browne told a conference in Oxford the US would be "completely independent of imported oil, probably by 2030".

He also said the amount of shale gas in the US was "effectively infinite".

Shale oil and gas is extracted using a method called fracking, but the process has been controversial because of the environmental risks associated with it.

Lord Browne is a director of fracking firm Cuadrilla.

He told the Resource 2012 forum on water, food and energy scarcity that the development of shale oil and gas was "quite extraordinary", and that the world was now entering the "latest age of primary energy".

Earthquakes

Under the process of fracking, oil or gas is extracted from shale rock by pumping a mixture of sand, water and chemicals into the rock at high pressure.

In April this year, a panel of experts appointed by the UK government ruled that test fracking could continue, but under strict conditions. The report came after a Cuadrilla site in Lancashire had to stop test fracking in 2011 after two small earthquakes were felt at the surface.

There are also concerns that fracking - which is banned in France and Switzerland - risks polluting water supplies. However, the practice is now widespread in the US.

Lord Browne said the sector needed stronger government regulation to prevent any bad practice.

He said: "Shale gas has a very bad reputation, as a result of the weak players cutting corners.

"Regulation tightening would be welcome."