The shale oil and gas boom has transformed energy markets so much that the US is on course to become the world's biggest combined oil-and-gas producer this year—if it hasn't already overtaken Russia.

In July, America produced around 22 million barrels a day of oil, natural gas and related fuels, the Wall Street Journal reports, compared to Moscow's 2013 forecast of around 21.8 barrels a day. Last year, the US tapped more natural gas than Russia for the first time in 30 years, and the gap in crude oil production is quickly narrowing.

American imports of natural gas and crude oil have fallen 32% and 15% since 2008 and are still dropping—but experts warn that the energy boom and the US lead in production may not last that much longer. Russia still has huge untapped shale reserves, and the US boom could be hit hard by swings in commodity prices, government regulation, and rising public opposition to fracking.

Drilling in shale is a lot more expensive than other kinds of exploration, meaning a global downturn and a fall in energy prices could dramatically slow production.