Shale oil. The US has a good bit. Started with minor players but the "great recession" caused major consolidation. Then there's tar sands from Alberta (primary pipeline goes south).

The US is now a net petro exporter. Not a huge one, but the days of depending on Persian Gulf energy are done. If you've wondered why the US is progressively disengaging from the knife fight that's the Middle East...

It's a stupidly-lot more complex than that, but that's a Cliff Notes of it.

BTW, Ukraine has a good bit of shale oil deposit and that might have been one of the factors in Putin's decision to try to take it over (consider what an alternative energy supplier to Europe might do to Russia's 'take').