I believe it was for Natural Gas.
Project Gasbuggy tests Nuclear Fracking
http://aoghs.org/technology/project-gasbuggy/
It did release more gas, but the gas was radioactive. Although a 50-kiloton nuclear explosion to fracture deep oil shale deposits Project Bronco was proposed, it never took place.
http://oilshalegas.com/greenriveroilshale.html
Here are some more interesting facts that I will soon forget.