I googled Helium and its a Radioactive breakdown of some parts in crude oil. Helium is so sparse in the atmosphere that, if you can find 10-50 times the atmospheric portion in the ground, then it is much cheaper to extract. This also seems to explain why seismic is not useful: all the crude or natural gas is gone but He remains.
At least, that was my understanding, possibly a misinterpretation, of some of the links.
