they clearly held some sort of gas fine for millions of years, so certainly they’re gonna store a bit of hydrogen too.

Not to rain on your parade, but hydrogen and natural gas aren't really comparable for storage. The natgas molecule is 8x heavier and MUCH larger than a molecule of hydrogen. Just on the size alone, hydrogen can slip through just about everything and needs to be stored at cryogenic temperatures. I don't think rock is going to be as good of a storage media as you'd assume.