The good oil

Actually, I don't really know if it's good or not.

Going on a short bushwalk after work, a section of the track has started seeping with a blue oily film on the top. I don't know what causes this - I've seen it a long way away from houses, so thought it may be from natural causes. I rather like the way it gets a kind of craquelure effect on the surface, and has a rather beautiful range of shiny blue tonings.

Any ideas? I'd love to know.