Youre right, ive run into something similiar stuff myself pulling out really old underground rusted conduits, never had the stuff analyzed for petroleum property's.
I didnt put two and two together until you just mentioned that right now, thats neat.
I always think when ripping these out that they used oil back in the day as a pulling lube. Its a nasty smelling black liquid it verys on thickness, runny to almost tar like really old ones, Interesting my dude, interesting.
Potentially under these natural occurrence, those pipes could of been,no way of knowing, definitely 30 - 50 - 75 years for it to get to that state like you see.
Under the ground with all that pressure where petroleum is created it would be a much faster process and wouldn't take so long to build up.