How about oil from the broken under-water pipelines?  I doubt they purge the lines before locking production in place.
I've read that they "pressure test" various pipelines before resuming production.  However, that may simply be to start the pump and see if it leaks anywhere.

Since they haven't gone anywhere near this spill (just flyovers) they are assuming its from the nearby storage tanks, it could easily be from a pipeline or more than one.