Fluorocarbons... I can just imagine the greenies screaming “Freon!”
Of course. But 99.99% of "greens" are technically illiterate (and probably politically illiterate, too), and don't understand that all fluorocarbons are NOT "ozone layer unfriendly", just a select few.
But the real point is that in order to trace the movement of the oil/gas leaks, one needs a "permanent" type of marker species with unique properties not found in nature, and which can be detected in extremely tiny concentrations.
I can't think of anything else that "fits the bill" as well the fluorocarbons. Other halocarbons won't do, as it turns out that there are natural sources of many of them (chlorine, bromine, and iodine analogs).