It's definitely not mercaptan, this is an artificially synthesised chemical made specially for it's distinct odour, low odour threshold. It's added to the natural gas at refineries, often from drums of small vessels. Helium is only 2% (max) of the mix. There's all sorts of potentially hydrocarbon, hydrogen sulphide and (minor) methane which will be the odours. You can get Draeger tubes which have sand in them and high will turn different colours depending on the chemical constituents present. It's qualitative, but reliable. Similarly you can get a photoionisation device that will prove an indicative concentration. Sounds like good signs to me though.
