Doesn't have to be H2S, can just be oxygen being displaced and you falling off the tank because of it. That's why when I pop a lid, I hit it and then walk away popping more lids connected to the same tanks while watching the wind sock and staying upwind of the stuff. Some of those tanks you can hear the whistle of the pressure going around the lid, that's when you know to hold your breath since it may not stop gassing off. Legally speaking you should have confined entry to be anywhere near an opening of a tank but that takes too much work to get, it's why in Texas you no longer do oil gauging. Oh and for the record anything that comes out of the ground is usually radioactive, usually Radon, there are maps that let you check the risk in your area. Just hold your breath and wash your hands before you eat.
"I've spent the vast majority of my working life in mines or oil. **They're all big on covering their keisters**, doesn't matter what company it is." Ftfy
