Nope. PROPANE sinks. NG rises.
Correct. When I was in college (Boston) we'd fill small weather balloons with a mixture of air and natural gas. We'd take a balloon to the roof of the apartment building (at night of course), tie a fuse on it, and let it go. Had one explode enroute to the Prudential Building. Another one blew up about 300 ft over a neighborhood in Roxbury.
Guess we could get into big trouble if we did that today.