Clay soil. The bubbles aren’t flammable, I held a long lighter over them and they actually blew it out. I got close and smelled the bubbles, they didn’t smell distinctive, maybe slightly sewery. There are no utilities that I know of under there, but the area has been occupied for a long time, and my house is 100 years old.
Might be an old, long abandoned pipe that finally cracked? Air that has been trapped underground gets this stale, sewery smell. They lighter might have just been blown out by the water spray ejected from the popping bubble.