Here is a hypothesis based on abiotic methane leaking out of the ground, I did some the figuring for this a while back:
Unless the methane were rapidly leaking out of the ground for an amount of time prior to ignition....that could cause methane-oxygen ratios in explosive proportions. But still would need the info on how much methane-oxygen at STP you would need for a 5 megaton explosion though.
By volume the ratio is 2(O2)-1(CH4)
1 megaton = 4.184x10^15 Joules
5-15% methane in air is considered explosive
Energy released by methane = 802 Kj/mol = 802x10^3 J/mol
20.92 |TJ| / 802 |Kj/mol| =2.608x10^10 mols of CH4
5.217x10^10 mols of O2
Oxygen in air is 20%, and assuming that 5-15% ratio displaces only the nitrogen.
2.608x10^11 mols of air molecules is involved.
If that is uniformly spread out, then assuming 41.4 mol/m^3 for dry air
You end up with an explosion requiring approximately 6.301x10^9 m^3 of air/fuel mixture at STP. That’s a cube of about 1.847 kilometers in all directions in order to give you 5 megatons of energy from this reaction.
Tunguska was an area of 2000 km^2 = 2x10^9 m^2
So the amount of methane/air mixture in such an explosion would have been about 3.15 meters high off of the ground (save for all the imprecision in my assumptions)....anyhow, that seems reasonable for the damage done.
(doesn’t it make sense that 1625 metric tons of methane gas needs to be combusted to yield 5 megatons energy equivalent of TNT?)
Source: Huge Tunguska Explosion Remains Mysterious 100 Years Later
...note, although it is fun to think about, like a gigantic barbecue grill being lit up, it probably was still most likely an asteroid event according to the sightings recorded in this Sibir newspaper article from July 2, 1908 from Wikipedia:
"On the 17th of June, around 9 a.m. in the morning, we observed an unusual natural occurrence. In the north Karelinski village [200 verst, or about 130 miles, north of Kirensk] the peasants saw to the north west, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a "pipe", i.e. a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground (forest), the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world."
