That would be my guess too. Natural gas, or more likely hydrogen sulfide. The tank was only partially full.
Hydrogen Sulfide
CAS RN: 7783-06-4
Explosive Limits / Potential
Explosive limits when mixed with air: lower limit 4.3% by vol, upper limit 46% by vol.
Moderately explosive when exposed to heat or flame.
FORMS EXPLOSIVE MIXTURES WITH AIR OVER WIDE RANGE.