Well had been cased off and they were waiting for cement to cure before suspending the well and moving off.
Failure was sudden and catastrophic -- sudden explosion and fire 10 pm Tuesday night. Full details pending.
Cement obviously failed and allowed gas into wellbore.
This was an oil prospect not natural gas, but pay zone was below 18,200' (beaucoup deep) and pressured. If oil-based mud were in use, gas could have dissolved imperceptibly in the drilling-mud column (no warning volume-gain in pits as gas displaces drilling fluid), and come to surface w/o warning. This happened in the big Ixtoc blowout in 1978/9. Sneaky, fast-breaking, very deadly.
Alternatively, gas came to surface behind casing, evolving off oil reservoir and escaping to surface around failed cement job w/o observation until last minute. Kaboom.
Rams couldn't be closed (blowout preventers). Don't have the full story on that. With casing run, a new wellhead and BOP stack should have been in place.
Many available ignition sources: generators, light switches.
Rig also had 700,000 gallons of diesel fuel aboard.
I think there is a good chance something other than a blowout occurred. Do we know what jobs the missing 11 were doing at the time? Were the Tool Pusher, driller and floor crew killed?
Based on your post, if sounds like a failed cementing job or failed casing.
Seen both occur, but in low pressure fields.