Formation KickDrilling operations use a special mix of mud to...

Formation Kick
Drilling operations use a special mix of mud to both lubricate the drill and to help push down; however, if the mud pressure is less then the oil pressure, the oil and mud can "kick" back up. Oil wells are particularly vulnerable to rupture during drilling operations because of the additional ports that have to be open. However, drillers cannot simply max out the mud pressure as it can lock up the drilling head. Some drilling operations actually rely on under pressure for drilling since the kick helps break up the rocks. There are specialized blowout preventers that minimize any danger of ruptures.

Better to play it safe and redrill lower portion with the proper weighted mud. As well as possibly losing the entire well if they had continued; the overpressued mud could have ruined the upper gas sands so they could not be produced.