I thought all the bad reviews and the insane budget would've killed a sequel.

...

it wasn’t clear that the story would be give a chance to continue if ticket sales didn’t pick up in the coming weeks. Unfortunately for Warner Brothers, the movie dropped a disastrous 66% in its second weekend, leaving its current grand total at $118 million worldwide. Taken from a $200 million budget, that’s not great news, and with more huge movies just around the corner, Green Lantern’s window of opportunity is rapidly closing. However, sources say that Warner Brothers is already hard at work planning the sequel and that they will stand behind the movie no matter what happens....

http://www.filmjunk.com/2011/06/28/green-lantern-2-already-has-a-green-light/