If you want to help Bhelen, you can talk to his agent, Vartag. He's in the Assembly Chamber by the entrance. You can just talk to him and accept his mission. It seems that Harrowmont has a dirty land deal in the works. You need to spread the word to the two bribed representatives before the vote. Now, if you want, you can go ahead and try to notify the Shaperate to learn that they're forgeries. If that matters for your role playing decision, then keep that in mind.
Step out and walk along the Diamond Quarter. Lady Dace is waiting in a little side alley near the exit to the Commons. You can give her the bad news, only to learn that Lord Dace is the only one that can void the deal. He's in the Deep Roads on a mission, so we've got a little bit of exploring to do. On the bright side, we've got early access to the Deep Roads thanks to her pass.
Move out into the commons and go into Tapster's Tavern. Lord Helmi is in the back area. Just talk to him and convince him to switch his support.
We still need Lord Dace's approval though. Move further through the commons and approach the group of guards watching the mine entrance. This is the access point for the Deep Roads. Before we go, you should stop by the nearby merchant and unload as much of your inventory as you can. There's an almost obscene amount of random loot in the deep roads. This is a good way to make some easy money in Dragon Age Origins. When you're ready, you can show the miners your pass to gain entry and then move inside. This is like the Denerim City Map. We only have one place to go though. Head on to Aeducan Thaig and get ready for a battle.