My Gas Fireplace won't stay on!?

I'm not sure of the make and model of my gas fireplace, but it will not stay on more than 15 minutes. I light the pilot then turn the unit on after the pilot is lit. It will stay on and heat the room for about 15 minutes 20 min. max and then it goes off completely. I then have start the process all over again starting with relighting the pilot! Does anyone have any ideas?