Jerome a Paris has an excellent summary article on natural Gas, with some impressive, though discouraging, graphs of production.  He cites two sources that are worth following up in their own right.  There is a pdf on California by David Maul, which has, in slide 10, the grim picture of the declining life of annual gas discoveries.

He also cites a paper on Texas production that covers much of the story from that part of the world.  Put together they emphasize how hard folks are having to scramble to keep us afloat in natural gas.  With decline rates of up to 45% the supply is critically dependant on new drilling and with smaller and smaller fields being tapped and lasting shorter periods of time, this is not a game with a foreseeable happy ending. (Thanks for the pointer jkissing)