This is interesting. The Energy Information Administration has put together a brief report on the impact of horizontal drilling in Pennsylvania. It begins: "Between 2009 and 2011, Pennsylvania's natural gas production more than quadrupled due to expanded horizontal drilling combined with hydraulic fracturing. This drilling activity, which is concentrated in shale formations that cover a broad swath of the state, mirrors trends seen in the Barnett shale formation in Texas."
You should read the whole thing (animations, graphics, charts, it has everything) (see here).