Barnett Shale in Texas is a Natural Gas Source, not a significant source of oil. Woodford Shale and Fayetteville Shale are also a Natural Gas fields.
"that's not entirely accurate" -CIA director Independence day.
On the western side of the Barnett we are finding oil at depth under the gas shales. Myself and some geosicence students from a major university are making a field trip to the shale in the next couple of weeks to do some practice seismic analysis. This oil is unconventional as its is in a tight pore formations that requires hydraulic fracturing to make the formations permeable. This technology is new and relatively expensive. That being said with oil prices they way they are it is still economical to use hydraulic sand fracturing to get at the oil since the Barnett is a relatively shallow formation coil on tube rigs can be used significantly lowering production costs. some history on the Barnett. The Barnett is a black shale of Mississippian age who's sediments were shed in deep marine anoxic waters off the coast from the former Oouachian mountains that ran from Mexico through Texas to Oklahoma and Arkansas. Being an anoxic basin the fossil carbon was not oxidized and under went petrogenesis. pretty much anywhere you had similar black shales laied down oil and gas would have been produced if the rocks were heated and pressurized to petrogenesis conditions; the source rocks also make a great petroclude since they are not permeable without stimulation. basicly the oil/Ng was produced and could not migrate away from the source rocks. It will just be sitting waiting for someone to fracture the formation and release the overburden pressure.
Have you looked at what is going on in Montague, Clay, and Archer Counties in Texas?
http://www.topix.com/content/kri/2008/02/in-shift-eog-interested-in-barnett-shale-oil