I don’t know about 200 Billion Bbls, but it is a very large play. The Bakken Shale was not explored until recently because of the lack of technology. EOG recently drilled a few wells that potentialed for 2000 bbls per day—considering the extent of the basin, it could be a very large field. In addition, the Barnett Shale in Texas, the Woodford Shale in Oklahoma and Texas, and the Fayetteville Shale in Arkansas are profitable now. Most of the source rocks in the US are being looked at as reservoirs because of rapidly advancing technology. The industry is shifting from conventional to unconvential plays. High product prices and improving drilling techniques have made that possible.
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.