For the first six months of 2012, more than 85 percent of Pennsylvania's production of Marcellus shale natural gas came from six counties: Bradford, Susquehanna, Lycoming, Tioga, Washington and Greene counties.
Bradford County is No. 1 and produced 234 million MCF of natural gas or 26 percent of the state total, the Marcellus Drilling News reported.
Susquehanna County is No. 2 with 188 million MCF or 21 percent.
The top-producing energy companies is Chesapeake Energy Corp. with 21 percent of the state's production.
No. 2 is Cabot Oil and Gas Corp. with 12 percent.
Talisman Energy USA Inc. is No. 3 with 11 percent.
A total of 59 companies are listed in the state records.
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Chesapeake Energy Corp,the Oklahoma-based firm is the No. 1 driller in Ohio.
Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.