Some shale formations in Poland and China have thwarted drilling efforts by Exxon Mobil Corp, Bloomberg reported on Thursday.
Drilling techniques that have worked well in the United States have failed, said Rex Tillerson, the firm's chief executive officer.
Exxon failed in its first two efforts to crack gas-rich shale fields in Poland and what gas was reached would not flow, he said.
"Some of the shales don't respond as well to hydraulic fracturing," he said. "It's going to take research and time in the lab to understand that.'
The company is looking at other methods to hydraulically fracture or frack the shales.
Exxon Mobil has shale leases on nearly 1.4 million acres, mostly in west Texas and New Mexico..
Click here to read the Bloomberg story.
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.