Haynesville Shale.

shreveporttimes.com: CEO: Haynesville Shale is fourth largest in the world.

Quotes:

The Haynesville Shale is likely to become America's largest natural gas field and perhaps the fourth largest in the world, Chesapeake Energy Chairman and CEO Aubrey McClendon disclosed Wednesday in a conference call with its newest partner, Plains Exploration and Production Co.

See, among others:

CHK & PXP

Encana & Royal Dutch Shell

Petrohawk

and..

GDP
PVA & GMXR
CRK
STR
EP
SM
XCO
FST
DVN
APC
HP (driller)
XTO & Hunt
BRY & O'Brien
COG & undisclosed

Did I forget anybody? Probably.