Two shale gas companies with roots in the energy-rich Appalachia region are set to combine forces, creating an energy giant that spans the Marcellus and Utica shale formations.

Pittsburgh-based EQT Corp. said Monday that it had reached a deal to acquire Canonsburg, Pa.-based Rice Energy for cash and stock worth about $6.7 billion.

Following their combination, EQT will have gas-producing assets spanning 1.5 million acres, primarily in Pennsylvania, Ohio and West Virginia.

“This transaction brings together two of the top Marcellus and Utica producers to form a natural gas operating position that will be unmatched in the industry,” EQT CEO Steve Schlotterbeck said in a statement.

Most of Rice’s energy assets are located adjacent to EQT assets, meaning that the new company will have greater capability to drill laterally for future wells, especially in Green and Washington counties in Pennsylvania.

EQT’s shares fell 5.3% to $55.68 shortly after the opening bell Monday. Rice’s stock…