Merging the Lower 48 portfolios of Chesapeake Energy Corp. and Southwestern Energy Co. is set to create the largest natural gas pure-play in the United States, becoming a “must-own” company with the resources to redefine the global energy reach of independent producers.

The $7.4 billion combination announced on Thursday was expected, as the tie-up has been rumored for months. The transaction, now set for completion by mid-year, would bring together two of the biggest natural gas independents in the United States. They built their assets across the gassy onshore basins using the unconventional drilling techniques that ushered in the golden age for U.S. natural gas in the 2010s. 

The merger “is the biggest gas-focused U.S. upstream deal in more than 10 years and reflects...