It was inevitable. Denver-based QEP Resources Inc. rolled onto stock traders’ screens and into the S&P 500 Index on July 1, taking with it a full two-thirds of the market cap of integrated-gas parent Questar Corp.

The spin-off was the result of its success.

Ten years ago, Salt Lake City-based Questar aimed to grow its unregulated E&P business—and substantially. At the time, the E&P unit held some 730 billion cubic feet (Bcf) of proved Rockies reserves, mostly natural gas, in the San Juan, Western Canadian, Green River and other basins. ...