America’s top shale field is becoming increasingly gassy as drilling slows down, undercutting profits for explorers at a time when investors are demanding better returns.

Natural gas has long been a nuisance in the Permian Basin in Texas and New Mexico, where a massive glut weighs on prices, with crude producers sometimes having to pay to get it hauled away or burn it off in a controversial practice known as flaring. Now the problem is intensifying as wells age and fewer new wells are drilled.