U.S. shale companies are poised to earn real money this year for the first time during the fracking boom.

Crude prices have surged almost 40% in the past six months. Yet shale producers are so far touting cautious spending plans for 2018—in contrast with past price spikes, when companies mounted aggressive drilling campaigns that quickly increased supplies and pushed prices back down. While the companies have posted profits in the past, they have never done so while limiting spending to the amount of cash they generate from...