Schlumberger saw its debt rating downgraded by S&P due to the unfolding slowdown in drilling by U.S. shale companies. The largest oilfield service company in the world has seen its earnings hit as the shale industry goes through a soft patch. S&P cut Schlumberger’s debt rating to A+, down from AA-. Meanwhile, Halliburton saw its outlook downgraded from “stable” to “negative.” “Oilfield services companies will no longer be able to generate the high operating margins they did in 2014,” Carin Dehne-Kiley,…