Schlumberger had its debt rating lowered by S&P Global Inc. (Bloomberg) — Schlumberger Ltd. had its debt rating lowered by S&P Global Inc. as belt-tightening in the U.S. shale patch translates into less drilling and fracking work for the world’s top oilfield services provider. The rating was cut a notch to A+, the fifth-highest invest grade, from AA-, S&P said on Friday. Its biggest rival, Halliburton Co., had its outlook revised to negative from stable by the ratings firm. Under pressure from shareholders, exploration and production companies are keeping spending in check, which is reducing demand for oilfield services, S&P said. “Oilfield services companies will no longer be able to generate the high operating margins they did in 2014,” Carin Dehne-Kiley, an analyst at S&P, wrote Friday in a report to investors. “The oilfield services industry has fundamentally changed due to permanent efficiency and productivity gains realized by E&P […]