The era of “exponential” growth in the U.S. oil and gas industry is over as most shale firms are returning cash to investors instead of going into debt to drill more, according to Halliburton, the world’s largest fracking services provider. “We’ll see growing investment, but quite frankly, nothing even close to what we saw from 2008 to 2014,” Halliburton’s CEO Jeff Miller said at a panel at the ADIPEC energy conference in Abu Dhabi on Tuesday, as carried by The National. “Companies were spending…