Recent years have offered an extraordinary set of conditions for US refining, but these market conditions may or may not last."Thanks to the advance of fracking, the US had a revival in refining," said Pedro Caruso, a partner in the Oil & Gas practice at Bain & Company. "The key question looking forward ... what is going to happen with that differential? Is that differential sustainable? Will it widen again to the levels we saw in the last few months? And that will determine whether the refineries in the US do well or not."