In what may be one of the largest oil spills ever on Alaska’s North Slope — and a poorly timed one for the oil industry — 92,400 gallons of saltwater and crude oil leaked from a pipeline on the Kuparuk oil field Sunday night. Phillips, which operates the field, said yesterday it had cleaned up most of the spill, which is the fourth major one to occur on the North Slope this winter. While the spill was more than 97 percent saltwater, the accident draws attention to the risks of drilling just as Big Oil and state leaders are trying to make the case that the industry has a positive environmental record and the Arctic National Wildlife Refuge should therefore be open to drilling.