The losses currently amount to less than 1% of total US crude production, which is around 13 million barrels a day, but they are expected to linger or even rise in North Dakota. In the city of Williston, at the center of the Bakken formation, below-freezing temperatures are expected throughout the end of the month, posing continued challenges. Oil producers there may need at least a month to restore output to normal levels after more than half of the state’s flows were cut off this week, state officials said.