Below $40 a barrel, the next likely production slowdown would come from Canadian tar sands fields, which start to lose money when the Brent price hits the high $30 range. But turning the flow off and restarting it again is a complex process, involving injecting steam into the ground, which makes it costly to restart, according to Wood Mackenzie. On the other hand, fuel represents a major cost for oil sands production, so lower oil prices could help lower overall production costs.