Q&A: Why is the attack in Saudi Arabia affecting oil prices?

The strike on the heartland of Saudi Arabia's oil industry, including damage to the world's biggest petroleum-processing facility, has driven oil prices to their highest level in nearly four months. Why is this so disruptive for global oil supplies? The attack on Saudi oil facilities on Saturday not only knocked out over half of the country's production, it also removed almost all the spare capacity available to compensate for any major disruption in oil supplies worldwide.