A few ideas from The Economist, "When oil is no longer in demand": 

Long-term demand is falling and the market could quickly become oversupplied. It was the Model T for millions of consumers that drove the horse off the road. It will be when the internal-combustion engine eventually loses its remarkable grip on the world’s roads that the age of oil will come to a screeching halt.

Oil companies will have to explore new lines of business, like building wind-farms.