It is not just electric cars that could impact the oil markets in the next decade or two. China and India have large populations that are now car buyers who are purchasing vehicles with much higher levels of efficiency than their current automobile fleet. Just a 20 or 30% improvement in fuel efficiency alone drops demand for oil substantially.
Further, new technology is on the horizon using super-capacitors to store electric charge. Once this tech becomes available, the short distance limitations of electric vehicles will no longer be a barrier.
An interesting company that is working on this technology is Sunvault Energy. I own a few shares myself.