<p>There is no need to double major. You will be able to keep your job as a Petroleum engineer for at least 45 more years because by then you will have a lot of experience and experienced petroleum engineers will stay in high demand.</p>
<p>Even if you did need to transition Petroleum engineers can also move into geothermal, mining, geological, natural gas, metallurgical, mineral and other similar engineering careers without much trouble.</p>
<p>Conventional Oil will last until about 2060, probably longer.</p>
<p>I recommend a masters in something instead like geophysics to compliment your petroleum degree.</p>