"a substantial buildup of liquid capacity over the next several years," from so-called nontraditional petroleum sources that will become more common over time.

what does he mean by this?

The only nontraditional petroleum source would be oil sands, I think.
Surely this means tar sands, oil shale, coal-to-liquids, biofuels and maybe even NGL's but i would imagine thats included under the umbrella of conventional sources.