I do not have a link, but I would hazard to guess that the majority of the propane on the market is actually from the NGL's (ethane, propane and butane) that are removed from natural gas. Separating the heavier components from methane is much more profitable.

I believe Robert mentioned that a lot of the lights in the crude are used in the refinery as fuel gas to fire various heaters.

Yes, I do know that the propane we buy is a mix from petroleum and NG. NG is peaking too, unfortunately. However, I do wonder about all that stranded NG that is just being flared - might it not be possible and economic to separate out the propane first and ship that?