There’s a very interesting article in the Pittsburgh Post-Gazette about the cracker plants that are planned for the Marcellus/Utica region, and why oil prices affect whether to build one or not.  What it boils down to is that you can use either oil or ethane to make the same product.  When oil prices are high, the ethane has a big price advantage over oil, so an ethane cracker plant is very attractive.  When oil prices are low, the advantage isn’t so great.  At $50/bbl, the cracker plants in this area would still be making money, but not enough to strongly attract the investment capital that is necessary to build.  Hence the current wait-and-see attitude of Shell and Odebrecht.