I'm not an economist, but my understanding is that the price of oil does not directly impact the valuation of these companies, per se, but that growth-oriented companies (especially) and oil sales depend on a strong global economy.

When the price of oil falls, that indicates that people are pulling back on oil/gas/etc purchases due to constrained budgets, reduced business/operations, etc. (and/or that people buying the oil predict that others will cut back on those things in the near future)