All three companies are contributors to the growing onshore U.S. oil and gas industry.The price of West Texas Intermediate (WTI) crude oil -- the U.S. benchmark -- just fell below $70 a barrel, sending ripples throughout the energy sector. Long-term investors may view lower oil prices as an opportunity to scoop up shares of quality energy stocks on sale.There are plenty of ways to invest in oil and gas, such as an integrated major like ExxonMobil (XOM -0.47%) or exploration and production (E&P) companies li ...