Which is a Pure play company in oil industry?

Frontier Oil Co, Continental Resources, ExxonMobil and Plains All American can all be considered to fall within the "Oil Industry" but to be a little more helpful, one should decide on which component of the oil industry to focus on i.e refiners, drilling/exploration/production, pipeline/transportation/marketing, or fully integrated oil companies.

The above examples would fall into these more specific classes of oil companies as follows:

Pure Play Refiner: Frontier Oil Company

Pure Play Driller/Producer: Continental Resources

Pure Play Tranporter/Pipeliner : Plains All American

Pure Play fully integrated oil company: ExxonMobil