If just six firms control over 90% of a state's oil refining capacity is this an example of an oligopoly?

Usually when you look at market poer for monomopoly and oligopoly you do the relabant market and concentration ratio of the top 4 or 8.





So the big question is more about the relavant market. Six firms with 90% sounds bad but if you can buy from the next state then it could be no big deal.





For example in Chicago one of the big refineries that supply the area is in Whiting Indiana....different state.If just six firms control over 90% of a state's oil refining capacity is this an example of an oligopoly?
To be an oligopoly the *market* (not the state) must be dominated by a small number of companies and generally the term only applies if the companies act in concert (collusion). Does this state import comparable refined oil products from other states ? Is there any fee or tariff for importing from other states ? Is the cost of product transport so high that import is impractical ?





Frankly I'm surprised that there are even 6 refiners in one state. How many refiners operate in the nation ? Is there any *real* evidence that they act in concert (illegal in the US) ? In the US there can be no interstate tariff. I seriously doubt you could call this an oligopoly.





For an example of an oligopoly .. consider the cell phone service companies. There are only a few and they control the market (no alternatives). They all have surprisingly similar cost structures. *IF* they acted in concert to create this cost structure we could consider them an oligopoly.If just six firms control over 90% of a state's oil refining capacity is this an example of an oligopoly?
Yes but what would 100 firms do for efficiency? How much does a refinery cost? Is a small refinery as efficient as a large one? How good are economists at evaluating technology?