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This works because refineries are in just a few hands now. This was big oil's plan back in the mid 1990s. The documents are online and I've linked to them before in the past. Collusion isn't required because cutting back is in their best interests without even needing to talk to the other guys. They know the other guys will cut back. They know that there isn't going to be any significant threat from independents looking to gain market share. They know nobody is going to go build a refinery to undercut them either. It simply isn't a free market and government regulation protects them from competition.
There can never be monopoly or effective collusion in a true free market because in a true free market someone can always come in and under cut their price.