Do you understand that oil has many grades, qualities and those are priced differently?
Do you also understand that much of our Gulf Coast had billions of dollars upgrading the refineries to use cheaper, heavier oil?
Do you understand that most of our increased oil production is light sweet and the Gulf Coast has essentially saturated the market available for that oil?
EIA tracking tool shows light-sweet crude oil imports to Gulf Coast virtually eliminated
http://www.eia.gov/todayinenergy/detail.cfm?id=19931
Market prices include transportation limitations. The pipeline bottle neck getting oil out of Cushing is the reason the WTI price separated from the Brent after beening nearly the same for many years, since their are essentially the same oil, but available in different locations.
So, in effect, were talking about a narrow market near saturation. From your data, it does appear that the price gap was closing as of the end of 2014. How much more could be expected?