I did some research last night on where the gas in Oregon comes from and I thought it was interesting. Based on the Wikipedia article below, almost all the petroleum comes from Washington refineries. There are a total of five refineries that are in Washington (Ferndale (Phillips 66), two in Anacortes (Tesoro and Shell), Tacoma (US Oil), Cherry Point (BP)). These refineries are supplied crude oil by a few places; Alaska's North Slope (by pipeline and ship), Alberta's oil sands (pipeline or rail), "tight oil plays" in the CONUS (by rail from the Bakken fields in Montana and North Dakota), or by ship from foreign sources (Russia, Middle East, South America, etc.). Approximately 80% of it comes from Alaska and Canada though, 58% and 21.5% respectively. Washington is a net exporter of refined products, mainly to Oregon, but also California, and British Columbia. Oregon has no refineries and gets almost all fuel from Washington via the Olympic Pipeline. In NW Oregon there are six terminals mostly in northwest Portland on Hwy 30 where the gas is put on trucks for transportation to the stations. I'm not sure where each individual brand then adds their additives though, I assume it is at the terminal facility that the fuel trucks are loaded.
https://en.wikipedia.org/wiki/Petroleum_refining_in_Washington_state
http://www.sightline.org/2013/12/04/washingtons-oil-where-does-it-come-from/
https://en.wikipedia.org/wiki/Alaska_North_Slope
http://www.akbizmag.com/Alaska-Business-Monthly/May-2012/Following-North-Slope-Crude-From-the-ground-to-the-gas-station/
https://en.wikipedia.org/wiki/Trans-Alaska_Pipeline_System
https://en.wikipedia.org/wiki/Athabasca_oil_sands
https://en.wikipedia.org/wiki/Bakken_Formation
https://en.wikipedia.org/wiki/Tight_oil
http://www.chevronpipeline.com/pdf/CPL-REPages506.pdf
https://www.bp.com/en_us/bp-us/what-we-do/bp-pipelines/our-pipelines-and-terminals/olympic-pipeline.html
https://en.wikipedia.org/wiki/Trans-Alaska_Pipeline_System
http://blogs.platts.com/2010/08/23/unbranded_vs_br/
https://www.oregon.gov/deq/Programs/Pages/Gasoline-Terminals.aspx
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=277725
https://en.wikipedia.org/wiki/Petroleum_refining_in_Washington_state
http://www.sightline.org/2013/12/04/washingtons-oil-where-does-it-come-from/
https://en.wikipedia.org/wiki/Alaska_North_Slope
http://www.akbizmag.com/Alaska-Business-Monthly/May-2012/Following-North-Slope-Crude-From-the-ground-to-the-gas-station/
https://en.wikipedia.org/wiki/Trans-Alaska_Pipeline_System
https://en.wikipedia.org/wiki/Athabasca_oil_sands
https://en.wikipedia.org/wiki/Bakken_Formation
https://en.wikipedia.org/wiki/Tight_oil
http://www.chevronpipeline.com/pdf/CPL-REPages506.pdf
https://www.bp.com/en_us/bp-us/what-we-do/bp-pipelines/our-pipelines-and-terminals/olympic-pipeline.html
https://en.wikipedia.org/wiki/Trans-Alaska_Pipeline_System
http://blogs.platts.com/2010/08/23/unbranded_vs_br/
https://www.oregon.gov/deq/Programs/Pages/Gasoline-Terminals.aspx
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=277725