Used to be very common in Illinois. The Clark refinery in Wood River was one that took sour crude for distillation, and cracked it all the way to coke. There were several mountainous piles of black coal like stuff along the perimeter of the plant.. Clark was usually a penny or two cheaper, and I never had any issues with it. Clark became Premcor, and dumped all of the retail operations in the midwest. I guess maybe the name is being resurrected??
When a teenager in Jefferson City, MO, there was a Clark station next to where I worked part time. Yes, they were alway 2 or 3¢ cheaper. My fondest memory was that they sold beer to minors if you promised to take the beer home and not drink while driving.