Crude oil is far from being one homogenous substance. Its physical characteristics differ depending on where in the world it's pulled out of the ground, and those variations determine its usage and price.
The Energy Information Administration (EIA) puts it succinctly: "not all crude is created equal." Some has a lot of sulphur, and it's called sour. Oil with less sulphur is called sweet. Crudes also vary in how dense they are. Sweet, light crude is the most valuable type of oil. Sour, heavy oil fetches the lowest prices. Here's why:
This is partly because gasoline and diesel fuel, which typically sell at a significant premium to residual fuel oil and other 'bottom of the barrel' products, can usually be more easily and cheaply produced using light, sweet crude oil. The light sweet grades are desirable because they can be processed with far less sophisticated and energy-intensive processes/refineries." (EIA)
Depending on these characteristics, crude ends up at different refineries:
Refining capacity in the Gulf Coast has large secondary conversion capacity including hydrocrackers, cokers, and desulfurization units. These units enable the processing of heavy, high sulfur (sour) crude oils like Mexican Maya that typically sell at a discount to light, low sulfur (sweet) crudes like Brent and Louisiana Light Sweet. Many East Coast refineries have less secondary conversion capacity, and in general they process crude oil with lower sulfur content and a lighter density. (EIA)*
The refining process itself -- fractional distillation, followed by further reprocessing and blending -- is how we extract from crude to create the different petro-products that we use:
Crude oil is made up of a mixture of hydrocarbons, and the distillation process aims to separate this crude oil into broad categories of its component hydrocarbons, or 'fractions.' Crude oil is first heated and then put into a distillation column, also known as a still, where different products boil off and are recovered at different temperatures. Lighter products, such as butane and other liquid petroleum gases (LPG), gasoline blending components, and naphtha, are recovered at the lowest temperatures. Mid-range products include jet fuel, kerosene, and distillates (such as home heating oil and diesel fuel). The heaviest products such as residual fuel oil are recovered at temperatures sometimes over 1,000 degrees Fahrenheit. (EIA)*
That's the rough overview of how crude gets from the ground to the gas station. In recent years, new extraction methods have made more crude available.