When petroleum engineers strike crude oil, they analyze it. There are two important qualities that are analyzed – the crude oil density, which determines whether the crude oil is classified as light or heavy and the sulfur content of the oil.
Heavy oil is slow when it flows and its constituents are also heavier. Refining heavy oil is more difficult and when it is used as a fuel, it also pollutes more. In contrast, during refining, light oil does not need as much special treatment, so the refining cost is also lower. Light oil does not pollute as much as heavy oil and you can sell it for a higher price.