Crude petroleum was created by dead plankton that sank to the bottom of the seabed millions of years ago. Over the course of time, sand and stones built up on top of it. Due to this impermeable layer and under oxygen exclusion, pressure and heat, the conversion of these "lifeforms" into crude petroleum took place. The basic building blocks of crude petroleum are hydrocarbon compounds, which can occur in various chain lengths (C5 - C100).