Petroleum (literally from the Greek, "rock oil") is a thick, dark brown or greenish inflammable liquid, which, at certain points, exists in the upper strata of the earth. It consists of a complex mixture of various hydrocarbons, largely of the methane series, but may vary much in appearance, composition, and properties. Biological material in rocks starts off largely as a waxy material known as kerogen. Under the influence of heat and pressure, kerogen breaks down first into liquids and to gases. Both the liquid (petroleum) and gas phases (natural gas) tend to migrate through porous rocks until they encounter impermeable beds where packets/pools will tend to collect. After a drilling and pumping process to extract it from the strata, petroleum is refined by distillation. The products include kerosene, benzene, gasoline, paraffin, etc.