What is petroleum? There are some places on earth where oil is not trapped underground, but actually seeps out at the earth's surface—Think Jed Clampett and a "bubbling crude!" This is one of the largest natural petroleum seeps, located in the upper Ojai Valley, California.

pe·tro·le·um petroleum

A thick, flammable, yellow-to-black mixture of gaseous, liquid, and solid hydrocarbons that occurs naturally beneath the earth's surface, can be separated into fractions including natural gas, gasoline, naphtha, kerosene, fuel and lubricating oils, paraffin wax, and asphalt and is used as raw material for a wide variety of derivative products. (American Heritage Dictionary)

The word petroleum comes from the Latin petra, meaning “rock,” andoleum, meaning “oil.”

The oil industry classifies "crude" by the location of its origin and by its relative weight or viscosity ("light", "intermediate" or "heavy"). The relative content of sulfur in natural oil deposits also results in referring to oil as "sweet," which means it contains relatively little sulfur, or as "sour," which means it contains substantial amounts of sulfur.