The world petroleum means literally rock oil. It was thought to be a completely different oil than that which comes from vegetable sources such as the olive, but modern research has traced its origin to the lipids (oils) of planktonic (free floating) plants and animals which live in brackish water such as blue-green algaes and foraminifera. The brackishness is essential because aerobic bacteria does not live in brackish water. Aerobic bacteria would decompose all of the organic matter. In brackish water the organic matter of the planktonic plants and animals sinks to the bottom and is incorporated in clay sediments which ultimately become sedimentary rocks. This is the origin of the oil shales.

Under high pressure and temperature the oil of clay shales can be squeezed out and into porous rock. In porous rock the oil can travel until it reaches an impervious barrier such as a salt dome.

The evidence for the organic origin of petroleum is overwhelming:

• Petroleum is found only in association with sedimentary rocks. There is no petroleum associated with igneous or metamorphic rocks.
• Polarized light passing through all petroleums undergoes a rotation that is the same as for the organic oils.
• Chlorophyll and hemin are found in petroleum.

(To be continued.)