March 9, 2015: What are the origins of oil?

Oil is a complicated mixture of different hydrocarbons. A hydrocarbon is a large organic molecule composed of hydrogen atoms attached to a backbone, or chain of carbon atoms.

Oil originates from fossil remains of microscopic plants and animals that lived millions of years ago which were deposited at the bottom of ancient oceans.  Some land plants contribute to this buildup of organic “mush” on the sea bed, eventually becoming part of the layers of sedimentary rock.

As the sediments accumulate, heat and pressure from the earth and rock cause chemical and physical changes.  At approximately 90°C, the “mush” changes into a liquid state called liquid oil.

The oil is less dense than water and therefore migrates upwards through the spaces between sand grains that make up rock. . The rising oil is trapped within the rock in pockets called reservoirs.

Once oil is identified, companies drill thousands of feet through layers of sand and sedimentary rock to reach the formations that contain oil. The oil is then pumped out and transported through pipelines or oil tankers to the refinery for processing.   Next week: the toxicity of oil refining.  Stay tuned.


  1. Department of Energy: Chemical Reaction Model for Oil and Gas Generation from Type 1 and Type 2 Kerogen.
  2. Geochemistry Research: Organic Origins of Petroleum.


This question prepared by: Elmang Nchako, MD, MPH  (Occupational Medicine Resident-PGY 3)   Department of Family and Community Medicine  Meharry Medical College


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