MadSci Network: Earth Sciences

Re: How do we know that petroleum oil comes from dead animals?

Area: Earth Sciences
Posted By: Nick Hoffman, Physics
Date: Sun Jun 15 23:11:00 1997
Area of science: Earth Sciences
ID: 865725975.Es
Well actually, Stephen, it doesn't....

Or at least, most of it doesn't. 

Hydrocarbons, (Oil and Gas) come from buried organic matter which is either 
derived from plants, bacteria, algae, simple organisms, or rarely from more 
complex wriggly things that you would actually call 'animals'.

This organic material (called Kerogen) gets buried in sediment (sometimes 
it IS the sediment) and preserved from oxidation by the nature of the
environment of deposition. 

Normally, wood and leaves rot down over a few years to re-enter the carbon
cycle, but if the groundwater/ lake water/ or seawater is stagnant and 
anoxic, then oxidation cannot take place. Anaerobic bacteria can live in 
these environments and do often modify the kerogens, but don't destroy 
them. Note that only the lower layers of the water need to be stagnant, 
the surface layers may be clean and well-oxygenated, overlying this 
stagnant smelly stuff.

With time and burial the rocks are compressed and heated (because the 
inside of the Earth is hot and heat slowly leaks out, so deeper buried 
rocks are hotter than shallow ones). The kerogens are gently cooked and oil 
sweats out of them.  Later, if burial continues, the rocks begin to bake 
and gas is released. Finally the rocks can get so hot that the organic 
material would "burn" if there was any oxygen. What happens insted is that 
the material is transformed into graphite (pure carbon) films and amorphous 

How do we know all this?

There is a clear progression from recently buried organic material through 
the whole process of kerogen transformation to oil and gas. Millions of 
samples have been recoved from rock outcrops and from drilling. In 
addition, special chemicals called BIOMARKERS are clearly derived directly 
from parent organic molecules in the original living plants, algae, 
bacteria, and animals.

We can even measure these biomarkers and match particular oils to a source 
rock of known age, environment, depositional facies, flora/fauna and degree of burial. 

For example, Indonesian Oils generally come from Tertiary age Pro-delta 
shales with a high land-plant content. North Sea Oils come from Jurassic
Marine shales from a strongly anoxic environment.

One aspect of my job is investigating these links between source rocks and 
oils, but we have specialists called GEOCHEMISTS who do it full-time.

Nick Hoffman
Petroleum Geophysicist

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