Page 12 - Practical Petroleum Geology, 2nd Edition

PRACTICAL PETROLEUM GEOLOGY Oil and Gas Accumulation

Petroleum Extension-The University of Texas at Austin

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In this chapter:

• The origins of petroleum
• Primary and secondary migration of petroleum

• The methods by which petroleum accumulates Oil and Gas
• Types of traps and their characteristics
Accumulation

For most of the time that humans have been aware of oil and natural gas,
these substances were thought of as minerals that had formed out of nonliv-
ing rock, such as gold, sulfur, and salt. Although oil had an odor suggesting
organic matter and although natural gas burned like swamp gas, most of
the gas and oil escaping from the ground seemed to come from solid rock
deep beneath the surface, where nothing lived.
However, beginning about two centuries ago, the geologic insights of
scientists such as James Hutton, Charles Lyell, and others showed that the
rocks in which oil was found were once loose sediment piling up in shallow
coastal waters where fish, algae, plankton, and corals had once lived. As a
result of such insights, it seemed possible that oil and gas had something
to do with the decay of dead organisms, just as coal, with its leaf and stem
imprints, seemed to be the fossilized remains of swamp plants.
Later advances in microscopy revealed that oil-producing and oil-
bearing rocks often contain fossilized creatures too small to be seen with
the unaided eye. Chemists discovered that carbon:hydrogen ratios in pe-
troleum are much like those in marine organisms and that certain complex
mole cules are found in petroleum source rocks that are otherwise known to
occur only in living cells. But it was the fact that most could be shown to
have originated in an environ ment rich with life that clinched the organic
theory of the origin of petroleum.

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