2.0 Origins of Oil and Gas
We are all aware that oil and gas are
recovered from deep below the earth's surface, but how did it get there in
the first place?
The most popular theory is known as the Organic
Theory. This theory states that oil and gas have biological origins.
Small sea creatures from the days when the earth was mostly covered in
water died and settled to the bottom of the ocean floor. Layer upon layer
of silt, sand and clay built up on top of them over time. Through the
process of decay, as well as ever increasing heat and pressure, the former
sea creatures were converted to oil and gas. Over millions of years,
continuous pressure actually compressed those layers of silt and clay into
layers of rock. This is known as "reservoir rock".
A very nice description of the generation,
migration and accumulation of petroleum is given in chapter 11 of Hyne
[7]. The temperature under the earth's surface increases the deeper you go
underground. At about 150 F, oil begins to form. Oil formation ceases at
about 300 F. Oil formed at lower temperatures (i.e. closer to the surface)
is called immature and is heavy. Oil formed deeper under the
surface is called mature and is light. At temperatures above 300 F,
oil is thermally cracked to produce light gases (i.e. natural gas). Since
temperature increases with depth, natural gas wells are typically drilled
much deeper than oil wells. Of interest in Canada is the formation of the
tar sands in Northern Alberta. They are thought to have formed from the
same oil that is recovered conventionally in other areas of the Western
Canada Sedimentary Basin, but at one period in the Earth's geological
history, the oil seeped to the surface where it degraded into tar.
This is probably the story of the origin of
oil and gas that we are all familiar with. However, there is an
interesting alternative theory. It is known as the Inorganic Theory and
was developed by Mendeleev (he of the periodic table) in the early 1800's.
He believed that petroleum came from deep within the earth, noting how
petroleum seemed to be associated with large features of the earth like
mountains and valleys rather than the finer scale sedimentary deposits.
There are many today who believe in this theory, and there is some
evidence to back them up. The following observations are taken from the
text "Modern Petroleum: A Basic Primer of the Industry", 3rd
Edition, by Berger and Andersen, Pennwell Publishers (1992):
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