did they come from?
of the Alberta oil sands is a controversial subject. Two primary theories
sands are the remnants of a once vast reserve of crude oil that, over
extremely long periods of time, has escaped or been destroyed
microbiologically; thus leaving behind some bitumen and also converting the
lighter crude oil into bitumen through bacterial processes.
bitumen evolved from highly organic cretaceous shales (similar to oil
shale). Underground pressure forced the bitumen out of the kerogen rich shales
where it soaked into existing silt grade sediments and sand bodies.
first theory, petroleum would be formed in the traditional manner, and then
converted to bitumen by some additional process. A summary of the oil
formation process is presented here, and a more complete version is
available on the PETROLEUM page.
process, in general, begins with the building of organic matter through the
action of PHOTOSYNTHESIS. In the case of marine deposits, this
begins with phytoplankton, which then enter the food chain that leads to
the sedimentation of zooplankton rich in proteins, carbohydrates, and
lipids. In terrestrial burial, the process begins in higher order organic
life such as trees and shrubs, which then deposit primarily carbohydrates
and lignin. The fraction of minerals to organic matter and the composition
of the organics themselves lead to differences in the way that organic
chemical reactions proceed and are catalyzed.
organic matter is buried, the process of diagenesis compacts it into
organic rich sedimentary rocks. Further degrees of diagenesis then lead to
the generation of kerogen which is then, through deeper burial and
catagenesis, transformed through various means into crude oil and natural
gas, or coal, depending on the precise conditions. In the case of oil shale
and, according to the second theory, bitumen sands, the process of
catagenesis is never allowed to be completed due to insufficient burial
(temperatures and pressures too low). For the first theory, we allow
complete catagenesis and petroleum formation; with a third component that
represents eventual decay to bitumen.
Step 1: Diagenesis forms
is a process of compaction under mild conditions of temperature and
pressure. When organic aquatic sediments (proteins, lipids, carbohydrates)
are deposited, they are very saturated with water and rich in minerals.
Through chemical reaction, compaction, and microbial action during burial,
water is forced out and proteins and carbohydrates break down to form new
structures that comprise a waxy material known as “kerogen” and a black tar
like substance called “bitumen”. All
of this occurs within the first several hundred meters of burial.
bitumen comprises the heaviest components of petroleum (i.e. asphalt), but
the kerogen will undergo further change to make hydrocarbons and, yes, more
Catagenesis (or “cracking”) turns kerogen into petroleum and natural gas
temperatures and pressures increase (deeper burial) the process of
catagenesis begins, which is the thermal degradation of kerogen to form
hydrocarbon chains. The conditions of catagenesis determine the product,
such that higher temperature and pressure lead to more complete “cracking”
of the kerogen and progressively lighter and smaller hydrocarbons.
Petroleum formation, then, requires a specific window of conditions; too
hot and the product will favor natural gas (small hydrocarbons), but too
cold and the organic matter will remain as kerogen.
second theory of oil sand formation, vast organic deposits underwent
diagenesis to form kerogen rich compacted sediments similar to oil shale.
The process of catagenesis may then have occurred to some extent, but in
general, the process did not reach sufficient temperature to undergo
complete alteration of the kerogen. Over extremely long periods of time,
some portion of this kerogen was transformed into bitumen products, which
then seeped into the surrounding sands.