There are two basic schools of thought surrounding the formation of petroleum deep
within the earth’s strata. There is the more widely accepted organic theory and the not so
popular inorganic theory.
Inorganic Theories
Deep seated terrestrial hypothesis
From as early as 1877, Dmitri Mendele'ev, a Russian who developed the periodic table,
postulated an inorganic origin when it became apparent that there were widespread
deposits of petroleum throughout the world. He reasoned that metallic carbides deep
within Earth reacted with water at high temperatures to form acetylene (C2H2). This
acetylene condensed to form heavier hydrocarbons. This reaction can be easily performed
under laboratory conditions.
This theory was modified by Berthelot in 1860 and by Mendele'ev in 1902. Their theory
was that the mantle of the earth contained iron carbide which would react with
percolating water to form methane:
FeC2 + 2H2O = CH4 + FeO2
The problem with this theory is the lack of evidence for the existence of iron carbide in
the mantle. These theories are referred to as the deep-seated terrestrial hypothesis.
Extraterrestrial hypothesis.
In 1890, Sokoloff proposed a cosmic origin for petroleum. His theory was that
hydrocarbons precipitated as rain from original nebular matter from which the solar
system was formed. The hydrocarbons were then ejected from earth's interior onto
surface rocks.
Interest in this inorganic theory heightened in the 20th Century as a result of two
discoveries: The existence of carbonaceous chondrites (meteorites) and the discovery that atmospheres containing methane exists for some celestial bodies such as Saturn, Titan,
Jupiter. The only known source for methane would be through inorganic reactions.
It has been postulated that the original atmosphere of earth contained methane, ammonia,
hydrogen and water vapor which could result is the creation of an oily, waxy surface
layer that may have been host to a variety of developing prebiotic compounds including
the precursors of life as a result of photochemical reactions (due to UV radiation).
The discovery (Mueller, 1963) of a type of meteorite called carbonaceous chondrites, also
led to a renewed interest in an inorganic mechanism for creating organic compounds.
Chondritic meteorites contain greater than 6% organic matter (not graphite) and traces of
various hydrocarbons including amino acids.
The chief support of an inorganic origin is that the hydrocarbons methane, ethane,
acetylene, and benzene have repeatedly been made from inorganic sources. For example,
congealed magma has been found on the Kola Peninsula in Russia (Petersil'ye, 1962)
containing gaseous and liquid hydrocarbons (90% methane, traces of ethane, propane,
isobutane). Paraffinic hydrocarbons have also been found in other igneous rocks (Evans,
Morton, and Cooper, 1964).
Problems with inorganic hypotheses.
Firstly, there is no direct evidence that will show whether the source of the organic
material in the chondritic meteorites is the result of a truly inorganic origin or was in an
original parent material which was organically created. Similar reasoning applies to other
celestial bodies.
Secondly, there is no field evidence that inorganic processes have occurred in nature, yet
there is mounting evidence for an organic origin.
And thirdly, there should be large amounts of hydrocarbons emitted from volcanoes,
congealed magma, and other igneous rocks if an inorganic origin is the primary
methodology for the creation of hydrocarbons. Gaseous hydrocarbons have been
recorded (White and Waring, 1963) emanating from volcanoes, with methane (CH4) the
most common. Volumes are generally less than 1%, but as high as 15% have been
recorded. But the large pools are absent from igneous rocks. Where commercial
accumulations do occur, they are in igneous rocks that have intruded into or are overlain
by sedimentary materials; in other words, the hydrocarbons probably formed in the
sedimentary sequence and migrated into the igneous material (more on this later when we
discuss traps).
Conclusion: There are unquestioned instances of indigenous magmatic oil, but the
occurrences are rare and the volumes of accumulated oil (pools) are low. Other
problematic issues: Commercial accumulations are restricted to sedimentary basins, petroleum seeps and accumulations are absent from igneous and metamorphic rocks, and
gas chromatography can fingerprint the organic matter in shales to that found in the
adjacent pool. Thus current theory holds that most petroleum is formed by the thermal
maturation of organic matter - An Organic Origin generated the vast reserves (pools) of
oil and gas.
Organic Theory:
There are a number of compelling reasons that support an organic development
hypothesis.
First and foremost, is the carbon-hydrogen-organic matter connection. Carbon and
Hydrogen are the primary constituents of organic material, both plant and animal.
Moreover, carbon, hydrogen, and hydrocarbons are continually produced by the life
processes of plants and animals. A major breakthrough occurred when it was discovered
that hydrocarbons and related compounds occur in many living organisms and are
deposited in the sediments with little or no change.
Second were observations dealing with the chemical characteristics of petroleum
reservoirs. Nitrogen and porphyrins (chlorophyll derivatives in plants, blood derivatives
in animals) are found in all organic matter; they are also found in many petroleums.
Presence of porphyrins also mean that anaerobic conditions must have developed early in
the formation process because porphyrins are easily and rapidly oxidized and decompose
under aerobic conditions. Additionally, low Oxygen content also implies a reducing
environment. Thus there is a high probability that petroleum originates within an
anaerobic and reducing environment.
Third were observations dealing with the physical characteristics. Nearly all petroleum
occurs in sediments that are primarily of marine origin. Petroleum contained in nonmarine sediments probably migrated into these areas from marine source materials
located nearby. Furthermore, temperatures in the deeper petroleum reservoirs seldom
exceed 300oF (141 oC) . But temperatures never exceeded 392oF (200oC) where
porphyrins are present because they are destroyed above this temperature. Therefore the
origin of petroleum is most likely a low-temperature phenomenon.
Finally, time requirements may be less than 1MM years; this is based on more recent oil
discoveries in Pliocene sediments.
However, physical conditions on the Earth may have been different in the geologic past
and therefore it may have taken considerably more time to develop liquid petroleum.
Organic Hypothesis - Summary.
The organic theory became the accepted theory about the turn of the century as the oil
and gas industry began to fully develop and geologists were exploring for new deposits.
Simply stated, the organic theory holds that the carbon and hydrogen necessary for the
formation of oil and gas were derived from early marine life forms living on the Earth
during the geologic past -- primarily marine plankton. Although plankton are
microscopic, the ocean contains so many of them that over 95% of living matter in the
ocean is plankton. The Sun's energy provides energy for all living things including
plankton and other forms of marine life.As these early life forms died, their remains were captured by the processes of erosion
and sedimentation
Successive layers of organic-rich mud and silt covered preceding layers of organic rich
sediments and over time created layers on the sea floor rich in the fossil remains of
previous lifeThermal maturation processes (decay, heat, pressure) slowly converted the organic matter
into oil and gas. Add additional geologic time (millions of years) and the organic rich
sediments were converted into layers of rocks. Add more geologic time and the layers
were deformed, buckled, broken, and uplifted; the liquid petroleum flowed upward through porous rock until it became trapped and could flow no further forming the oil and
gas reservoirs that we explore for at present But the chemistry of the hydrocarbons found in the end product (oil, gas) differ
somewhat from those we find in living things. Thus changes, transformation, take place
between the deposition of the organic remains and the creation of the end product. The
basic formula for the creation of petroleum (oil, gas) is:
Petroleum End Product = ([Raw Material + Accumulation + Transformation + Migration]
+ Geologic Time)
Petroleum, according to the organic theory, is the product of altered organic material
derived from the microscopic plant and animal life, which are carried in great volumes by
streams and rivers to lakes or the sea, where they are deposited under deltaic, lacustrine
and marine conditions with finely divided clastic sediments.
These environments produce their own microscopic plant and animal life, which are
deposited with the organic materials introduced by the streams and rivers. As deposition
of the organic material takes place in these environments, burial and protection by clay
and silt accompany it. This prevents decomposition of the organic material and allows it
to accumulate.
Conversion of the organic material is called catagenesis. It is assisted by pressure caused
by burial, temperature and thermal alteration and degradation. These factors result from
depth, some bacterial action in a closed nonoxidising chemical system, radioactivity and
catalysis. Temperature, as thermogenic activity, appears to be the most important
criterion, with assistance other factors as applicable. Accumulation of organic and clastic
material on a sea or lake bottom is accompanied by bacterial action. If there is abundant
oxygen, aerobic bacteria act upon the organic matter and destroy it.
Plant and animal remains contain abundant carbon and hydrogen, which are fundamental
elements in petroleum. Shale and some carbonates contain organic material that bears
hydrocarbons of types similar to those in petroleum. These rocks are not reservoir rocks
and could be considered ultimately to be source beds. The hydrocarbons are of the same
type as those found in living plants and animals and consist of asphalt, kerogen and liquid forms. The best source rocks are considered to be organically rich, black-coloured shales,
deposited in a non-oxidising, quiet marine environment.
Generation of crude oil
Organic material in shale averages approximately one (1) percent of the shale rock
volume. Clay mineral constituents comprise the remaining 99 percent.
Kerogen is an insoluble, high molecular weight, polymeric compound which comprises
about 90 percent of the organic material in shale. The remaining 10 percent comprises
bitumens of varying composition, which, according to some researchers, is thermally
altered kerogen. As alteration occurs, kerogen is developed by the increasing temperature
in the closed system.
Temperature increases with depth. Normal heat flow within the earth’s crust produces an
average geothermal gradient of approximately 1.5 oF for each 100 feet of depth.
Maturation studies on various crude oil types indicate that temperatures required to
produce oil occur between the depth of approximately 5,000 feet and 20,000 feet under
average heat-flow conditions.
Pressure, like temperature, is a function of depth and increases 1 psi for each foot of
depth. Pressure is caused by the weight of the sedimentary overburden.
Bacterial action is important in the conversion of organic material to petroleum at shallow
depths. It is involved in the process of breaking down the original material into
hydrocarbon compounds, which eventually become biogenic gas.
Kerogen is a primary factor in forming bitumens that increase and migrate to accumulate
as crude oil. Thermal conversion of kerogen to bitumen is the important process of crude
oil formation. Thermal alteration increases the carbon content of the migratable
hydrocarbons, which leaves the unmigratable kerogen components behind.
Maturation of kerogen is a function of increased burial and temperature and is
accompanied by chemical changes. As kerogen thermally matures and increases in carbon
content, it changes from an immature light greenish-yellow color to an overmature black,
which is representative of a higher coal rank. Generation of Natural Gas
Natural gas comprises biogenic gas and thermogenic gas with differences contingent
upon conditions of origin.
Biogenic gas forms at low temperatures at overburden depths of less than 3,000 feet
under anaerobic or conditions associated with high rates of marine sediment
accumulation. Oxygen in the sediments is consumed or eliminated early. And before
reduction of sulfates in the system. Methane, the most common of natural gas
constituents, forms after the sulfates are eliminated by hydrogen reduction of carbon
dioxide. Anaerobic oxidation of carbon dioxide produces methane. Current estimates
suggest that approximately 20 percent of the world’s known natural gas is biogenic.
Thermogenic gas forms at significantly higher temperatures and overburden pressures. It
contains methane and significantly larger amounts of heavier hydrocarbons than biogenic
gas. As time and temperature increase, progressively lighter hydrocarbons form as wet
gas and condensate in the latter stages of thermogenesis.
