the minimum overburden required to initiate petroleum generation will be correspondingly greater
(4–6 km).
In general one can say that petroleum can not be
generated near the surface except locally through the
influence of hydrothermal and igneous activity. Shallow deposits of oil and gas which we find today were
actually formed at great depths and either the overburden has been removed by erosion or the hydrocarbons
have migrated upwards considerable distances. As we
have already seen, however, large amounts of natural
gas, chiefly methane (CH 4 ), may be formed near the
surface by biochemical processes.
Temperature increases with increasing overburden,
causing the carbon-carbon bonds of the organic
molecules in the kerogen to rupture. This results in
smaller hydrocarbon molecules. When kerogen maturation reactions are completed, the kerogen’s
“organic” components, which may be derived from
lipids, fatty acids and proteins, have been converted
into hydrocarbons.
As the temperature rises, more and more of the
bonds are broken, both in the kerogen and in the
hydrocarbon molecules which have already been
formed. This “cracking” leads to the formation of
lighter hydrocarbons from the long hydrocarbon
chains and from the kerogen. The removal of gas,
mainly CH 4 , leaves the residual kerogen relatively
enriched in carbon. At the outset kerogen (Type I
and II) has an H/C ratio of 1.3–1.7. Humic kerogen
(Type III), which has high initial oxygen contents,
gives off mostly CO 2 gas and so its oxygen/carbon
ratio gradually diminishes. This diagenetic alteration
begins at 70–80
C and as water and CH 4 are removed
the H/C ratio will fall to about 0.6 and the O/C ratio
will become less than 0.1 at about 150–180
C.
In the North Sea basin, most of the oil is generated
at temperatures around 130–140
C, which equates
with a depth of about 3.5 km. If temperatures higher
than 170–180
C persist for a few million years, all the
longer hydrocarbon chains will already have been
broken (cracked), leaving us only with gas – mainly
methane (dry gas). The kerogen composition will
gradually be depleted in hydrogen and move towards
pure carbon (graphite) (H/C!0).
Source rock can in some cases be mapped out on
seismic. Source rocks have in most cases lower density
and velocity (low AI) than sandstones, limestones and
shales. When the kerogen becomes mature, generation
of petroleum changes the solid/fluid ratio and particularly gas will reduce velocity and density. Mapping
out source rocks and their maturity has become a new
and important exploration technique in some sedimentary basins (Løseth et al. 2011). Seismic data may also
be used as indications of the degree of maturity.
14.2 What Factors Influence the
Maturation of Kerogen?
The term “maturity” refers here to the degree of thermal transformation of kerogen into hydrocarbons and
ultimately into gas and graphite. The conversion of
kerogen into hydrocarbons is a chemical process
which takes place with activation energies of around
50–60 kcal/mol. This energy is required to break
chemical bonds in the kerogen which consists of
very large molecules (polymers) so that smaller hydrocarbon molecules can be formed.
It has been assumed that formation of oil is a first
order reaction, the rate of which is an exponential
function of time. Understanding the factors which
influence the rate of this reaction is of great interest.
Four factors are thought to contribute:
1. Temperature
2. Pressure
3. Time
4. Minerals or other substances which increase the
rate of reaction (catalysts) or which inhibit
reactions (inhibitors).
Temperature is clearly the most important factor,
and hydrocarbons can be produced experimentally
from kerogen by heating it (pyrolysis). This reaction
is time-dependent and in laboratory experiments,
where time is more limited than it is in nature, fairly
high temperatures (350–550
C) have to be used in
pyrolysis. This is the case when oil and gas is produced from oil shales by pyrolysis of immature kerogen in ovens after quarrying (see chapter 21). Pressure
appears to play a minor role but increasing pressure
should reduce the rate of the reaction because of the
increase in volume involved in the formation of
hydrocarbons (Le Chatelier’s rule).
There is a relatively small volume increase when
kerogen becomes oil, even though oil is lighter than
kerogen. This is due to the residual (coke) which
remains unaltered.
364
K. Bjørlykke
(4–6 km).
In general one can say that petroleum can not be
generated near the surface except locally through the
influence of hydrothermal and igneous activity. Shallow deposits of oil and gas which we find today were
actually formed at great depths and either the overburden has been removed by erosion or the hydrocarbons
have migrated upwards considerable distances. As we
have already seen, however, large amounts of natural
gas, chiefly methane (CH 4 ), may be formed near the
surface by biochemical processes.
Temperature increases with increasing overburden,
causing the carbon-carbon bonds of the organic
molecules in the kerogen to rupture. This results in
smaller hydrocarbon molecules. When kerogen maturation reactions are completed, the kerogen’s
“organic” components, which may be derived from
lipids, fatty acids and proteins, have been converted
into hydrocarbons.
As the temperature rises, more and more of the
bonds are broken, both in the kerogen and in the
hydrocarbon molecules which have already been
formed. This “cracking” leads to the formation of
lighter hydrocarbons from the long hydrocarbon
chains and from the kerogen. The removal of gas,
mainly CH 4 , leaves the residual kerogen relatively
enriched in carbon. At the outset kerogen (Type I
and II) has an H/C ratio of 1.3–1.7. Humic kerogen
(Type III), which has high initial oxygen contents,
gives off mostly CO 2 gas and so its oxygen/carbon
ratio gradually diminishes. This diagenetic alteration
begins at 70–80
C and as water and CH 4 are removed
the H/C ratio will fall to about 0.6 and the O/C ratio
will become less than 0.1 at about 150–180
C.
In the North Sea basin, most of the oil is generated
at temperatures around 130–140
C, which equates
with a depth of about 3.5 km. If temperatures higher
than 170–180
C persist for a few million years, all the
longer hydrocarbon chains will already have been
broken (cracked), leaving us only with gas – mainly
methane (dry gas). The kerogen composition will
gradually be depleted in hydrogen and move towards
pure carbon (graphite) (H/C!0).
Source rock can in some cases be mapped out on
seismic. Source rocks have in most cases lower density
and velocity (low AI) than sandstones, limestones and
shales. When the kerogen becomes mature, generation
of petroleum changes the solid/fluid ratio and particularly gas will reduce velocity and density. Mapping
out source rocks and their maturity has become a new
and important exploration technique in some sedimentary basins (Løseth et al. 2011). Seismic data may also
be used as indications of the degree of maturity.
14.2 What Factors Influence the
Maturation of Kerogen?
The term “maturity” refers here to the degree of thermal transformation of kerogen into hydrocarbons and
ultimately into gas and graphite. The conversion of
kerogen into hydrocarbons is a chemical process
which takes place with activation energies of around
50–60 kcal/mol. This energy is required to break
chemical bonds in the kerogen which consists of
very large molecules (polymers) so that smaller hydrocarbon molecules can be formed.
It has been assumed that formation of oil is a first
order reaction, the rate of which is an exponential
function of time. Understanding the factors which
influence the rate of this reaction is of great interest.
Four factors are thought to contribute:
1. Temperature
2. Pressure
3. Time
4. Minerals or other substances which increase the
rate of reaction (catalysts) or which inhibit
reactions (inhibitors).
Temperature is clearly the most important factor,
and hydrocarbons can be produced experimentally
from kerogen by heating it (pyrolysis). This reaction
is time-dependent and in laboratory experiments,
where time is more limited than it is in nature, fairly
high temperatures (350–550
C) have to be used in
pyrolysis. This is the case when oil and gas is produced from oil shales by pyrolysis of immature kerogen in ovens after quarrying (see chapter 21). Pressure
appears to play a minor role but increasing pressure
should reduce the rate of the reaction because of the
increase in volume involved in the formation of
hydrocarbons (Le Chatelier’s rule).
There is a relatively small volume increase when
kerogen becomes oil, even though oil is lighter than
kerogen. This is due to the residual (coke) which
remains unaltered.
364
K. Bjørlykke