When kerogen is converted directly into gas, or
from oil which has been formed first, there is a marked
volume increase. This should lead to slower reaction
rates under high pressure in a closed system and retard
generation of gas. Generation of petroleum, particularly gas, may contribute to the formation of overpressure but in a sedimentary basin the pressure will for the
most part be controlled by the flow of water which is
the dominant fluid phase. In the source rock however
overpressure is likely to develop, causing
hydrofracturing which helps to expel the generated
petroleum. The main cause for overpressuring is, however, not only the increase in fluid volume but the
transformation of solid into fluids. When solid kerogen
is transformed into fluid oil or gas the ratio between
the solid phase and the fluid phase is changed, as
expressed by the porosity and the void ratio.
Temperature is however the most important factor
controlling petroleum generation.
It has long been suspected that minerals, particularly clay minerals, might affect the rate of hydrocarbon generation. A number of laboratory experiments
have been carried out in which kerogen is mixed with
various minerals but the results have not been
conclusive.
The conversion of organic matter begins at
70–80
C, given long geological time. Between 70
and 90
C the transformation of kerogen proceeds
very slowly, and it is only in ancient, organic-rich
sediments that significant amounts are formed. Most
of the maturation process occurs between 100 and
150
C. Here the degree of kerogen transformation is
also a function of time. This means that rocks which
have been subjected to 100
C for 50 million years are
more mature than rocks which have been exposed to
this temperature for 10 million years. As the organicrich sediment (source rock) is buried in a sedimentary
basin, it will normally be subjected to increasing temperature as a function of increasing burial depth. If we
know the stratigraphy of the overlying sediment
sequence and the geothermal gradient and the subsidence curve, we can calculate the temperature as a
function of time.
At low degrees of maturity we find more of the
alkenes (olefins) and cykloalkenes (naphtenes),
which have high H/C ratios, while with greater maturity there is an increase in the proportion of aromates
and polyaromates (low H/C ratio). Oil thus acquires
increasing gas content with increasing maturity.
During this transformation of organic matter, water
and oxygen-rich compounds are liberated first, then
compounds which are rich in hydrogen. This conversion results in enrichment of carbon and the colour of
the residual kerogen changes from light yellow to
orange, brown and finally black. These gradations
can best be registered by measuring light absorption
of fossil pollen and spores (palynomorphs). It is also
possible to analyse colour changes in other kinds of
fossils, for example conodonts.
For application in exploration a rapid semiquantitative method has been developed whereby
these colour changes are estimated from smooth
spores examined under transmitted light and compared
with a standard colour scale. This parameter is called
the “Thermal alteration index” (TAI) and will give a
rough idea of the thermal maturity of the sediments
and their temperature history.
Another way of assessing palaeotemperatures at
which alteration took place in sedimentary rocks is to
record the degree of carbonisation of other plant
remains which are usually present. Vitrinite, which
originally was fragments of woody tissue, is a common component of coal but is also found in smaller
amounts in marine source rocks. This material is
analysed by measuring the amount of light it reflects.
It becomes shinier and reflects light better as the
degree of carbonisation increases. By measuring the
reflectivity of vitrinite particles under a reflected light
microscope an exact value is obtained for this maturity
parameter, expressed by the reflectivity coefficient R 0
(% vitrinite reflectance). If R 0 is less than 0.5% in a
shale it can not have generated much oil and is classed
as immature. Shales with R 0 ¼ 0.9–1.0 have been
exposed to temperatures corresponding to maximum
oil generation. R 0 ¼ 1.3 represents the upper limit for
oil generation, above which the shale will only produce condensate (light oils) or gas.
For certain source rocks the ratio between extractable alkenes (paraffins) with an even number of carbon
atoms per molecule and those with an odd number
may also be an expression of maturity. In plant material and in marine algae, one finds a higher abundance
of alkenes with an odd number of carbon atoms than in
transformed organic matter like waxes and fatty acids.
The decrease in this predominance of odd over even in
source rocks with increasing maturity is due to the
dilution of the original biologically derived n-alkanet
mixture with a newly generated mixture which has a
14 Source Rocks and Petroleum Geochemistry
365
from oil which has been formed first, there is a marked
volume increase. This should lead to slower reaction
rates under high pressure in a closed system and retard
generation of gas. Generation of petroleum, particularly gas, may contribute to the formation of overpressure but in a sedimentary basin the pressure will for the
most part be controlled by the flow of water which is
the dominant fluid phase. In the source rock however
overpressure is likely to develop, causing
hydrofracturing which helps to expel the generated
petroleum. The main cause for overpressuring is, however, not only the increase in fluid volume but the
transformation of solid into fluids. When solid kerogen
is transformed into fluid oil or gas the ratio between
the solid phase and the fluid phase is changed, as
expressed by the porosity and the void ratio.
Temperature is however the most important factor
controlling petroleum generation.
It has long been suspected that minerals, particularly clay minerals, might affect the rate of hydrocarbon generation. A number of laboratory experiments
have been carried out in which kerogen is mixed with
various minerals but the results have not been
conclusive.
The conversion of organic matter begins at
70–80
C, given long geological time. Between 70
and 90
C the transformation of kerogen proceeds
very slowly, and it is only in ancient, organic-rich
sediments that significant amounts are formed. Most
of the maturation process occurs between 100 and
150
C. Here the degree of kerogen transformation is
also a function of time. This means that rocks which
have been subjected to 100
C for 50 million years are
more mature than rocks which have been exposed to
this temperature for 10 million years. As the organicrich sediment (source rock) is buried in a sedimentary
basin, it will normally be subjected to increasing temperature as a function of increasing burial depth. If we
know the stratigraphy of the overlying sediment
sequence and the geothermal gradient and the subsidence curve, we can calculate the temperature as a
function of time.
At low degrees of maturity we find more of the
alkenes (olefins) and cykloalkenes (naphtenes),
which have high H/C ratios, while with greater maturity there is an increase in the proportion of aromates
and polyaromates (low H/C ratio). Oil thus acquires
increasing gas content with increasing maturity.
During this transformation of organic matter, water
and oxygen-rich compounds are liberated first, then
compounds which are rich in hydrogen. This conversion results in enrichment of carbon and the colour of
the residual kerogen changes from light yellow to
orange, brown and finally black. These gradations
can best be registered by measuring light absorption
of fossil pollen and spores (palynomorphs). It is also
possible to analyse colour changes in other kinds of
fossils, for example conodonts.
For application in exploration a rapid semiquantitative method has been developed whereby
these colour changes are estimated from smooth
spores examined under transmitted light and compared
with a standard colour scale. This parameter is called
the “Thermal alteration index” (TAI) and will give a
rough idea of the thermal maturity of the sediments
and their temperature history.
Another way of assessing palaeotemperatures at
which alteration took place in sedimentary rocks is to
record the degree of carbonisation of other plant
remains which are usually present. Vitrinite, which
originally was fragments of woody tissue, is a common component of coal but is also found in smaller
amounts in marine source rocks. This material is
analysed by measuring the amount of light it reflects.
It becomes shinier and reflects light better as the
degree of carbonisation increases. By measuring the
reflectivity of vitrinite particles under a reflected light
microscope an exact value is obtained for this maturity
parameter, expressed by the reflectivity coefficient R 0
(% vitrinite reflectance). If R 0 is less than 0.5% in a
shale it can not have generated much oil and is classed
as immature. Shales with R 0 ¼ 0.9–1.0 have been
exposed to temperatures corresponding to maximum
oil generation. R 0 ¼ 1.3 represents the upper limit for
oil generation, above which the shale will only produce condensate (light oils) or gas.
For certain source rocks the ratio between extractable alkenes (paraffins) with an even number of carbon
atoms per molecule and those with an odd number
may also be an expression of maturity. In plant material and in marine algae, one finds a higher abundance
of alkenes with an odd number of carbon atoms than in
transformed organic matter like waxes and fatty acids.
The decrease in this predominance of odd over even in
source rocks with increasing maturity is due to the
dilution of the original biologically derived n-alkanet
mixture with a newly generated mixture which has a
14 Source Rocks and Petroleum Geochemistry
365