101
Kerogen
geothermal gradient varies from place to place, it is generally on the order of 25°C–30°C/km
(15°F/1000 ft or 120°C/1000 ft, that is, 0.015°C/ft of depth or 0.012°C/ft of depth). This leaves
serious questions about whether or not the material has been subjected to temperatures in excess
of 250°C (>480°F).
Such experimental work is interesting insofar as it shows similar molecular moieties in kerogen and petroleum (thereby confirming similar origins for kerogen and petroleum). However, the
absence of geologic time in the laboratory is not a reason to increase the temperature and it must be
remembered that application of high temperatures (>250°C, <480°F) to a reaction not only increases
the rate of reaction (thereby making up for the lack of geologic time) but can also change the
nature and the chemistry of a reaction. In such a case, the geochemistry is altered. Furthermore, the
introduction of a pseudoactivation energy in which the activation energies of kerogen conversion
reactions are reduced leave much to be desired because of the assumption required to develop this
pseudo-activation energy equation(s). In fact, not only will the oil window (the oil-producing phase)
vary from kerogen-type to kerogen-type, it is not valid to use a fixed set of kinetic parameters within
each of these groups (Whelan and Farrington, 1992).
Therefore, a thorough investigation is needed to determine the chances of such high temperatures being present during the main phase, or even at various phases, of petroleum generation.
By far the major part of the organic material present in most rocks (or shales) occurs as kerogen,
an insoluble organic solid of variable composition, which is usually finely dispersed throughout the
mineral matrix. Typical kerogen-containing shales are nonporous, impermeable strata containing
approximately 5%–20% w/w organic material and the remainder (80%–95% w/w) as the mineral
matrix. The organic material is often further defined as insoluble material (kerogen) and material
(bitumen) extractable from the mineral matrix by organic solvents. The term bitumen, as used here,
is a term of convenience and should not be confused with naturally occurring bitumen (Chapter 1).
It would be more appropriate to add a qualifier so that the name of the extractable material becomes
kerogen-bitumen.
In order to explain the role of kerogen in the formation of petroleum, the biogenic theory of petroleum formation is espoused as opposed to the abiogenic theory of petroleum formation (Chapter 3).
In simple terms petroleum has formed throughout much of the Earth’s history, in fact, oil is
being formed in some parts of the Earth today. Almost all oil and gas comes from tiny decayed
plants, algae, and bacteria. At certain times in the Earth’s history conditions for oil formation
have been particularly favorable. For example, in the Jurassic period (one of the oil-forming
periods about 150 million years ago), the seas and swampy areas were rich in microscopic plants
and animals.
When these died they slowly sank to the bottom forming thick layers of organic material. This,
in turn, became covered in layers of mud that trapped the organic material. The layers of mud prevented air from reaching the organic material. Without air, the organic material did not decay in the
same way as organic material decays in the presence of air such as, for example, in a compost heap.
As the layers of mud grew in thickness, pressure increased and the temperature also increased.
The increasing temperature, pressure, and anaerobic bacteria (microorganisms that can live without oxygen) started acting on the organic material. As this happened, the material was changed
(matured) into petroleum and natural gas.
The Van Krevelen diagram (a plot of the atomic hydrogen–carbon ratio vs. the atomic oxygen–
carbon ratio) (Figure 5.2), derived from the elemental analysis of kerogen and coal, is a very practical means of studying kerogen composition. The position of kerogen in the H/C–O/C diagram is
related to the total quantity of hydrocarbons, which in turn is a function of the relative amounts of
aromatic hydrocarbon structures. The data for kerogen analysis in the H/C–O/C diagram can be
considered to describe the evolutionary path for kerogen from different precursors. Oil and gas are
believed to be formed during this evolutionary path. Analysis of the minor elements, sulfur and
nitrogen, is much more difficult to simulate and may require a more detailed framework.
Kerogen
geothermal gradient varies from place to place, it is generally on the order of 25°C–30°C/km
(15°F/1000 ft or 120°C/1000 ft, that is, 0.015°C/ft of depth or 0.012°C/ft of depth). This leaves
serious questions about whether or not the material has been subjected to temperatures in excess
of 250°C (>480°F).
Such experimental work is interesting insofar as it shows similar molecular moieties in kerogen and petroleum (thereby confirming similar origins for kerogen and petroleum). However, the
absence of geologic time in the laboratory is not a reason to increase the temperature and it must be
remembered that application of high temperatures (>250°C, <480°F) to a reaction not only increases
the rate of reaction (thereby making up for the lack of geologic time) but can also change the
nature and the chemistry of a reaction. In such a case, the geochemistry is altered. Furthermore, the
introduction of a pseudoactivation energy in which the activation energies of kerogen conversion
reactions are reduced leave much to be desired because of the assumption required to develop this
pseudo-activation energy equation(s). In fact, not only will the oil window (the oil-producing phase)
vary from kerogen-type to kerogen-type, it is not valid to use a fixed set of kinetic parameters within
each of these groups (Whelan and Farrington, 1992).
Therefore, a thorough investigation is needed to determine the chances of such high temperatures being present during the main phase, or even at various phases, of petroleum generation.
By far the major part of the organic material present in most rocks (or shales) occurs as kerogen,
an insoluble organic solid of variable composition, which is usually finely dispersed throughout the
mineral matrix. Typical kerogen-containing shales are nonporous, impermeable strata containing
approximately 5%–20% w/w organic material and the remainder (80%–95% w/w) as the mineral
matrix. The organic material is often further defined as insoluble material (kerogen) and material
(bitumen) extractable from the mineral matrix by organic solvents. The term bitumen, as used here,
is a term of convenience and should not be confused with naturally occurring bitumen (Chapter 1).
It would be more appropriate to add a qualifier so that the name of the extractable material becomes
kerogen-bitumen.
In order to explain the role of kerogen in the formation of petroleum, the biogenic theory of petroleum formation is espoused as opposed to the abiogenic theory of petroleum formation (Chapter 3).
In simple terms petroleum has formed throughout much of the Earth’s history, in fact, oil is
being formed in some parts of the Earth today. Almost all oil and gas comes from tiny decayed
plants, algae, and bacteria. At certain times in the Earth’s history conditions for oil formation
have been particularly favorable. For example, in the Jurassic period (one of the oil-forming
periods about 150 million years ago), the seas and swampy areas were rich in microscopic plants
and animals.
When these died they slowly sank to the bottom forming thick layers of organic material. This,
in turn, became covered in layers of mud that trapped the organic material. The layers of mud prevented air from reaching the organic material. Without air, the organic material did not decay in the
same way as organic material decays in the presence of air such as, for example, in a compost heap.
As the layers of mud grew in thickness, pressure increased and the temperature also increased.
The increasing temperature, pressure, and anaerobic bacteria (microorganisms that can live without oxygen) started acting on the organic material. As this happened, the material was changed
(matured) into petroleum and natural gas.
The Van Krevelen diagram (a plot of the atomic hydrogen–carbon ratio vs. the atomic oxygen–
carbon ratio) (Figure 5.2), derived from the elemental analysis of kerogen and coal, is a very practical means of studying kerogen composition. The position of kerogen in the H/C–O/C diagram is
related to the total quantity of hydrocarbons, which in turn is a function of the relative amounts of
aromatic hydrocarbon structures. The data for kerogen analysis in the H/C–O/C diagram can be
considered to describe the evolutionary path for kerogen from different precursors. Oil and gas are
believed to be formed during this evolutionary path. Analysis of the minor elements, sulfur and
nitrogen, is much more difficult to simulate and may require a more detailed framework.