-
CHAPTER 5: RELATIONSHIP BETWEEN ELEMENTAL
COMPOSITION AND GENETIC POTENTIAL
In the introduction of this thesis, methods for assessing the petroleum potential of coal
were discussed. This and succeeding chapters provide new insights using a multi parameters
approach. The yields, compositions and generation characteristics of New Zealand coal are all
addressed. Inferences for feeding the deep biosphere are also presented.
In this and succeeding chapters, the first sample set was used exclusively because its
maturity increases continuously, and its facies are less variable in comparison with those of
the second sample set (FIGURES 2-2; 3-5; 3-8). The more detailed analysis of macromolecules
have been performed on the first sample set. However, the risk of oxidation caused by long
time stored in room conditions should be noticed.
5.1 EVOLUTION PATHWAY OF NEW ZEALAND COALS ON THE BASIS OF C, H, O
ANALYSIS
The most important factors controlling the quality of organic matter, which have a
decisive influence on the hydrocarbon potential of possible source rocks, are the relation of
elemental composition expressed as the atomic hydrogen-to-carbon as well as oxygen-to-
carbon ratios. These ratios are used to classify the insoluble portion of organic matter because
C, H and O make up most of kerogen structures (DURAND AND MONIN, 1980). Diagrams used for
interpretation of the elemental analysis are variations in combinations of these elements
Among these diagrams, the one proposed by vAN KREVELEN (1961), consisting of an H/C versus
O/C atomic ratios, is the most suitable for processing elemental analysis. VAN KREVELEN has
plotted elemental analyses for macerals in his diagram and showed that they located in
separated bands, namely alginite, exinite, vitrinite and fusinite.
115
-
TISSOT ET AL. (1974) introduced a simpler nomenclature. Type I refers to kerogen with a
high initial H/C atomic (c.a. 1.5 and more) and a low initial O/C ratio (lower than 0.1). Such
kerogen contains a high proportion of aliphatic chains which are mainly derived from alga
lipids and algaenan. The content of polyaromatic and hetero compounds is low in comparison
with other types. Therefore, this type of kerogen is highly oil-prone and generates most of its
hydrocarbons over a narrow maturity range (TISSOT ET AL., 1974). Type II kerogen is
characterized by the relatively high H/C (1.2 to 1.5) and low O/C (compared to type III)
atomic ratios. It is usually related to marine sediments where an autochthonous organic
matter, derived from a mixture of phytoplankton, zooplankton and micro-organisms, and has
been deposited in a reduction environment. This kind of kerogen is oil-prone, but lower than
that of type I. Type III, the one of central importance in this thesis, is derived from continental
plants and contains much identifiable vegetal debris. It is rich in humic macerals of the
vitrinite group. The relatively low initial H/C ratio is usually less than 1.0 and high initial O/C
atomic ratio as high as 0.2 or 0.3. The hydrogen content is lower due to a higher relative
abundance of condensed aromatic and oxygen-containing structures. The chemical changes in
humic coals during its evolution through the different rank stages fall along the type III
kerogen track, because they consist mainly of vitrinite. HARWOOD (1977) introduced kerogen
Type IV which have initial atomic H/C ratios smaller than 0.8. Type IV kerogen, containing
high-carbon residue organic matter, can be deposited under either marine or non-marine
conditions, but they have been regarded principally as gas producers with much less oil
potential than type I and type II kerogen (MURCHISON ET AL., 1987).
In this study, elemental composition data was provided by RICHARD SYKES (GNS). The
investigated samples had H/C values ranging from 0.7 to 0.9, and O/C ranging from 0.07 to
0.34. Elemental compositions of samples G001985 and G001992 were not available. From
the observation on organic matter types in general, POWELL (1978; 1988) suggested that in order
for a source rock to be effective, 10-20% of its organic matter must equate with Type I
organic matter, or 20-30% must equate to Type II organic matter. That means the bulk atomic
H/C ratios would fall in the range 0.8-0.9 (POWELL 1988). Similarly, HUNT (1991) reported that
there are quite strong evidences to show that coal is contributing to accumulations of liquid
petroleum, and once organic matter has an H/C ratio larger than about 0.9 it usually indicates
some liquid generating capability. Additionally, SAXBY (1980) has showed a robust systematic
relationship between the elemental chemical composition of coal and oil yield which is given
as:
116
-
3.33O/C0.57H/C7.66free) mineraldry;(wt%; % −×−×=Oil (EQUATION 5-1)
The author shown that coals having atomic H/C values higher than 0.8 is capable to
generate liquid hydrocarbons, e.g. 20 percentage of oil on a coal weight basis that can be
generated from coals having atomic H/C value around 0.8. Based on this simple criterion, it
can be said that the studied New Zealand coals have the ability to generate liquid
hydrocarbons. However, it should be noted that not simply generation but also
retention/explusion plays crucial roles in differentiating oil from gas generating source,
especially in the case of coals.
FIGURE 5-1 presents the changing elemental composition with samples are numbered
according to their relative maturity basing on their Rank(Sr) values. FIGURE 5-1 obviously
shows that these samples follow the evolution path of kerogen type III, and it is marked by a
significant decrease in O/C ratio (0.34- 0.07), despite a minor change in H/C values (0.9- 0.7)
with increasing maturity (TABLE 5-1). This presents a progressive shift along each coalification
track from high to low H/C and O/C values (TISSOT ET AL., 1974; DURAND AND ESPITALIÉ, 1976; DURAND AND MONIN, 1980; DURAND AND PARATTE, 1983; TISSOT AND WELTE, 1984; BOREHAM AND POWELL,
1993). Since, the evolution of organic matter results in carbon enrichment in the solid phase,
and the formation of volatile products that are enriched in hydrogen and oxygen compared
with the starting material (TISSOT ET AL., 1974; TISSOT AND WELTE, 1984; HORSFIELD, 1984 AND
REFERENCES THEREIN; BOREHAM AND POWELL, 1993). Furthermore, this figure also show the elemental
composition changes corresponding to the first maturation stage of DURAND AND MONIN (1980;
P.132- 133), where C=O functions (IR) rapidly disappear, resulting in the formation of CO2,
H2O and heavy heteroatomic products, e.g., resins, asphaltenes. Neither the second stage
described by these authors, the principal oil and gas formation phase, characterized by the
notable decrease of H/C and the constancy or slightly decrease of O/C, nor the third
metagenesis stage, are represented in the sample set.
Briefly, based the basis of C, H and O elemental analysis and the using of van
Krevelen diagram it shows that (1) the New Zealand coals follow the evolution pathway of
kerogen type III in H/C vs. O/C diagram and (2) these coals have the potential to generate
liquid hydrocarbons according to elemental composition only. More importantly, it presents a
significant decrease of O/C ratio with increasing maturity that is a convinced evidence of the
oxygen loss during coalification. This loss of oxygen during diagenesis and its applications
117
-
not only explaining the increasing of hydrogen index values of low rank coals with maturation
but also evaluating the feeding potential for deep biosphere are discussed herein following.
0 0.1 0.2 0.3 0.4 0.5O/C (atomic ratio)
0
0.5
1
1.5
2
H/C
(ato
mic
ratio
)
8 610
73
15 1311 912
5 42
222123 2019
1618 17
New Zealand coals
Mean evolution path of type I
Mean evolution path of type II
Mean evolution path of type III
0 0.1 0.2 0.3 0.4 0.5O/C (atomic ratio)
0
0.5
1
1.5
2
H/C
(ato
mic
ratio
)
8 610
73
15 1311 912
5 42
222123 2019
1618 17
New Zealand coals
Mean evolution path of type I
Mean evolution path of type II
Mean evolution path of type III
Figure 5-1: The kerogen type and evolution paths of New Zealand coals
based on atomic ratios (after van Krevelen, 1961; Type I-III Tissot et al., 1974; the evolution paths (bands) and mean evolution paths for Type I-III are after Durand and Monin (1980).
118
-
Table 5-1: The bulk geochemical data of New Zealand coals from Rock-Eval and elemental composition analysis. Samples are ordered corresponding to their relative maturity, based on Suggate rank
S1 S2 S3 HI OI H/C O/C H/C O/C Sa les Short lables R0 (%) (mg/g sediment) (mg/g TOC) Atomic ratios Weigh ratios
G00 5 1 0.25 22.4 75.5 46.0 156 95 n.d n.d n.d n.dG00 8 2 0.27 19.6 64.8 43.5 133 89 0.90 0.34 0.075 0.457G00 9 3 0.25 13.8 72.6 52.3 148 107 0.87 0.34 0.072 0.451G00 7 4 0.26 13.0 58.0 45.4 124 97 0.86 0.33 0.071 0.438G00 6 5 0.27 18.7 70.3 41.4 141 83 0.86 0.32 0.072 0.423G00 6 6 0.29 16.4 82.6 44.5 152 82 0.84 0.30 0.070 0.405G00 8 7 0.28 7.5 69.3 39.4 129 73 0.82 0.28 0.068 0.375G00 5 8 0.29 9.8 69.6 39.5 135 77 0.86 0.29 0.072 0.390G00 3 9 0.41 7.3 94.0 28.4 161 49 0.81 0.23 0.067 0.306G00 7 10 0.39 6.2 101.3 21.4 170 36 0.78 0.23 0.065 0.302G00 2 11 0.4 4.0 105.2 23.3 175 39 0.81 0.22 0.068 0.290G00 4 12 0.45 3.7 108.5 17.0 170 27 0.80 0.20 0.067 0.269G00 1 13 0.45 3.0 94.5 19.9 154 32 0.78 0.21 0.065 0.277G00 2 14 0.49 4.5 113.9 20.5 176 32 n.d n.d n.d n.dG00 0 15 0.44 3.0 104.0 15.5 173 26 0.79 0.18 0.066 0.239G00 5 16 0.52 3.2 134.9 10.6 198 16 0.78 0.17 0.065 0.230G00 7 17 0.52 2.9 141.1 9.9 209 15 0.82 0.17 0.068 0.232G00 6 18 0.52 4.0 150.5 8.4 230 13 0.83 0.14 0.069 0.190G00 4 19 0.61 6.2 179.9 9.1 283 14 0.84 0.12 0.070 0.161G00 3 20 0.76 6.9 193.3 3.9 248 5 0.78 0.09 0.065 0.126G00 0 21 0.71 7.6 198.3 4.3 267 6 0.80 0.10 0.067 0.131G00 9 22 0.69 10.9 190.8 2.4 260 3 0.81 0.11 0.067 0.148G00 1 23 0.8 13.7 194.0 3.7 259 5 0.82 0.07 0.069 0.091
119
mp
198198197198198197197197198197198198198199198199199199199199199198199
-
5.2 EVOLUTION PATHWAY OF NEW ZEALAND COALS USING ROCK-EVAL PYROLYSIS
The evolution pathways and types of kerogen can also be characterized based on two
indices: the hydrogen index (S2/ organic carbon) and the oxygen index (S3/ organic carbon)
obtained on whole-rock samples via Rock-Eval pyrolysis. Since, ESPITALIÉ ET AL., (1977) have
proved a good correlation between hydrogen index and H/C ratio, as well as oxygen index
and O/C ratio (FIGURE 5-2). These indices are therefore can be plotted in place of the normal
van Krevelen diagram, and interpreted in the same way. It is because pyrolysis does not
require kerogen isolation, this method therefore proved more rapid and less expensive than
elemental analysis to classify easily the three principal types of organic matter. The genetic
potential of the source rock can be quantitatively expressed in mg of hydrocarbons per g
organic carbon.
Figure 5-2: Correlation of hydrogen and oxygen indices measured by pyrolysis of
rock with H/C and O/C ratios (Figure of Espitalié et al., 1977)
A pseudo-van Krevelen diagram containing data for New Zealand coal is presented in
FIGURE 5-3. The New Zealand coal falls between the type II and III reference curves, but closer
to that of kerogen type III. Most coals originated from Northland, Eastern Southland and
120
-
0
150
300
450
600
750
900
0 50 100 150
Hyd
roge
n In
dex
Northland
Eastern Southland
Waikato
West Coast
Taranaki
I
II
III
Waikato basins show a close
affinity for the type III reference
curve. Meanwhile, samples from
West Coast and Taranaki basins
are closely aligned with the
higher hydrogen index values of
the type II reference curve. It
seems that they may consist of
variable mixtures of hydrogen-
rich and hydrogen-poor organic
matter (C.F. KATZ ET AL., 1991) and
presenting better hydrocarbon
potential than samples from
Northland, Eastern Southland
and Waikato basin do (C.F. TISSOT ET AL., 1974).
From elemental analysis
discussed hereinbefore and as
have been illustrated in FIGURE 5-1 (VAN KREVELEN,
are defined as kerogen type III and plot around
New Zealand coals do not have abnormally h
using whole rock whereas elemental analysis re
high hydrogen index values demonstrate that th
only the hydrocarbons generated from pyrolysis
with solid bitumen (C.F. CLEMENTZ, 1978; SEE
overestimates the liquid-hydrocarbon generati
Zealand coals might be due to the differing met
FID of the Rock-Eval only responds to carbo
pyrolysis products such as hydrogen or wate
important product from immature samples, is
elemental analysis (PETERS, 1986).
Figure
Oxygen Index
5-3: Classification of the kerogen type and its evolution pathways of New Zealand coals using hydrogen and oxygen indices (Scheme of Espitalié et al., 1977)
1961; TISSOT ET AL., 1974) New Zealand coals
the mean evolution curve. It means that the
igh hydrogen contents. As pyrolysis is done
quires isolated kerogen, therefore, the relative
e hydrocarbon yield in S2 peak presenting not
of kerogen but also hydrocarbons associated
MORE DETAIL IN CHAPTER 7). Additionally, the
ve potential of coals in general and New
hods of product detection (PETERS, 1986). The
n mass and C-H bonds whilst the common
r, which would be expected to be a more
not included in the HI but is measured by
121
-
5.3 THE EVOLUTION OF HYDROCARBON GENERATION POTENTIAL OF LOW RANK
COALS AS FUNCTION OF MATURATION
LANGFORD & BLANC- VALLERON, (1990) have suggested a graph of S2, S3 vs. TOC (%) to
determine HI and OI values more accurately. Their original motivation was to recognise
mineral mater effects (HORSFIELD AND DOUGLAS, 1980; ESPITALIÉ ET AL., 1980; KATZ, 1983). Once
samples are plotted on this diagram, they form a linear regression with a high degree of
correlation. Based on the slope of that regression line, the true value of HI and OI will be
given, thus the more precise petroleum potential of the rocks can be assessed. In this study,
the S2 versus TOC% and S3 versus TOC% diagrams have been plotted, samples are numbered
with increasing Suggate rank, and coloured/ zoned in different groups corresponding to
different basins. Discussions in this part are related with S2 vs. TOC % plot, whereas the S3
vs. TOC% plots will be discussed later on in SECTION 5.4.
The S2 versus TOC% plot, FIGURE 5-4, shows that the HI evolution of samples from
Northland, Eastern Southland and Waikato Basins, those with Rank(Sr) values smaller than 7,
0 20 40 60 80TOC %
0
40
80
120
160
200
S 2 (m
g/g
Sedi
men
t)
8
6
10
73
15
13
11
9
12
15
42
222123
14
20
19
16
18
17
New Zealand coals Taranaki
Waikato
West Coast
HI~ 130
HI~ 250
Northland Eastern Southland
Rank(Sr)0-7
Rank(Sr)> 7
0 20 40 60 80TOC %
0
40
80
120
160
200
S 2 (m
g/g
Sedi
men
t)
8
6
10
73
15
13
11
9
12
15
42
222123
14
20
19
16
18
17
New Zealand coals Taranaki
0 20 40 60 80TOC %
0
40
80
120
160
200
S 2 (m
g/g
Sedi
men
t)
8
6
10
73
15
13
11
9
12
15
42
222123
14
20
19
16
18
17
New Zealand coals TaranakiTaranaki
Waikato
West Coast
HI~ 130
HI~ 250
Northland
Rank(Sr)0-7
Rank(Sr)> 7
Eastern Southland
Figure
5-4: S2 against TOC plot for New Zealand coals (scheme of Langford and Blanc-Valleron, 1990)122
-
begins at HI value of c.a. 130 mg hydrocarbons/g TOC, their S2 values increase with
increasing TOC values. But some samples from West Coast Basin (16, 17, 18) and sample 19
(Taranaki Basin) have S2 values increasing with slightly decreasing in TOC% values with
only a small change in OI values of those samples (FIGURE 5-7). The remaining samples from
West Coast and Taranaki Basins fall in the line representing HI ~ 270 (mg hydrocarbons/g
TOC). Their HI values slightly increase, together with a small fall in OI values.
The Rock-Eval measured hydrogen index values range from 120 to 280 (mg
hydrocarbons/g TOC; SEE TABLE 5-1) increasing with maturation, i.e., increasing vitrinite
reflectance from 0.2 to 0.8%, are illustrated in FIGURE 5-5.
10.80.60.40.2Vitrinite reflectance (R0%)
120
160
200
240
280
320
HI (
mg/
gSe
d)
NorthlandEastern SouthlandWaikatoWest CoastTaranaki
HI (
mg/
gTO
C)
10.80.60.40.2Vitrinite reflectance (R0%)
120
160
200
240
280
320
HI (
mg/
gSe
d)
NorthlandEastern SouthlandWaikatoWest CoastTaranaki
HI (
mg/
gTO
C)
Figure 5-5: The increasing of hydrogen index values of New Zealand coals with maturation from peat to high volatile
These hydrogen indexes fall within the New Zealand hydrogen index trend presented
by SYKES AND SNOWDON (2002; FIGURE 5-6). The authors have reported that the New Zealand Coal
Band is well defined on cross plots of HI vs. Rank(Sr), and displays clear rank-related trends.
Hydrogen index values increase pronouncedly from a range of ~ 75- 225 at Rank(Sr)~ 4
123
-
(the lignite/sub-bituminous boundary; R0 ~ 0.35-0.4%) to ~ 220- 335 at its peak at
Rank(Sr) ~ 11- 12 (R0 ~ 0.65- 0.8%). They also pointed out that a coal lying at the base of the
Coal Band at Rank(Sr) ~ 3, for instance, has a measured HI of ~ 145 less than a coal of
equivalent type (i.e., also lying at the base of the Coal Band) at Rank(Sr) ~ 12. It consequently
leads to a mistake classifying many New Zealand immature coals as gas-prone based solely
on their HI values < 200. This results in gross underestimates of the oil potential of New
Zealand coals. To avoid that, they suggested the effective HI values for New Zealand coals,
denoted as HI0, which can be estimated by translating the measured HI values of less (or
more) mature samples to their respective positions within/ above or below the Coal Band on
the line of effective HI at Rank(Sr) ~11-12 (FIGURE 5-6). According to PETERS AND MOLDOWAN
(1993), coals with HI0 > 300 mg hydrocarbons/g TOC are considered to be mainly oil-prone,
those with HI0 range from 200-300 are considered as gas- and oil-prone, others with HI0 from
50- 200 mg hydrocarbons/g TOC are considered as mainly gas-prone, and the rest with HI
smaller than 50 are inert. Base on this, the investigated New Zealand coals were classified as
mixed gas- and oil-prone (SEE FIGURE 5-6).
3.0%
Rank(Sr)
~R0
HI (
mg/
g TO
C)
0
100
200
300
400
500
0 2 4 6 8 10 12 14 16 18 20
NZ Coal BandNZ coals (n=728)
0.25 0.35 0.5 0.8 1.6
Investigated HI values
Gas- & oil-prone
Oil-prone
Gas-prone
Line of effective HI
HI increases by up to ~150 mg/g prior
to oil expulsion
3.0%
Rank(Sr)
~R0
HI (
mg/
g TO
C)
0
100
200
300
400
500
0 2 4 6 8 10 12 14 16 18 20
NZ Coal BandNZ coals (n=728)
0.25 0.35 0.5 0.8 1.6
Investigated HI values
Gas- & oil-prone
Oil-prone
Gas-prone
Line of effective HI
HI increases by up to ~150 mg/g prior
to oil expulsion
Rank(Sr)
~R0
HI (
mg/
g TO
C)
0
100
200
300
400
500
0 2 4 6 8 10 12 14 16 18 20
NZ Coal BandNZ coals (n=728)NZ Coal BandNZ coals (n=728)NZ coals (n=728)
0.25 0.35 0.5 0.8 1.6
Investigated HI values
Gas- & oil-prone
Oil-prone
Gas-prone
Line of effective HI
HI increases by up to ~150 mg/g prior
to oil expulsion
HI increases by up to ~150 mg/g prior
to oil expulsion
Figure
5-6: Cross-plot of hydrogen index (S2/TOC) against Rank(Sr) presenting rank-related increase in HI prior to the onset of oil expulsion (Rank(Sr) ~ 11-12). The kerogen quality classification is that of Peters and Moldowan (1993). The hydrogen index values of investigated coals following the green dotted line
124
-
It seems that the true petroleum generation potential of coals is equivalent to the HI0
values. It means that the coals with approximately similar hydrogen index values should
theoretically contain the same proportion of hydrogen and thus have potentially the same
petroleum generation capacity. However, it has been argued that the HI0 value may not
necessarily be an expression of the ability to generate oil. The type of generated petroleum is
dependent on the chain length of the n-alkanes in the coal structures (HORSFIELD ET AL., 1988; HORSFIELD, 1989; POWELL ET AL., 1991; POWELL AND BOREHAM, 1994; ISAKSEN ET AL., 1998; PETERSEN,
2005). The HI is a measure of the hydrogen in the coal, but this may not be present as long-
chain n-alkanes, but rather as shorter chains with a potential to form only gas or condensate
PETERSEN (2005). The author based on a worldwide database consisting of more than 500 coals,
found that Carboniferous coals contain low proportions of n-alkanes with a carbon
number > C19, whereas Cenozoic coals in particular are much richer in long-chain aliphatics.
The difference in the chemical structure between them can be related to the original coal-
forming vegetation, and may explain why Carboniferous coals are principally gas- or
condensate-prone, whereas Cenozoic coals can be highly oil-prone. In contrast to the
Carboniferous woody, gymnospermous mire vegetations, the Cenozoic coals were formed
from an advance and diverse vegetation, which may have produced a more aliphatic-rich
vitrinitic organic matter upon deposition. He concluded that coals could act as source rocks
for oil accumulations, their generation potentials are related to the depositional conditions of
coals-forming mires, with marine influence having a positive effect by increasing the
hydrogen content in the vitrinite organic matter. From this, it could be deduced that an overall
vegetational control seems to be additionally exerted on the source rock potential.
The increase in HI prior to oil expulsion has been explained by SYKES AND SNOWDON
(2002) as being due to the structural rearrangement of the coal macromolecular matrix during
diagenesis and catagenesis. It results in the formation of the significant amounts of new bonds
with different generation potential (KILLOPS ET AL., 1998; SCHENK AND HORSFIELD, 1998;
SYKES & SNOWDON, 2002). SYKES AND SNOWDON (2002) based their explanation on the scheme
presented by SCHENK AND HORSFIELD (1998) for Carboniferous coals (R0 = 0.7- 6.1%). SCHENK AND
HORFIELD (1998) found that (1) data of bulk petroleum formation rate vs. temperature curves
(0.70 K/min, open-system pyrolysis) of artificially matured shales and coals (i.e., samples
have been pyrolyzed using non-isothermal heating (0.7K/min) up to end temperature between
375- 470°C) result in an overlap of many individual curves corresponding to individual
product generating reactions. This phenomenon is explained by the same reactions taking
125
-
place during pyrolysis measure (as used for determining the kinetic parameters) and the
artifical maturation processes. (2) Similarly, a same pattern of generation rate vs. temperature
curves is displayed by the natural maturation sequence of Toarcian shale in the maturity range
of 0.53 to 1.44% vitrinite reflectance, which suggests a satisfactory correspondence of open-
system pyrolysis and natural evolution in case of shale. (3) Conversely, a quite different
pattern is produced by the natural maturation sequence of Carboniferous German coals (0.74-
2.81% vitrinite reflectance). The rate curves of mature samples partly step out beyond the less
mature envelopes. This indicates that natural coalification produces bulk petroleum precursor
structures which are released at temperatures where pyrolytic product generation should have
already come to an end (according to the rate curves for less mature coals). These results led
them to conclude that parts of the numerous reactions involved in natural coalification do not
take place under the very different time temperature conditions of laboratory pyrolysis, which
may partly be linked to oxygen functionalities (C.F. BOUDOU ET AL., 1994). The enhanced
appearance of pyrolytic petroleum precursors at high temperatures points to a progressive
structural reorganization of the organic matter during natural evolution. These structural
changes retain considerable petroleum potentials up to high levels of maturity whereas
artificial maturation leads to quite a rapid decreases of pyrolysis yields. Consequently, these
results which were obtained by SCHENK AND HORSFIELD (1998), not only indicate an obvious
failure of open laboratory pyrolysis to simulate natural coalification, but more importantly
also present a progressive structural reorganization of the solid organic matter during natural
evolution. These structural rearrangements gain considerable petroleum potentials up to high
levels of maturity and help explain the increase in HI prior to oil expulsion, according to
KILLOPS ET AL. (1998; 2002), SYKES & SNOWDON, (2002).
However, the question remains as to whether the release of large amounts of CO2, CO
and other functionalised groups at low levels of maturity can explain the enrichment of
hydrogen relative to carbon (DURAND AND PARATTE, 1983) without having to bring in the structural
rearrangements presented above. In contrast, SYKES AND SNOWDON (2002) found that the New
Zealand Coal Band shows no appreciable increase in atomic H/C over the Rank(Sr) range of
4-12 (THEIR FIGURE 2B). Moreover, the mass balance calculation done by KILLOPS ET AL. (1996;
1998) suggested the loss of CO2 could only account for an increase in HI of smaller than 10
mg/g TOC, which is less than 10% of the increase of up to 150 mg/g TOC observed for New
Zealand coals. The following two sections (SECTION 5.4 AND 5.5) consider this problem again.
126
-
5.4 THE LOSS OF OXYGEN DURING EARLY DIAGENESIS
The loss of oxygen during diagenesis is indicated firstly in FIGURE 5-7A showing a drop
of S3 continuously with both increasing TOC% values and maturity. FIGURE 5-7A shows an
obvious decreasing of oxygen index values with maturation. Additionally, data from infrared
spectrometry shows the loss of carbonyl carbon (K-1700 (cm/mg TOC) with vitrinite
reflectance range from 0.2 to 0.8% R0 (FIGURE 5-7B). This result is in agreement with the
literature review presented in SECTION 5.1, which is described above that the first stage of
organic matter evolution is characterized by the rapid disappearance of C=O functions.
0 20 40 60 80TOC %
0
20
40
60
S 3 (m
g/g
Sedi
men
t) 8
6
10
7
3
15
13
11
9
12
1
5
42
222123
14
20
1916
1817
New Zealand coals
Northland
Eastern Southland
Waikato
West Coast& Taranaki
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1
vitrinite reflectance (%)
K-1
700
(cm
/mg
TOC
)
AA BB
0 20 40 60 80TOC %
0
20
40
60
S 3 (m
g/g
Sedi
men
t) 8
6
10
7
3
15
13
11
9
12
1
5
42
222123
14
20
1916
1817
New Zealand coals
Northland
Eastern Southland
Waikato
West Coast& Taranaki
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1
vitrinite reflectance (%)
K-1
700
(cm
/mg
TOC
)
0 20 40 60 80TOC %
0
20
40
60
S 3 (m
g/g
Sedi
men
t) 8
6
10
7
3
15
13
11
9
12
1
5
42
222123
14
20
1916
1817
New Zealand coals
Northland
Eastern Southland
Waikato
West Coast& Taranaki
0 20 40 60 80TOC %
0
20
40
60
S 3 (m
g/g
Sedi
men
t) 8
6
10
7
3
15
13
11
9
12
1
5
42
222123
14
20
1916
1817
New Zealand coals
Northland
Eastern Southland
Waikato
West Coast& Taranaki
0
20
40
60
80
100
120
0 0.2 0.4 0.6 0.8 1
vitrinite reflectance (%)
K-1
700
(cm
/mg
TOC
)
AA BB
Figure
content
ratio, w
decarbo
SUGGATE
diagene
rupture
5-7: The loss of oxygen presented as the drop of S3 values with increasing in TOC values (A; scheme of Langford and Blanc-Valleron, 1990), and as the loss of carbonyl carbon as a function of rank (B)
Similarly BOUDOU ET AL, (1984) also reported that several oxygenated functional group
s, such as –OH, -COOH, C=O groups, decrease clearly as a function of O/C atomic
hich is used as a rank parameter of Mahakam coal. This loss of oxygen is related with
xylation and dehydration reactions forming H2O and/ or CO2 (DURAND AND MONIN, 1980;
AND BOUDOU, 1993). Because as mentioned in TISSOT & WELTE (1984) that in the stage of
sis, heteroatomic bonds are broken successively and roughly in order of ascending
energy, starting with some labile carbonyl and carboxyl groups (e.g., ketones and
127
-
acids). Heteroatoms, especially oxygen, are partly removed as volatile products: H2O and
CO2, still the loss of oxygen in form of CO was considered to be negligible (HIGGS, 1986).
Evidently, FIGURE 3-2 (CHAPTER 3) presenting the content of moisture vs. vitrinite reflectance
shows that with increasing of maturation the measured content of moisture linearly decreases
from c.a. 30% to 3% (proximate air-dried; DATA FROM SYKES GNS).
Secondly, there is a considerable diminution of oxygen relative to hydrogen during
coalification in the range of vitrinite reflectance about 0.2- 0.8%. This is confirmed by
FIGURE 5-8A which shows that H/C weight ratios vary between a narrow range of 0.072- 0.064,
meanwhile FIGURE 5-8B shows an obviously linear decreasing of O/C weight ratios from 0.45
to 0.1 with maturation. This phenomenon is related to which has been discussed in SECTION 5.1
that is during maturation, the evolution of organic matter is indicated by reduction of O/C and
then of H/C because most of the chemical structures that bind oxygen are less stable than
those that bind hydrogen (DURAND AND MONIN, 1980; P.132- 133).
10.50.30.2Vitrinite Reflectance (Ro%)
0.064
0.068
0.072
0.076
H/C
- wei
ght r
atio
AA
10.50.30.2Vitrinite Reflectance (Ro%)
0.064
0.068
0.072
0.076
H/C
- wei
ght r
atio
AA
10.50.30.2Vitrinite Reflectance (Ro%)
0
0.1
0.2
0.3
0.4
0.5
O/C
- wei
ght r
atio
BB
10.50.30.2Vitrinite Reflectance (Ro%)
0
0.1
0.2
0.3
0.4
0.5
O/C
- wei
ght r
atio
BB
Figure
The loss
presenting the o
smaller than 7 (O
higher than 7 (O
5-8: The variation of H/C weight ratio (A), O/C weight ratio (B) of New Zealand coals with maturation from peat to high volatile bituminous
of oxygen has here been divided into two stages as shown in FIGURE 5-9
xygen index values against O/C atomic ratios. The samples with Suggate rank
/C > 0.15) follow the regression line 1 (solid line), and those with Rank(Sr)
/C < 0.15) follow the regression line 2 (dotted line) corresponding to the loss
128
-
of CO2 and CO (?). The strong linear correlations between O/C and OI indicate that the
proportion of released oxygen (in form of CO2 or CO) is proportional to the total oxygen
content. Therefore, it might be necessary to take into account the significant loss of oxygen in
form of not only H2O, CO2 but also CO. Similarly, BOUDOU ET AL. (1994) found the correlation
between the content of oxygen and the amount of released oxygen. They divided this
correlation into two stages, where O/C > 0.15 is resulted from some selectivity thermal
decarboxylation. Conversely, for O/C < 0.15, the linear correlation was attributed to the
fraction of CO2 which results from the disproportionation of CO from the decomposition of
non-carboxylic groups (-OH, -O-, -COO-).
Northland
Eastern Southland
Waikato
West Coast& Taranaki
0 0.1 0.2 0.3 0.4O/C (mmf)
0
30
60
90
120
OI (
mg/
gTO
C) 8
6
10
7
3
15
13
11
9
12
5
4
2
222123 20
19 161817
New Zealand coals
CO loss (?)CO2 loss
Northland
Waikato
West Coast& Taranaki
Eastern Southland
Northland
Waikato
West Coast& Taranaki
Eastern Southland
0 0.1 0.2 0.3 0.4O/C (mmf)
0
30
60
90
120
OI (
mg/
gTO
C) 8
6
10
7
3
15
13
11
9
12
5
4
2
222123 20
19 161817
New Zealand coals
CO2 lossCO loss (?)
Figure
5-9: The O/C atomic ratio versus OI value cross-plot for New Zealand coals. The loss of oxygen due to maturation can be divided into two stages resulting in releasing different products (CO or CO2).
129
-
5.5 OXYGEN LOSS DURING EARLY DIAGENESIS CAUSING INCREASE HYDROGEN
INDEX VALUES
The loss of oxygen during diagenesis, discussed in SECTION 5.1 AND SECTION 5.4, is one of
the major causes of the increase in HI prior to oil expulsion. This conclusion is learned from
the relationship between elemental compositions and Rock-Eval hydrogen index values. It has
been shown by ESPITALIE´ET AL (1977; FIGURE 5-2) that there is a good correlation between
hydrogen index and H/C ratio, as well as oxygen index and O/C ratio. Hydrocarbon yields
increase with increasing atomic H/C ratios and decrease with increasing O/C ratios.
Furthermore, ORR (1981) has also examined pyrolytic hydrocarbon yields as a function of
elemental composition, with a broad range of atomic H/C (0.71- 1.55) and a more restricted
O/C (0.08- 0.19) ratio range. As an outstanding result, they provided an equation showing a
systematic relation between elemental compositions and pyrolytic hydrocarbon yields- EQUATION 5-2.
)O/C(800)29.0H/C(694HI ×−−×= (EQUATION 5-2)
In other words, pyrolysis yield is controlled by both hydrogen and oxygen containing
moieties in coals and kerogen in general. The equation has been applied for non-extracted
New Zealand coals, which have atomic O/C ratios in range of 0.07- 0.34, and H/C ratios
between 0.7- 0.9, in order to calculate hydrogen index values from elemental-atomic H/C and
O/C ratios. These calculated hydrogen index values (FROM EQUATION 5-2) were compared with
measured ones. FIGURE 5-10 shows a good fit, the regression line expressed as xy ×= 93.0
(R2= 0.92). It can be pre-concluded that the EQUATION 5-2 works quite well with New Zealand
samples and there is a strong correlation between elemental compositions, i.e., H/C, O/C with
measured hydrogen index from Rock-Eval analysis. Additionally as discussed in SECTION 5.1
AND SECTION 5.4 that there is a significant loss of oxygen during coalification with increase in
maturity rank within peat through high-volatile-bituminous range. The evolution of New
Zealand coals is marked by a significant decrease in O/C (0.34- 0.07), despite a minor change
in H/C (0.9- 0.7). All things are considered, it can be stated that in case of coals with O/C
ranging from 0.07 to 0.34, the evolution of hydrogen index is strongly controlled by O/C
atomic ratios. In other words, the loss of oxygen during diagenesis results in the increase of
hydrogen index of low rank coals. Whereas in case of coals with O/C < 0.1, the evolution of
hydrogen index is related with not only the depleted of O/C but mainly with the remarkable
130
-
decrease H/C arising from the generation of hydrocarbons. These findings show that simple
oxygen loss can largely explain the increase in hydrogen index occurring up to rank 10
(C.F. KILLOPS ET AL., 1996). At higher ranks, the aromatics at reactions discussed by SCHENK AND
HORSFIELD (1998) take on an overriding importance.
100 200 300 400
0
100
200
300
400M
easu
red
Hyd
roge
n In
dex
(mg/
gTO
C)
New Zealand coals
8
6
10
7
3
15
13
119
12
5
42
2221
2320
19
16
18
17
100 200 300 400
0
100
200
300
400M
easu
red
Hyd
roge
n In
dex
(mg/
gTO
C)
New Zealand coals
8
6
10
7
3
15
13
119
12
5
42
2221
2320
19
16
18
17
Figure
Although, P
impossible to corre
extensive set of A
Tertiary. They fou
there is a clear-c
composition. Mean
Zealand coals hav
POWELL ET AL. (1991)
only H/C atomic
pyrolysate hydroca
Calculated Hydrogen Index (c.f. Orr, 1983)Calculated Hydrogen Index (c.f. Orr, 1983) 5-10: Comparison between the measured HI from Rock-Eval analysis and the calculated HI values based on the O/C and H/C atomic ratios (Equation of Orr, 1981)
OWELL ET AL. (1991) AND BOREHAM & POWELL (1993) pointed out that it was
late pyrolysis yields to atomic H/C for values between 0.8 and 1.0 for an
ustralian coals and associated shales ranging in age from Permian and
nd out that only with an atomic H/C ratio of above 1.0 or below 0.8 then
ut relationship between pyrolysate hydrocarbon yield and elemental
while, this correlation seems to work quite well for the investigated New
ing H/C ~ 0.7- 0.9. This contrary result does not disclaim the results of
AND BOREHAM & POWELL (1993) but it does suggest the important role of not
ratio but also O/C atomic ratio in defining the relationship between
rbon yields and elemental compositions.
131
-
5.6 OXYGEN LOSS DURING EARLY DIAGENESIS: POTENTIAL LINK WITH THE DEEP
BIOSPHERE
The fate of the CO2 released from coal structures is of great potential importance. It
may create secondary porosity by dissolving feldspar (SURDAM ET AL., 1989) or, as discussed
below, it may be utilised by microorganism as a carbon and energy source.
Nowadays, many bacterial populations with considerable diversity have been detected
in deep sedimentary rocks (PARKES ET AL., 1994; 2000; ZLATKIN ET AL., 1996; RINGELBERG ET AL.,
1997), where are believed to be extremely hard for microorganisms to live. For example, they
can be found in hydrothermal vents whose temperature is up to 113°C (PARKES ET AL., 1994) that
by far higher than the conventional temperature limit for life in the biosphere (50°C; TISSOT
AND WELTE, 1984; KILLOPS AND KILLOPS, 1993). Microbial communities have been detected at
depths approaching 1000m (REVIEW IN HORSFIELD AND KIEFT, 2007) where overburden thickness is
one of many other factors that reduce the “living space” of microorganisms as it makes
porosity, pore interconnections, and permeability of sedimentary rocks decrease. Importantly,
living in such depth, these microbial communities are fully detached from surfaces processes
so that they are also detached from energy and food supplies. However, it has been suggested
that nutrients may be provided by abiotic reactions in deeper sediments (RICE & CLAYPOOL, 1981;
PARKES ET AL., 2000). It is assumed that the small, functionalised molecules are not only the
fermentation products of bacterial degraders in the shallow biosphere but also the products of
abiotic chemical reactions. These compounds can sustain life for ecosystems in deep
sedimentary rocks. According to the publication of HORSFIELD AND OTHERS (2006), who have
deduced that abiotically driven degradation reactions can provide substrates for microbial
activity in deep sediments in the Nankai Through (Japan) at the convergent continental
margin. Low rank coals appear to be essential well suited for feeding the deep subsurface
microbes. As has been discussed in previous sections (SECTION 5.1 AND SECTION 5.3), there is
dramatic fall in elemental O/C ratio, oxygen index and the loss of carbonyl carbon of
immature New Zealand coals as function of maturation. The products are released as either
CO2 or CO that could be substrates for microbial activity. Thus, now in this section, we try to
quantify the loss of CO2 during diagenesis to give the quantitative feeding potential link to
deep biosphere, using a mass balance model.
132
-
PELET (1985) stated that the mobile products formed during the geochemical evolution
of organic matter must be quantitatively evaluated because these products can never be found
in place in the rock where they were formed. He formulated a simple equation (TRHC; EQUATION.
5-3) for calculating the extent of conversion of precursors into hydrocarbons which utilises the
fall in hydrogen index taking place during progressive maturation of any given source rock.
( )( )⎥⎦
⎤⎢⎣
⎡−×−×
=x
xHC HIHI
HIHITR1200
1200
0
0 (EQUATION 5-3)
where HI0 = hydrogen index of immature sample
HIx = hydrogen index of mature sample
1200 represents the reciprocal (times 1000) of 0.83, the assumed
proportion of carbon in Rock- Eval pyrolysis products
For calculating actual yields as a function of TRHC, the author pointed out that the total
petroleum yields of mature sample must be normalised to the original organic carbon content
of the original sample (i.e. prior to hydrocarbon generation and migration). This normalisation
of total petroleum yield to the original organic carbon content is expressed by
)( ) ⎥⎦
⎤⎢⎣
⎡−
−×=
xi HI
HIHIHI1200
(1200 0x (EQUATION 5-4)
where HIi = hydrogen index of mature sample normalised to original TOC
HI0 = hydrogen index of immature sample
HIx = hydrogen index of mature sample
1200 represents the reciprocal (times 1000) of 0.83, the assumed
proportion of carbon in Rock- Eval pyrolysis products
For the least mature sample in the interested series, its renormalisation is not necessary
as its value HI0 represents the un-matured starting point with which all other samples are
compared. As the fact that hydrogen index decrease during maturation, this actual decrease
corresponds to the yield of hydrocarbons generated from the kerogen during maturation. This
yield is given as subtracted yield of HI0 and HIi and changes as a function of TRHC.
133
-
The approach of PELET (1985) can be extended to the oxygen index. A new
Transformation Ratio for CO2 generation (TRCO2) has been formulated here in order to
quantify loss during maturation:
( )( )⎥⎦
⎤⎢⎣
⎡−×−×
=x
xCO OIOI
OIOITR3600
3600
0
02 (EQUATION 5-5)
where OI0 = oxygen index of immature sample
OIx = oxygen index of mature sample
3600 represents the reciprocal (times 1000) of 0.27, the proportion of
carbon in CO2
The Transformation Ratio of CO2 was plotted versus vitrinite reflectance showed in
FIGURE 5-11 AND TABLE 5-2. It presents that these values do follow a rank-related trend that means
Transformation Ratios of CO2 increase with increasing in maturity. About 40% of conversion
of precursors into CO2 is already by 0.3% R0, about 80% of this conversion is already by
0.5% R0. The conversion becomes slowly and almost completes (96%) at 0.8% R0. It
obviously shows CO2 generation is one of the major features of diagenesis that might feed the
deep biosphere.
In order to calculate the yields of CO2 generated during maturation, measured input
data were renormalized to the original organic content of the original sample. The
normalisation is expressed by:
)( ) ⎥⎦
⎤⎢⎣
⎡−
−×=
xi OI
OIOIOI3600
(3600 0x (EQUATION 5-6)
where OIi = oxygen index of mature sample normalised to original TOC
OI0 = oxygen index of immature sample
OIx = oxygen index of mature sample
3600 represents the reciprocal (times 1000) of 0.27, the proportion of
carbon in CO2
134
-
10.50.30.2Vitrinite Reflectance (R0%)
0
0.2
0.4
0.6
0.8
1
Tran
sfor
mat
ion
Rat
io (T
RC
O2)
New Zealand coals3
41
2
5 6
87
9
1110
13 141215
1617 1918
21 202322 OI Transformation Ratio Samples Short label Rank(Sr) R0(%)
(mg/g TOC) TR CO2
G001979 3 0.1 0.25 107 0.00G001987 4 0.4 0.26 97 0.09G001985 1 0 0.25 95 0.12G001988 2 0 0.27 89 0.17G001986 5 0.6 0.27 83 0.23G001976 6 1.6 0.29 82 0.24G001975 8 3.4 0.29 77 0.29G001978 7 3 0.28 73 0.32G001983 9 4.7 0.41 49 0.55G001982 11 5.6 0.40 39 0.64G001977 10 5.4 0.39 36 0.67G001981 13 6.6 0.45 32 0.70G001992 14 6.9 0.49 32 0.71G001984 12 6.1 0.45 27 0.76G001980 15 7 0.44 26 0.76G001995 16 7.4 0.52 16 0.86G001997 17 7.8 0.52 15 0.87G001994 19 9.5 0.61 14 0.87G001996 18 8.3 0.52 13 0.88G001990 21 10.8 0.71 6 0.95G001993 20 10.1 0.76 5 0.95G001991 23 11.8 0.80 5 0.96G001989 22 11.6 0.69 3 0.97
Figure 5-11: The CO2 Transformation Ratio evolution as function of rank Table 5-2: The oxygen index and CO2 Transformation Ratio values of New Zealand coals
135
-
The loss of CO2 as function of TRCO2 was illustrated in FIGURE 5-12. This figure shows
that CO2 is released from kerogen structures from 10 to 105 (mg/g TOC) with increasing
TRCO2 due to maturation processes from peat to high volatile bituminous. This is equivalent to
0.23 to 2.4 millimoles CO2 per gram of total organic carbon. For methanogenesis via CO2 reduction, four moles of hydrogen are required:
OH2CHH4CO 2422 +→+
Thus, between 0.92 and 9.6 millimoles hydrogen would be required for complete CO2 reduction during diagenesis. Future work must determine if this is feasible or not.
0 0.2 0.4 0.6 0.8 1Transformation Ratio (TRCO2)
0
30
60
90
120
CO
2 lo
ss (
mg/
g TO
C)
New Zealand coals4 1
2
568
7
9
1110
13 1412 15
16 171918
21 2023 22
Figure 5-12: The CO2 loss of New Zealand coals during maturation as function of Transformation Ratio of CO2
136