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