Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:
Organic Geochemistry 31 (2000) 1363±1373
www.elsevier.nl/locate/orggeochem
Recognising biodegradation in gas/oil accumulations
through the d13C compositions of gas components
R.J. Pallasser *
CSIRO Petroleum, PO Box 136, North Ryde, NSW 1670, Australia
Abstract
A large suite of natural gases (93) from the North West Shelf and Gippsland and Otway Basins in Australia have
been characterised chemically and isotopically resulting in the elucidation of two types of gases. About 26% of these
gases have anomalous stable carbon isotope compositions in the C1±C4 hydrocarbons and CO2 components, and are
interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations
between successive n-alkane homologues (up to +29% PDB) and isotopically heavy CO2 (up to +19.5% PDB). Irrespective of geographic location, these anomalous gases are from the shallower accumulations (600±1700 m) where
temperatures are lower than 75 C. The secondary biogenic gases are readily distinguishable from thermogenic gases
(74% of this sample suite), which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon
accumulations are under 2000 m. While dissolution eects may have contributed to the high 13C enrichment of the CO2
component in the secondary biogenic gases, the primary signature of this CO2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis. Correlation between biodegraded oils and biodegraded ``dry'' gas supports the concept that gas is formed from the bacterial destruction of oil, resulting in ``secondary
biogenic gas''. Furthermore, the prominence of methanogenic CO2 in these types of accumulations along with some
isotopically-depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are
linked. It is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to
and supports methanogenesis. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Natural gas; Secondary biogenic gas; CO2; Carbon isotopes; Anaerobic biodegradation; Methanogenesis
1. Introduction
Natural gases are frequently studied to understand
their origins or that of associated oils. While the overall
molecular composition is a re¯ection of the formation
history, the stable carbon isotopes (d13C) of the individual components of natural gases are of particular value
because they are the product of precursor (source)
composition, overprinted by maturation and alteration
processes (Whiticar, 1994). However, examination of
natural gases is fairly limited in analytical scope so that
interpretation relies mainly on the chemical distribution
and the isotopic composition of perhaps four or ®ve
* Corresponding author. Tel.: +61-2-9887-8666; fax: +612-9887-8197.
E-mail address: robert.pallasser@syd.dpr.csiro.au
components. Furthermore, two commercially important
types of natural gas are possible and these form via two
distinct mechanisms; thermogenic or bacterial (Whiticar, 1994). These have been distinguished on the basis of
d13C composition, where values for bacterial methane
range from ÿ50 to ÿ120% PDB. The substrate or
organic precursor for bacterial gas has been generally
assumed to be immature organic matter (recent or
ancient) that is not necessarily related to any potential
source rock (Schoell, 1983; Rice, 1989). The formation
pathway for bacterial gas, essentially methane (i.e. C1/
(C1±C5)> 0.95), is often assigned as being due to either
fermentation or carbon dioxide (CO2) reduction. However, in many cases, attributing a bacterial gas to
the CO2 reduction pathway is a gross simpli®cation
because it usually avoids the issues related to oxidation
reduction potential and energy source.
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00101-7
1364
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
The signi®cance that pre-existing or associated reservoir oils actually represent the organic substrate for
many gas occurrences with a bacterial ``®ngerprint'' is
gaining momentum (Sweeney and Taylor, 1999). Compelling ®eld evidence to support this type of bacterial
gas, which was termed ``secondary biogenic gas'' by
Scott et al. (1994), is presented in this study.
Natural gases (totalling 93) were characterised chemically and isotopically from the major Australian petroleum producing provinces: the North West Shelf
region, comprising several basins and the Gippsland and
Otway Basins in south east Australia (Fig. 1). Gas samples were obtained from oil companies operating in
these regions. The main reservoirs on the North West
Shelf occur in Lower Cretaceous sandstones and the
unconformably underlying beds of the Triassic (Mungaroo Formation) and Jurassic sandstones (Delfos and
Boardman., 1994; Baillie and Jacobson, 1997), with the
deepest accumulations being found below 3000 m.
Accumulations in the gas prone Otway Basin are located within Late Cretaceous±Eocene sandstones and are
generally fairly ``dry'' (Mehin and Link, 1994). Oil and
gas accumulations in the adjacent Gippsland Basin
occur in Late Cretaceous to Early Tertiary sandstones
within the Latrobe group (geology described by Gilbert
and Hill, 1994). Aquifers underlying near-shore gas/oil
accumulations seem to be widespread in all of these
areas, meaning hydrogeological factors such as water
2ÿ
temperature, presence of oxidant anions (NOÿ
3 , SO4
ÿ
and HCO3 ) and the potential for biological contamination are probably more critical in the evolution of many
gases in these basins.
2. Analytical methods
conductivity detector (TCD) and a ¯ame ionisation
detector (FID) connected in series. Gas samples were
introduced via a 1ml sample loop. The C2±C5 hydrocarbons and carbon dioxide (CO2) were resolved on a
1.5 m 3 mm (od) stainless steel column packed with
silica gel. Permanent gases and methane (C1) were
resolved on a 1 m 3 mm (od) stainless steel column
packed with 5 AÊ molecular sieve medium. The temperature of the molecular sieve column was held constant at 60 C. The silica gel column was programmed
from 40 C (5 min) to 140 C at 10 C/min, held for 6 min
then heated to 220 C at 10 C/min and ®nally held for 11
min. Air components, CO2 and gaseous hydrocarbons
up to C5 were detected by TCD. Abundances were calculated from peak areas using weight factors based on
the relative thermal conductivities of the individual
gases.
Stable carbon isotope analysis of the C1±C5 normal
hydrocarbons and CO2 was achieved by (i) preparative
chromatographic separation, conversion and trapping
of individual gas species on a custom-built, computerautomated system using a Porapak column (4 m 3
mm od) and a cupric oxide furnace operated at 900 C,
(ii) sample puri®cation and drying by cryo-distillation at
ÿ78 C and, (iii) isotope ratio mass spectrometry
(IRMS) for the individual compounds by dual inlet
stable isotope MS on a Finnigan MAT 252. GC cryotrapping methods followed by trapping box/micro
volume MS was employed for wet gases occurring in
trace amounts. Some earlier analyses were determined
on a VG 602D IRMS. The carbon analytical system has
been maintained against the international isotopic standards for natural gases NGS #1 and NGS #2. The
stable carbon isotope compositions are expressed in
parts per thousand (%) relative to PeeDee Belemnite
(PDB), according to the expression:
Natural gas compositions were determined on a SRI
8610 gas chromatograph (GC) ®tted with a thermal
13 C% 1000
(PDB has a
Fig. 1. Map of Australia showing the basins where the natural
gases for this study were obtained.
13
13 C= 12 C
sample ÿ
13
12
C= C
13
C= 12 C
reference
reference
C/12C ratio 0.0112372).
The oil sample was separated from formation water
using a separating funnel. The dry oil was then fractionated on alumina over silica with 100 ml petroleum
ether to elute the aliphatic hydrocarbons, followed by
150 ml of a 4:1 mixture of dichloromethane and petroleum ether to elute the aromatic hydrocarbons and
®nally elution with 80 ml of 1:1 dichloromethane and
methanol to yield the polar compounds. The aliphatic
and aromatic fractions were analysed on a HewlettPackard 5973 GCMS. Chromatography was carried out
after split injection (10:1) on a fused silica (60 m 0.25
mm i.d.) DB5MS column operated from 40oC (2 min)
then heated at a rate of 4 C/min to 300 C (25 min hold).
1365
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
3. Results and discussion
3.1. Identi®cation of two gas types
The analytical data for the 93 gases are presented in
summarised form on Table 1. The gases (separated
according to the two main regions, the North West
Shelf and the Gippsland and Otway Basins) were further subdivided into two ``types'' of gas identi®ed during
this work. In addition to diagnostic chemical-compositional gas ratios, a typical gas analysis for each of the
de®ned data sets is provided in Table 1. Ranges and
means of stable carbon isotopic compositions for the
C1±C4 normal hydrocarbons and CO2 are also listed on
Table 1, as well as the mean isotopic separations
between ethane±methane and propane±ethane pairs for
the four data sets.
The designations, type 1 and type 2, are based on the
combination of isotopic traits judged to be anomalous,
i.e. comparatively greater isotopic separations between
successive gaseous alkanes and isotopically heavy CO2.
Most type 2 gases have d13C (C2±C1) values greater
than 15% and a d13C of CO2 more positive than 0%
However, several exceptions, where samples meet only
one or other of these criteria, are also included in this
category on the basis of other indicative information,
e.g. shallow depth. Gases of type 2 are generally also
drier than type 1 gases as shown by the C1/(C1±C5)
averages in Table 1.
The majority of the 93 gases studied are assigned as
type 1, but a signi®cant proportion (24, 26% of the total),
are the anomalous type 2 gases. Type 1 gases are interpreted to be typical thermogenic gases, based on the
comparatively ordered distributions of their chemical
Table 1
Summary of gas ratios, typical compositions, stable carbon isotope data and average isotopic separations for the two de®ned gas types
from the North West Shelf and the Gippsland and Otway Basins
North West Shelf Basins
Gippsland and Otway Basins
Type 1
Type 2
Type 1
Type 2
51
1221±3790
2836
19
593±1274
880
18
1358±2966
2110
5
1250±1714
1440
Typical gas compositions (MOL%)
CH4
82.93
C2H6
5.83
C3H8
2.23
i-C4H10
0.25
0.36
n-C4H10
i-C5H12
0.07
n-C5H12
0.04
7.63
CO2
N2
0.66
91.12
0.82
0.04
0.01
0.01
±
±
2.25
5.75
72.42
6.82
3.21
0.42
0.61
0.13
0.12
15.88
0.39
97.17
0.34
0.02
±
±
0.01
±
0.69
1.77
Molecular ratios
C2/C3 range
C2/C3 mean
i-C4/n-C4 range
i-C4/n-C4 mean
C1/(C1-C5) range
C1/(C1-C5) mean
1.24±10.29
3.29
0.22±1.33
0.63
0.77±0.99
0.91
0.47±44.44
15.68
0.33±18.30
4.39
0.91±1.00
0.99
1.27±3.45
2.52
0.59±1.25
0.79
0.83±0.91
0.91
1.39±39.75
12.45
0.70±12.00
3.96
0.89±1.00
0.94
Isotopic compositions
(13C %, PDB)
CH4 range
CH4 mean
C2H6 range
C2H6 mean
C3H8 range
C3H8 mean
n-C4H10 range
n-C4H10 mean
CO2 range
CO2 mean
d13C (C2ÿC1) mean
d13C (C3ÿC2) mean
ÿ55.5 to ÿ34.0
ÿ40.3
ÿ45.3 to ÿ26.2
ÿ30.5
ÿ31.1 to ÿ24.2
ÿ28.7
ÿ30.6 to ÿ23.5
ÿ28.2
ÿ18.9 to ÿ3.1
ÿ9.4
9.9
1.5
ÿ50.4 to ÿ32.3
ÿ45.3
ÿ32.8 to ÿ14.6
ÿ26.4
ÿ30.8 to ÿ14.7
ÿ23.1
ÿ30.7 to ÿ19.0
ÿ23.5
ÿ6.9 to +13.9
3.4
19.2
5.5
ÿ38.9 to
ÿ35.3
ÿ33.5 to
ÿ27.7
ÿ32.0 to
ÿ27.1
ÿ30.1 to
ÿ26.1
ÿ26.1 to
ÿ9.4
7.5
0.6
ÿ44.7 to ÿ33.0
ÿ38.6
ÿ27.4 to ÿ15.9
ÿ22.4
ÿ26.7 to ÿ23.0
ÿ24.4
ÿ26.4 to ÿ20.1
ÿ23.3
+1.0 to +19.5
10.8
16.2
7.7
Number of wells
Depth ranges (m)
Mean depth (m)
ÿ31.2
ÿ22.8
ÿ22.8
ÿ20.9
ÿ2.9
1366
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
and isotopic compositions for their geographic areas
(Table 1). Information on mean depths and ranges
(Table 1) also indicate dierences between the two gas
types with a general trend for type 1 gases to originate
from deeper reservoirs. More signi®cant than any
observed regional trends was the ubiquity of type 2
gases to shallow reservoirs in both the North West Shelf
and the Gippsland and Otway Basins, although its prevalence was greater on the North West Shelf.
The type 1 (thermogenic) gases from the Gippsland
and Otway Basins have more enriched isotopic compositions than their North West Shelf counterparts (Table
1), which could be partly source related. Additionally,
the lower d13C (Cn±Cn-1) averages of 7.5 and 0.6%
suggest higher maturities for the Gippsland and Otway
Basin gases in general (James, 1983; Xiao et al., 1997)
compared to the North West Shelf gases (9.9 and 1.6%),
despite the generally lower reservoir depths in the
Gippsland and Otway Basins. The slightly greater average i-C4/n-C4 ratio of 0.79 for the Gippsland and Otway
Basins compared to 0.63 for the North West Shelf gases
is also consistent with higher maturity (Alexander et al.,
1983). Over-mature source rocks related to higher geothermal gradients underlie parts of the Otway Basin (K.
Mehin, pers. comm.), which may account for these
values.
3.2. Interpretation of type 2 gases
The entirety of the gas data (summarised in Table 1)
has been treated graphically to contrast the anomalous
compositional and isotopic dierences which distinguish
type 2 gases from the norm of thermogenic natural gases
(type 1). Samples with greater d13C (C2ÿC1) values
(>15) tend to be the ``drier'' gases with C1/(C1±C5) over
0.95 (Fig. 2a). Similarly these gases also have the most
13
C enriched CO2 (Fig. 2b). Unusually high d13C
(CnÿCn-1) values were also found in natural gases studied by James and Burns (1984) and James (1990),
which were interpreted as evidence for biodegraded
natural gas accumulations. Interestingly, the CO2
reported in those samples was isotopically depleted,
quite unlike those found in the current study (see Section 3.6). Large d13C (C2ÿC1) separations (30%) were
also reported more recently for ``microbially reformed''
natural gases from the Liaohe Basin, China (Jianfa et
al., 1999).
The comparatively greater d13C (C2ÿC1) values
and, where measurable, the greater d13C (C3ÿC2)
values in type 2 versus type 1 gases are consistent with
biodegradation having produced the type 2 gases. Isotopic enrichment of residual hydrocarbon gas substrate
can be predicted from the ``kinetic isotope eect''
(Hoefs, 1973) and re¯ects the lower bond strengths of
12
Cÿ12C relative to 13Cÿ12C. The isotopic enrichment
of individual compounds results in wider isotopic dif-
Fig. 2. Methane±ethane carbon isotope separations for type 1
and type 2 gases and their corresponding (a)wet-gas indices and
(b) d13C of CO2. The process of designation of the gases into
type 1 and type 2 is described in the text (Section 3.1).
ferences between higher molecular weight hydrocarbons, due to what is seen to be the greater
susceptibility of higher gas alkanes to biodegradation
(James and Burns, 1984). This is re¯ected in the type 2
gases by the high C2/C3 ratios and the absence of C4 and
C5 from the typical analyses (Table 1). However in some
type 2 gas accumulations, biodegradation has also produced the other extreme, i.e. the lowest C2/C3 ratio. In
addition, type 2 ethane exhibits the most positive d13C
values of all the hydrocarbon gases (see ranges in Table
1). This supports the ®ndings of Clayton et al. (1997),
who also noted the greater isotopic eect in ethane
relative to propane. This could be due to ethane-speci®c
degrading bacteria as reported by Davis (1967). Whiticar (1994) stated that bacteria can metabolise methane
more easily than C2+ gases, however, in the present
study the type 2 gases convey the opposite trend and are
compositionally ``dry'' with very few exceptions. It is
not clear whether the apparent greater stability of
methane in the presence of the trophic bacteria
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
1367
(responsible for the destruction of C2+ hydrocarbons) is
related to the types of chemical bonds or environmental
factors such as methane-speci®c ¯ora or the type of
oxidant available. In some compositionally ``dry''
examples of type 2 gas accumulations however, methane
is found to be isotopically enriched (see ranges on Table 1),
which is discussed further in Section 3.6.
C2/C3 and i-C4/n-C4 gas ratios (where measurable) for
both gas types are plotted relative to d13C (C2ÿC1) on
Fig. 3a and relative to the d13C of CO2 on Fig. 3b.
Samples show correspondingly elevated C2/C3 and i-C4/
n-C4 ratios when isotope separations exceed 15%. Due
to the greater relative utilisation of n-alkanes by biodegradation, the proportion of branched to straight
chained alkanes is commonly used to gauge the degree
of biodegradation in oils (Palmer, 1993) as well as biodegraded gas accumulations (Larter et al., 1999). Similarly the predominance of high i-C4/n-C4 ratios where
the corresponding CO2 in the natural gas is enriched
(d13C>0%) provides evidence for the bacterial origin of
CO2 in type 2 gas accumulations. The sharply increased
(10 to 20 fold) i-C4/n-C4 ratios are clearly the result of
biodegradation (complemented by sharply increased C2/
C3 ratios and anomalous isotopic compositions) and
cannot, therefore, be misinterpreted as an indicator of
increased maturity amongst the type 2 gases (see Alexander et al., 1983; Larter et al., 1999).
Fig. 3. The relationship of gas ratios indicative of biodegradation to (a) methane±ethane carbon isotopic separations and (b)
d13C of CO2.
Fig. 4. Total ion chromatograms (TIC) of the aliphatic and
aromatic fractions separated from a North West Shelf oil typically aected by biodegradation (API
www.elsevier.nl/locate/orggeochem
Recognising biodegradation in gas/oil accumulations
through the d13C compositions of gas components
R.J. Pallasser *
CSIRO Petroleum, PO Box 136, North Ryde, NSW 1670, Australia
Abstract
A large suite of natural gases (93) from the North West Shelf and Gippsland and Otway Basins in Australia have
been characterised chemically and isotopically resulting in the elucidation of two types of gases. About 26% of these
gases have anomalous stable carbon isotope compositions in the C1±C4 hydrocarbons and CO2 components, and are
interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations
between successive n-alkane homologues (up to +29% PDB) and isotopically heavy CO2 (up to +19.5% PDB). Irrespective of geographic location, these anomalous gases are from the shallower accumulations (600±1700 m) where
temperatures are lower than 75 C. The secondary biogenic gases are readily distinguishable from thermogenic gases
(74% of this sample suite), which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon
accumulations are under 2000 m. While dissolution eects may have contributed to the high 13C enrichment of the CO2
component in the secondary biogenic gases, the primary signature of this CO2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis. Correlation between biodegraded oils and biodegraded ``dry'' gas supports the concept that gas is formed from the bacterial destruction of oil, resulting in ``secondary
biogenic gas''. Furthermore, the prominence of methanogenic CO2 in these types of accumulations along with some
isotopically-depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are
linked. It is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to
and supports methanogenesis. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Natural gas; Secondary biogenic gas; CO2; Carbon isotopes; Anaerobic biodegradation; Methanogenesis
1. Introduction
Natural gases are frequently studied to understand
their origins or that of associated oils. While the overall
molecular composition is a re¯ection of the formation
history, the stable carbon isotopes (d13C) of the individual components of natural gases are of particular value
because they are the product of precursor (source)
composition, overprinted by maturation and alteration
processes (Whiticar, 1994). However, examination of
natural gases is fairly limited in analytical scope so that
interpretation relies mainly on the chemical distribution
and the isotopic composition of perhaps four or ®ve
* Corresponding author. Tel.: +61-2-9887-8666; fax: +612-9887-8197.
E-mail address: robert.pallasser@syd.dpr.csiro.au
components. Furthermore, two commercially important
types of natural gas are possible and these form via two
distinct mechanisms; thermogenic or bacterial (Whiticar, 1994). These have been distinguished on the basis of
d13C composition, where values for bacterial methane
range from ÿ50 to ÿ120% PDB. The substrate or
organic precursor for bacterial gas has been generally
assumed to be immature organic matter (recent or
ancient) that is not necessarily related to any potential
source rock (Schoell, 1983; Rice, 1989). The formation
pathway for bacterial gas, essentially methane (i.e. C1/
(C1±C5)> 0.95), is often assigned as being due to either
fermentation or carbon dioxide (CO2) reduction. However, in many cases, attributing a bacterial gas to
the CO2 reduction pathway is a gross simpli®cation
because it usually avoids the issues related to oxidation
reduction potential and energy source.
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00101-7
1364
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
The signi®cance that pre-existing or associated reservoir oils actually represent the organic substrate for
many gas occurrences with a bacterial ``®ngerprint'' is
gaining momentum (Sweeney and Taylor, 1999). Compelling ®eld evidence to support this type of bacterial
gas, which was termed ``secondary biogenic gas'' by
Scott et al. (1994), is presented in this study.
Natural gases (totalling 93) were characterised chemically and isotopically from the major Australian petroleum producing provinces: the North West Shelf
region, comprising several basins and the Gippsland and
Otway Basins in south east Australia (Fig. 1). Gas samples were obtained from oil companies operating in
these regions. The main reservoirs on the North West
Shelf occur in Lower Cretaceous sandstones and the
unconformably underlying beds of the Triassic (Mungaroo Formation) and Jurassic sandstones (Delfos and
Boardman., 1994; Baillie and Jacobson, 1997), with the
deepest accumulations being found below 3000 m.
Accumulations in the gas prone Otway Basin are located within Late Cretaceous±Eocene sandstones and are
generally fairly ``dry'' (Mehin and Link, 1994). Oil and
gas accumulations in the adjacent Gippsland Basin
occur in Late Cretaceous to Early Tertiary sandstones
within the Latrobe group (geology described by Gilbert
and Hill, 1994). Aquifers underlying near-shore gas/oil
accumulations seem to be widespread in all of these
areas, meaning hydrogeological factors such as water
2ÿ
temperature, presence of oxidant anions (NOÿ
3 , SO4
ÿ
and HCO3 ) and the potential for biological contamination are probably more critical in the evolution of many
gases in these basins.
2. Analytical methods
conductivity detector (TCD) and a ¯ame ionisation
detector (FID) connected in series. Gas samples were
introduced via a 1ml sample loop. The C2±C5 hydrocarbons and carbon dioxide (CO2) were resolved on a
1.5 m 3 mm (od) stainless steel column packed with
silica gel. Permanent gases and methane (C1) were
resolved on a 1 m 3 mm (od) stainless steel column
packed with 5 AÊ molecular sieve medium. The temperature of the molecular sieve column was held constant at 60 C. The silica gel column was programmed
from 40 C (5 min) to 140 C at 10 C/min, held for 6 min
then heated to 220 C at 10 C/min and ®nally held for 11
min. Air components, CO2 and gaseous hydrocarbons
up to C5 were detected by TCD. Abundances were calculated from peak areas using weight factors based on
the relative thermal conductivities of the individual
gases.
Stable carbon isotope analysis of the C1±C5 normal
hydrocarbons and CO2 was achieved by (i) preparative
chromatographic separation, conversion and trapping
of individual gas species on a custom-built, computerautomated system using a Porapak column (4 m 3
mm od) and a cupric oxide furnace operated at 900 C,
(ii) sample puri®cation and drying by cryo-distillation at
ÿ78 C and, (iii) isotope ratio mass spectrometry
(IRMS) for the individual compounds by dual inlet
stable isotope MS on a Finnigan MAT 252. GC cryotrapping methods followed by trapping box/micro
volume MS was employed for wet gases occurring in
trace amounts. Some earlier analyses were determined
on a VG 602D IRMS. The carbon analytical system has
been maintained against the international isotopic standards for natural gases NGS #1 and NGS #2. The
stable carbon isotope compositions are expressed in
parts per thousand (%) relative to PeeDee Belemnite
(PDB), according to the expression:
Natural gas compositions were determined on a SRI
8610 gas chromatograph (GC) ®tted with a thermal
13 C% 1000
(PDB has a
Fig. 1. Map of Australia showing the basins where the natural
gases for this study were obtained.
13
13 C= 12 C
sample ÿ
13
12
C= C
13
C= 12 C
reference
reference
C/12C ratio 0.0112372).
The oil sample was separated from formation water
using a separating funnel. The dry oil was then fractionated on alumina over silica with 100 ml petroleum
ether to elute the aliphatic hydrocarbons, followed by
150 ml of a 4:1 mixture of dichloromethane and petroleum ether to elute the aromatic hydrocarbons and
®nally elution with 80 ml of 1:1 dichloromethane and
methanol to yield the polar compounds. The aliphatic
and aromatic fractions were analysed on a HewlettPackard 5973 GCMS. Chromatography was carried out
after split injection (10:1) on a fused silica (60 m 0.25
mm i.d.) DB5MS column operated from 40oC (2 min)
then heated at a rate of 4 C/min to 300 C (25 min hold).
1365
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
3. Results and discussion
3.1. Identi®cation of two gas types
The analytical data for the 93 gases are presented in
summarised form on Table 1. The gases (separated
according to the two main regions, the North West
Shelf and the Gippsland and Otway Basins) were further subdivided into two ``types'' of gas identi®ed during
this work. In addition to diagnostic chemical-compositional gas ratios, a typical gas analysis for each of the
de®ned data sets is provided in Table 1. Ranges and
means of stable carbon isotopic compositions for the
C1±C4 normal hydrocarbons and CO2 are also listed on
Table 1, as well as the mean isotopic separations
between ethane±methane and propane±ethane pairs for
the four data sets.
The designations, type 1 and type 2, are based on the
combination of isotopic traits judged to be anomalous,
i.e. comparatively greater isotopic separations between
successive gaseous alkanes and isotopically heavy CO2.
Most type 2 gases have d13C (C2±C1) values greater
than 15% and a d13C of CO2 more positive than 0%
However, several exceptions, where samples meet only
one or other of these criteria, are also included in this
category on the basis of other indicative information,
e.g. shallow depth. Gases of type 2 are generally also
drier than type 1 gases as shown by the C1/(C1±C5)
averages in Table 1.
The majority of the 93 gases studied are assigned as
type 1, but a signi®cant proportion (24, 26% of the total),
are the anomalous type 2 gases. Type 1 gases are interpreted to be typical thermogenic gases, based on the
comparatively ordered distributions of their chemical
Table 1
Summary of gas ratios, typical compositions, stable carbon isotope data and average isotopic separations for the two de®ned gas types
from the North West Shelf and the Gippsland and Otway Basins
North West Shelf Basins
Gippsland and Otway Basins
Type 1
Type 2
Type 1
Type 2
51
1221±3790
2836
19
593±1274
880
18
1358±2966
2110
5
1250±1714
1440
Typical gas compositions (MOL%)
CH4
82.93
C2H6
5.83
C3H8
2.23
i-C4H10
0.25
0.36
n-C4H10
i-C5H12
0.07
n-C5H12
0.04
7.63
CO2
N2
0.66
91.12
0.82
0.04
0.01
0.01
±
±
2.25
5.75
72.42
6.82
3.21
0.42
0.61
0.13
0.12
15.88
0.39
97.17
0.34
0.02
±
±
0.01
±
0.69
1.77
Molecular ratios
C2/C3 range
C2/C3 mean
i-C4/n-C4 range
i-C4/n-C4 mean
C1/(C1-C5) range
C1/(C1-C5) mean
1.24±10.29
3.29
0.22±1.33
0.63
0.77±0.99
0.91
0.47±44.44
15.68
0.33±18.30
4.39
0.91±1.00
0.99
1.27±3.45
2.52
0.59±1.25
0.79
0.83±0.91
0.91
1.39±39.75
12.45
0.70±12.00
3.96
0.89±1.00
0.94
Isotopic compositions
(13C %, PDB)
CH4 range
CH4 mean
C2H6 range
C2H6 mean
C3H8 range
C3H8 mean
n-C4H10 range
n-C4H10 mean
CO2 range
CO2 mean
d13C (C2ÿC1) mean
d13C (C3ÿC2) mean
ÿ55.5 to ÿ34.0
ÿ40.3
ÿ45.3 to ÿ26.2
ÿ30.5
ÿ31.1 to ÿ24.2
ÿ28.7
ÿ30.6 to ÿ23.5
ÿ28.2
ÿ18.9 to ÿ3.1
ÿ9.4
9.9
1.5
ÿ50.4 to ÿ32.3
ÿ45.3
ÿ32.8 to ÿ14.6
ÿ26.4
ÿ30.8 to ÿ14.7
ÿ23.1
ÿ30.7 to ÿ19.0
ÿ23.5
ÿ6.9 to +13.9
3.4
19.2
5.5
ÿ38.9 to
ÿ35.3
ÿ33.5 to
ÿ27.7
ÿ32.0 to
ÿ27.1
ÿ30.1 to
ÿ26.1
ÿ26.1 to
ÿ9.4
7.5
0.6
ÿ44.7 to ÿ33.0
ÿ38.6
ÿ27.4 to ÿ15.9
ÿ22.4
ÿ26.7 to ÿ23.0
ÿ24.4
ÿ26.4 to ÿ20.1
ÿ23.3
+1.0 to +19.5
10.8
16.2
7.7
Number of wells
Depth ranges (m)
Mean depth (m)
ÿ31.2
ÿ22.8
ÿ22.8
ÿ20.9
ÿ2.9
1366
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
and isotopic compositions for their geographic areas
(Table 1). Information on mean depths and ranges
(Table 1) also indicate dierences between the two gas
types with a general trend for type 1 gases to originate
from deeper reservoirs. More signi®cant than any
observed regional trends was the ubiquity of type 2
gases to shallow reservoirs in both the North West Shelf
and the Gippsland and Otway Basins, although its prevalence was greater on the North West Shelf.
The type 1 (thermogenic) gases from the Gippsland
and Otway Basins have more enriched isotopic compositions than their North West Shelf counterparts (Table
1), which could be partly source related. Additionally,
the lower d13C (Cn±Cn-1) averages of 7.5 and 0.6%
suggest higher maturities for the Gippsland and Otway
Basin gases in general (James, 1983; Xiao et al., 1997)
compared to the North West Shelf gases (9.9 and 1.6%),
despite the generally lower reservoir depths in the
Gippsland and Otway Basins. The slightly greater average i-C4/n-C4 ratio of 0.79 for the Gippsland and Otway
Basins compared to 0.63 for the North West Shelf gases
is also consistent with higher maturity (Alexander et al.,
1983). Over-mature source rocks related to higher geothermal gradients underlie parts of the Otway Basin (K.
Mehin, pers. comm.), which may account for these
values.
3.2. Interpretation of type 2 gases
The entirety of the gas data (summarised in Table 1)
has been treated graphically to contrast the anomalous
compositional and isotopic dierences which distinguish
type 2 gases from the norm of thermogenic natural gases
(type 1). Samples with greater d13C (C2ÿC1) values
(>15) tend to be the ``drier'' gases with C1/(C1±C5) over
0.95 (Fig. 2a). Similarly these gases also have the most
13
C enriched CO2 (Fig. 2b). Unusually high d13C
(CnÿCn-1) values were also found in natural gases studied by James and Burns (1984) and James (1990),
which were interpreted as evidence for biodegraded
natural gas accumulations. Interestingly, the CO2
reported in those samples was isotopically depleted,
quite unlike those found in the current study (see Section 3.6). Large d13C (C2ÿC1) separations (30%) were
also reported more recently for ``microbially reformed''
natural gases from the Liaohe Basin, China (Jianfa et
al., 1999).
The comparatively greater d13C (C2ÿC1) values
and, where measurable, the greater d13C (C3ÿC2)
values in type 2 versus type 1 gases are consistent with
biodegradation having produced the type 2 gases. Isotopic enrichment of residual hydrocarbon gas substrate
can be predicted from the ``kinetic isotope eect''
(Hoefs, 1973) and re¯ects the lower bond strengths of
12
Cÿ12C relative to 13Cÿ12C. The isotopic enrichment
of individual compounds results in wider isotopic dif-
Fig. 2. Methane±ethane carbon isotope separations for type 1
and type 2 gases and their corresponding (a)wet-gas indices and
(b) d13C of CO2. The process of designation of the gases into
type 1 and type 2 is described in the text (Section 3.1).
ferences between higher molecular weight hydrocarbons, due to what is seen to be the greater
susceptibility of higher gas alkanes to biodegradation
(James and Burns, 1984). This is re¯ected in the type 2
gases by the high C2/C3 ratios and the absence of C4 and
C5 from the typical analyses (Table 1). However in some
type 2 gas accumulations, biodegradation has also produced the other extreme, i.e. the lowest C2/C3 ratio. In
addition, type 2 ethane exhibits the most positive d13C
values of all the hydrocarbon gases (see ranges in Table
1). This supports the ®ndings of Clayton et al. (1997),
who also noted the greater isotopic eect in ethane
relative to propane. This could be due to ethane-speci®c
degrading bacteria as reported by Davis (1967). Whiticar (1994) stated that bacteria can metabolise methane
more easily than C2+ gases, however, in the present
study the type 2 gases convey the opposite trend and are
compositionally ``dry'' with very few exceptions. It is
not clear whether the apparent greater stability of
methane in the presence of the trophic bacteria
R.J. Pallasser / Organic Geochemistry 31 (2000) 1363±1373
1367
(responsible for the destruction of C2+ hydrocarbons) is
related to the types of chemical bonds or environmental
factors such as methane-speci®c ¯ora or the type of
oxidant available. In some compositionally ``dry''
examples of type 2 gas accumulations however, methane
is found to be isotopically enriched (see ranges on Table 1),
which is discussed further in Section 3.6.
C2/C3 and i-C4/n-C4 gas ratios (where measurable) for
both gas types are plotted relative to d13C (C2ÿC1) on
Fig. 3a and relative to the d13C of CO2 on Fig. 3b.
Samples show correspondingly elevated C2/C3 and i-C4/
n-C4 ratios when isotope separations exceed 15%. Due
to the greater relative utilisation of n-alkanes by biodegradation, the proportion of branched to straight
chained alkanes is commonly used to gauge the degree
of biodegradation in oils (Palmer, 1993) as well as biodegraded gas accumulations (Larter et al., 1999). Similarly the predominance of high i-C4/n-C4 ratios where
the corresponding CO2 in the natural gas is enriched
(d13C>0%) provides evidence for the bacterial origin of
CO2 in type 2 gas accumulations. The sharply increased
(10 to 20 fold) i-C4/n-C4 ratios are clearly the result of
biodegradation (complemented by sharply increased C2/
C3 ratios and anomalous isotopic compositions) and
cannot, therefore, be misinterpreted as an indicator of
increased maturity amongst the type 2 gases (see Alexander et al., 1983; Larter et al., 1999).
Fig. 3. The relationship of gas ratios indicative of biodegradation to (a) methane±ethane carbon isotopic separations and (b)
d13C of CO2.
Fig. 4. Total ion chromatograms (TIC) of the aliphatic and
aromatic fractions separated from a North West Shelf oil typically aected by biodegradation (API