Recognising biodegradation in gas/oil accumulations
through the
d
13
C 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±C4hydrocarbons and CO2components, and are
interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations between successiven-alkane homologues (up to +29%PDB) and isotopically heavy CO2(up to +19.5%PDB).
Irre-spective of geographic location, these anomalous gases are from the shallower accumulations (600±1700 m) where temperatures are lower than 75C. 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 high13C enrichment of the CO
2
component in the secondary biogenic gases, the primary signature of this CO2is attributed to biochemical
fractiona-tion associated with anaerobic degradafractiona-tion and methanogenesis. Correlafractiona-tion between biodegraded oils and biode-graded ``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 CO2in 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
indivi-dual 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
components. Furthermore, two commercially important types of natural gas are possible and these form via two distinct mechanisms; thermogenic or bacterial (Whiti-car, 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.
How-ever, 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. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 0 1 - 7
www.elsevier.nl/locate/orggeochem
* Corresponding author. Tel.: 2-9887-8666; fax: +61-2-9887-8197.
The signi®cance that pre-existing or associated reser-voir oils actually represent the organic substrate for many gas occurrences with a bacterial ``®ngerprint'' is gaining momentum (Sweeney and Taylor, 1999). Com-pelling ®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 che-mically and isotopically from the major Australian pet-roleum producing provinces: the North West Shelf region, comprising several basins and the Gippsland and Otway Basins in south east Australia (Fig. 1). Gas sam-ples 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 (Mun-garoo 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 loca-ted 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 temperature, presence of oxidant anions (NO3ÿ, SO42ÿ
and HCO3ÿ) and the potential for biological
contamina-tion are probably more critical in the evolucontamina-tion of many gases in these basins.
2. Analytical methods
Natural gas compositions were determined on a SRI 8610 gas chromatograph (GC) ®tted with a thermal
conductivity detector (TCD) and a ¯ame ionisation detector (FID) connected in series. Gas samples were introduced via a 1ml sample loop. The C2±C5
hydro-carbons and carbon dioxide (CO2) were resolved on a
1.5 m3 mm (od) stainless steel column packed with silica gel. Permanent gases and methane (C1) were
resolved on a 1 m3 mm (od) stainless steel column packed with 5 AÊ molecular sieve medium. The tem-perature of the molecular sieve column was held con-stant at 60C. The silica gel column was programmed
from 40C (5 min) to 140C at 10C/min, held for 6 min
then heated to 220C at 10C/min and ®nally held for 11
min. Air components, CO2 and gaseous hydrocarbons
up to C5were detected by TCD. Abundances were
cal-culated 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 CO2was achieved by (i) preparative
chromatographic separation, conversion and trapping of individual gas species on a custom-built, computer-automated system using a Porapak column (4 m 3 mm od) and a cupric oxide furnace operated at 900C,
(ii) sample puri®cation and drying by cryo-distillation at
ÿ78C and, (iii) isotope ratio mass spectrometry
(IRMS) for the individual compounds by dual inlet stable isotope MS on a Finnigan MAT 252. GC cryo-trapping methods followed by cryo-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 stan-dards 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:
13C%1000
The oil sample was separated from formation water using a separating funnel. The dry oil was then fractio-nated 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 petro-leum 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 Hewlett-Packard 5973 GCMS. Chromatography was carried out after split injection (10:1) on a fused silica (60 m0.25 mm i.d.) DB5MS column operated from 40oC (2 min)
then heated at a rate of 4C/min to 300C (25 min hold).
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 fur-ther subdivided into two ``types'' of gas identi®ed during this work. In addition to diagnostic chemical-composi-tional 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±C4normal hydrocarbons and CO2are 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 1andtype 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 (C
2±C1) values greater
than 15% and a d13C of CO
2 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 inter-preted 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
Number of wells 51 19 18 5
Depth ranges (m) 1221±3790 593±1274 1358±2966 1250±1714
Mean depth (m) 2836 880 2110 1440
Typical gas compositions (MOL%)
CH4 82.93 91.12 72.42 97.17
C2H6 5.83 0.82 6.82 0.34
C3H8 2.23 0.04 3.21 0.02
i-C4H10 0.25 0.01 0.42 ±
n-C4H10 0.36 0.01 0.61 ±
i-C5H12 0.07 ± 0.13 0.01
n-C5H12 0.04 ± 0.12 ±
CO2 7.63 2.25 15.88 0.69
N2 0.66 5.75 0.39 1.77
Molecular ratios
C2/C3range 1.24±10.29 0.47±44.44 1.27±3.45 1.39±39.75
C2/C3mean 3.29 15.68 2.52 12.45
i-C4/n-C4range 0.22±1.33 0.33±18.30 0.59±1.25 0.70±12.00
i-C4/n-C4mean 0.63 4.39 0.79 3.96
C1/(C1-C5) range 0.77±0.99 0.91±1.00 0.83±0.91 0.89±1.00
C1/(C1-C5) mean 0.91 0.99 0.91 0.94
Isotopic compositions (13C%, PDB)
CH4range ÿ55.5 toÿ34.0 ÿ50.4 toÿ32.3 ÿ38.9 toÿ31.2 ÿ44.7 toÿ33.0
CH4mean ÿ40.3 ÿ45.3 ÿ35.3 ÿ38.6
C2H6range ÿ45.3 toÿ26.2 ÿ32.8 toÿ14.6 ÿ33.5 toÿ22.8 ÿ27.4 toÿ15.9
C2H6mean ÿ30.5 ÿ26.4 ÿ27.7 ÿ22.4
C3H8range ÿ31.1 toÿ24.2 ÿ30.8 toÿ14.7 ÿ32.0 toÿ22.8 ÿ26.7 toÿ23.0
C3H8mean ÿ28.7 ÿ23.1 ÿ27.1 ÿ24.4
n-C4H10range ÿ30.6 toÿ23.5 ÿ30.7 toÿ19.0 ÿ30.1 toÿ20.9 ÿ26.4 toÿ20.1
n-C4H10mean ÿ28.2 ÿ23.5 ÿ26.1 ÿ23.3
CO2range ÿ18.9 toÿ3.1 ÿ6.9 to +13.9 ÿ26.1 toÿ2.9 +1.0 to +19.5
CO2mean ÿ9.4 3.4 ÿ9.4 10.8
d13C (C2ÿC
1) mean 9.9 19.2 7.5 16.2
d13C (C3ÿC
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 pre-valence was greater on the North West Shelf.
The type 1 (thermogenic) gases from the Gippsland and Otway Basins have more enriched isotopic compo-sitions than their North West Shelf counterparts (Table 1), which could be partly source related. Additionally, the lower d13C (C
n±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 aver-agei-C4/n-C4ratio 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 geo-thermal 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 (C
2ÿC1) values
(>15) tend to be the ``drier'' gases with C1/(C1±C5) over
0.95 (Fig. 2a). Similarly these gases also have the most
13C enriched CO
2 (Fig. 2b). Unusually high d13C
(CnÿCn-1) values were also found in natural gases
stu-died 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 Sec-tion 3.6). Larged13C (C
2ÿC1) 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 (C
2ÿC1) values
and, where measurable, the greater d13C (C 3ÿC2)
values in type 2 versus type 1 gases are consistent with biodegradation having produced the type 2 gases. Iso-topic enrichment of residual hydrocarbon gas substrate can be predicted from the ``kinetic isotope eect'' (Hoefs, 1973) and re¯ects the lower bond strengths of
12Cÿ12C relative to 13Cÿ12C. The isotopic enrichment
of individual compounds results in wider isotopic
dif-ferences between higher molecular weight hydro-carbons, 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/C3ratios and the absence of C4and
C5from the typical analyses (Table 1). However in some
type 2 gas accumulations, biodegradation has also pro-duced the other extreme, i.e. the lowest C2/C3ratio. In
addition, type 2 ethane exhibits the most positived13C
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). Whiti-car (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
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 CO
2. The process of designation of the gases into
(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/C3andi-C4/n-C4gas ratios (where measurable) for
both gas types are plotted relative tod13C (C
2ÿC1) on
Fig. 3a and relative to the d13C of CO
2 on Fig. 3b.
Samples show correspondingly elevated C2/C3andi-C4/
n-C4ratios when isotope separations exceed 15%. Due
to the greater relative utilisation ofn-alkanes by biode-gradation, the proportion of branched to straight chained alkanes is commonly used to gauge the degree of biodegradation in oils (Palmer, 1993) as well as bio-degraded gas accumulations (Larter et al., 1999). Simi-larly 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
CO2in 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 Alex-ander et al., 1983; Larter et al., 1999).
3.3. Type 2 gases and biodegraded oils
The type 2 gases are often accompanied by underlying oils that show evidence of biodegradation. A sample of heavy oil (API <20) recovered from one of the more
shallow accumulations (600 m) on the North West Shelf has the characteristic unresolved complex mixture in both the aliphatic and aromatic fractions (Fig. 4). Bio-degraded oils have been correlated to bioBio-degraded hydrocarbon gases (James and Burns, 1984) and to13C
enriched CO2 (Dimitrakopoulos and Muehlenbachs,
1987). Valyayev and Grinchenko (1985) also noted the presence of isotopically ``ultra heavy'' CO2where oils/
gases were within a few hundred metres of the surface. On the North West Shelf (current study), gases showing signs of biodegradation tend to exist at present depths of 600 to 1300m, which can be correlated with tem-peratures generally under 75C. In the Gippsland and
Otway Basins this type of alteration appears at depths less than 1700 m depending on well location in the basin in relation to elevated geothermal gradients. Based on information from well data and/or geothermal gra-dients for these localities, the temperatures for these gas
Fig. 4. Total ion chromatograms (TIC) of the aliphatic and aromatic fractions separated from a North West Shelf oil typi-cally aected by biodegradation (API <20).
Fig. 3. The relationship of gas ratios indicative of biodegrada-tion to (a) methane±ethane carbon isotopic separabiodegrada-tions and (b)
reservoirs were in the order of 50±65C. Temperature
(related to depth) is probably the most important environmental constraint (Connan et al., 1997) that determines whether type 1 gases become biodegraded to type 2 gases. This is re¯ected in a plot of depth against thed13C of CO
2(Fig. 5), which shows that type 2 gases
occur within 2000 m of the surface whereas thermogenic (type 1) gases tend to occur deeper.
3.4. Application of indicators for exploration and appraisal
Assessing natural gases in terms of their wet-gas index (C1/(C1ÿC5)) has been widely used for maturity
com-parisons because this ratio increases with increased source maturity (Hunt, 1979). Although this index may provide a fair guide in general terms, it fails to address physical and chemical processes at work during migra-tion and in the reservoir, including PVT alteramigra-tion. This may explain some poor correlations found between stable carbon isotope data and gas compositions in many provinces. A good example is a condensate which has a ``wet'' gas composition but isotopic compositions in keeping with high maturities (James, 1983).
Assessing natural gases on the basis of absoluted13C
data of individual hydrocarbons can also lead to ambi-guities since both biodegradation and maturation increase these values. The type 2 gases in this study
could be misinterpreted as over-mature because they are ``dry'' and have highd13C values, a point emphasised by
James and Burns (1984). Thed13C of C
1ÿC4gases have
been shown to become more positive systematically with increasing thermal maturity (James, 1983; Galimov, 1988; Xiao et al., 1997). Biodegradation alters or destroys any maturity relationship imprinted on d13C.
To illustrate this thed13C for four thermogenic and ®ve
biodegraded gases from the North West Shelf have been plotted on Fig. 6 (after Chung et al., 1988) who pro-posed thatd13C values are linearly related to the inverse
of their respective carbon numbers (1/Cn). The gradients
of thermogenic gases plotted in this way are a function of maturity (mature gases produce ¯atter curves), while biodegraded gases are evident by their oset and mark-edly increased slopes.
A ranking in relative maturity (RM) obtained from these gradients can therefore provide a convenient way to compare a large number of gases since it is given by a single value:
RM1= ÿG
where ÿG is the negative gradient determined by the decrease ind13C over the change in 1/C
nbetween C4and
C1. RM for all the North West Shelf gases have been
plotted against the d13C for methane (Fig. 7). RM
values range from 0.02 to 0.18, the latter representing
Fig. 5. Relationship of d13C of CO
2 versus depth for type 1
(thermogenic) gases and type 2 (biodegraded) gases which mostly occur under 1500 m.
Fig. 6. Natural gas plot ofd13C(C
n) versus 1/Cn,, for type 1
high maturity. This shows ®rstly the systematic correla-tion between increasing RM and increasing d13C for
thermogenic methane (type 1), and secondly that type 2 gases plot separately, lack a clear trend, and are iso-topically depleted in comparison to type 1 gases. The latter observation suggests methanogenic input into type 2 natural gases. The apparent low maturity of type 2 gases is partly due to isotopically-enriched ethane, and partly due to isotopically-depleted methane.
Integration of all indicators (Section 3.1±3.4), espe-cially the co-occurrence of biodegraded oils, provides a useful tool in predicting hydrocarbon type and quality irrespective of geographic location. Understanding how type 2 signatures develop should assist in clarifying the distinction between natural gases. Any data found to report intermediate between the two gas types could represent incipient bacterial degradation, thus threshold values or boundaries need to be assessed against ther-mogenic gases in other regions.
3.5. The signi®cance of CO2
The CO2 associated with type 2 gases is usually in
fairly low concentration (indicated by the typical com-positions on Table 1) and tends to be isotopically
enri-ched to a signi®cant extent (up to +19.5% PDB; Fig.
5). Whilst this correlation is only very general, a review of the processes that potentially lead to the ®nal isotopic composition is warranted, since this is a very important indicator for gas in biodegraded accumulations.
3.5.1. Physical fractionation
Given the noticeably lower concentrations of CO2
(<3% of total gas) and the prominence of aquifer sys-tems associated with this type of gas accumulation, it is possible that aqueous dissolution and removal of this gas might alter its ®nald13C composition. Fundamental
studies into the isotopic equilibrium between gaseous and dissolved CO2have consistently shown that there is
a 1.03% enrichment (") between the phases at 25C
(Szaran, 1998), with the lighter isotope having the greater solubility. This direction of isotopic change means that loss of the very soluble CO2 through the
groundwater system should result in the13C enrichment
of the remaining CO2. Theoretically this exceeds 1%
because of the continuum of removal of CO2 over an
integrated time period of biodegradation occurring at the oil±water contact. In a similar approach to that of Fuex (1980) the magnitude of carbon isotopic enrich-ment for the remaining CO2(d13Cf) was modelled using
the following relationship (see Clark and Fritz, 1997);
13Cf 13C0"Ln f
whered13C
0is its isotopic composition before dissolving
away, " is the isotope enrichment (" decreases about 0.16% at 60C), and Ln(f) is the natural log of the
remaining fraction.
Assuming that 99.9% of all reservoired CO2was lost
into an adjacent aquifer, the remaining fraction would be enriched by around 4±5% PDB. While alteration
through this mechanism is limited to 5%, it could pos-sibly be a component of the ®nal isotopic composition in some cases. Even though some biodegraded oils have very low gas/oil ratios (100 cubic feet gas per barrel of oil), the possible eects of methane dissolution ond13C
were regarded to be even less (2%), based on the work
of Fuex (1980).
3.5.2. Bacterially generated CO2andd13C
The concentrations and isotopic signatures of CO2
found in type 1 and 2 gases are plotted in Fig. 8. Inclu-ded on this scheme for comparison are data points representing (i) secondary carbonates found near shal-low oil accumulations that have been oxidised to CO2
by microorganisms (Gould and Smith, 1978), (ii) aero-bic oxidation of methane leaking from coal seams into sandy soil and aerobic oxidation of methane in ground-waters (CSIRO unpublished data) and (iii) Sydney metropolitan land®ll gases (CSIRO unpublished data). The broad trend of thermogenic CO2 resulting from Fig. 7. Natural gases from The North West Shelf distinguished
according to type using relative maturity (RM) and thed13C of
decarboxylation reactions during source maturation shown by line A±B, can be seen to blend into that of inorganic origins on Fig. 8. CO2 generated by aerobic
respiration, represented by (i) and (ii), appears to re¯ect more closely the substrate or food source used by the microorganisms, in this case oil (ÿ27%) and methane (ÿ45%). Similarly the oxidation of oil spills in soils
produces CO2 with values around ÿ25% (R. Krouse,
pers. comm.). Conversely, the CO2associated with
bio-degraded oils and type 2 gases discussed in this study have very enriched d13C values, in the range +6.9 to
+13.9% and +1.0 to +19.5% for the North West
Shelf and the Gippsland and Otway Basins, respectively. The source distributions presented here relate very well to those described by Jenden et al. (1993). In compar-ison, investigations by Carothers and Kharaka (1980) found soluble bicarbonate in shallow oil-®eld waters up to +28%PDB. Also,13C enriched carbonates that were
attributed to biodegradation of petroleum were recor-ded withd13C values as positive as +14%PDB
(Dimi-trakopoulos and Muehlenbachs, 1987). In both of these cases the origin of the CO2was thought to be the result
of bacterial fermentation processes, oil being considered the source for this CO2in the latter. On the other hand,
CO2 ranging up to +17% associated with shallow
(<500 m) coal seams was found to be consistent with CO2 reduction (Smith and Pallasser, 1996). Notably,
CO2associated with anaerobic organic degradation and
methanogenesis recovered from land®lls denoted by (iii), also shares the isotopic region occupied by type 2 gases. In summary, aerobic oil and gas biodegradation can be correlated with CO2 in a fairly negative d13C
range (ÿ25 to ÿ45% PDB) while anaerobic
biode-gradation appears to result in residual CO2with a fairly
positived13C range (0 to +20%PDB).
3.6. Biodegradation of gas/oil accumulations and methanogenesis.
There is increasing evidence that major ``dry'' gas ®elds have arisen from the bacterial degradation of oil (Dimitrakopoulos and Muehlenbachs, 1987; Sweeney and Taylor, 1999; this study). As shown earlier, type 2 gases are often marked by bacterial methane and methanogenic CO2(Figs. 7 and 8) and are accumulated
in relatively shallow reservoirs (600 to 1700 m) at tem-peratures of 50 to 75C. Reassessment of the giant
bio-genic gas reserves described by Rice and Claypool (1981) is probably warranted considering the interpreted importance of oil as the source material for this ``dry'' gas.
Hydrocarbon alteration within reservoirs has com-monly been attributed to aerobic biodegradation. In this study, anaerobic oil biodegradation appears to be more signi®cant than aerobic processes (Fig. 8) where viable temperatures and the availability of anaerobic oxidants (NO3ÿ, SO42ÿor HCO3ÿ) are perhaps the main
determi-nants (Connan et al., 1997). It is not clear whether oxi-dation via either NO3ÿ or SO42ÿ results in isotopically
depleted CO2as in the case of aerobic oxidation [shown
by (i) and (ii) in Fig. 8], but the CO2with values ofÿ8
andÿ38%PDB interpreted by James and Burns (1984) to have resulted from biodegradation (reservoired >2200 m and temperature 70C) may be two examples.
The frequently discussed alternatives of fermentation versus CO2reduction to account for bacterial methane
genesis, requires clari®cation. Fermentation can proceed in isolation to produce methane and CO2which may be
important in groundwater systems. However CO2
reduction to methane requires an electron donor (i.e. higher redox potential) such as H2. The provision of H2
is energy intensive and necessitates the breakdown of some other substrate such as organic matter. For example, in land ®lls anaerobic processes account for the breakdown of complex organic materials and con-current production of gas (typicallyÿ51%for methane
and +16% for CO2). In circumstances where CO2
reduction is suspected, a secondary source of energy that supports this process must be sought.
It is proposed that methanogenesis plays a vital part in the biodegradation of oil. While methane can be
Fig. 8. Comparison of CO2(d13C versus mol%) from type 1
and type 2 gases (in this study) with aerobic and anaerobic CO2
produced through fermentation on certain organic sub-strates, this mechanism has not been demonstrated on the components of oils. On the other hand, oil can be degraded by anaerobic oxidation with electron accep-tors such as nitrate (NO3ÿ), sulphate (SO42ÿ) or
bicarbo-nate (HCO3ÿ), the latter referring to CO2reduction. Oil
biodegradation is a good example of the way the CO2
reduction process requires a complementary degrada-tional mechanism to provide an electron donor. The electron donor is probably in the form of H2and
pre-sumably the substrate (degraded oil/water) becomes depleted in H. It is suggested here that through a bac-terial partnership or coupling, hydrocarbons are oxi-dised by HCO3ÿ (CO2) resulting in the hydroxylation
and possible hydrolysis of oil compounds. The H2
released is simultaneously taken up in the transforma-tion of CO2(or residual from HCO3ÿ) to methane (Fig.
9). In this case it is the oil that provides the energy for the overall redistribution of C, H and O. Interspecies H2
transfer, as is proposed here, has been demonstrated in other systems (Marty, 1989; Cord-Ruwisch et al., 1998) where hydrolytic oxidative processes and CO2reduction
occur alongside. Strong isotopic enrichment of CO2very
likely accompanies its complex involvement during the coupled HCO3ÿoxidation±reduction stages. A
Rayleigh-type continuum operative during this biogeochemical
processing would amplify this even further. Metallic electron acceptors Fe(III) and Mn(IV) can also degrade oil hydrocarbons but are thought to be less important than CO2and SO42ÿ(Hunkeler et al., 1998).
The bacterial methane apparent with type 2 gases (Fig. 7), are not strongly indicative (ÿ51 toÿ32%PDB)
of methanogenesis according to published data (Whiti-car, 1994). In fact the isotopically enriched values amongst this group of gases (ÿ32%) indicate methano-trophism (Zyakun, 1989), which is the utilisation of methane by bacteria. Isotopic dierences between CO2
and methane have been used to distinguish bacterial gas origins where fractionations are 55±80% (Smith and
Pallasser, 1996, and references therein). Therefore, using the fractionation between these two gases as evidence for methanogenic processes is misleading in this study where values as low as 37%arise because of
methano-trophism. These observations suggest that the methane has undergone degradation in addition to the higher hydrocarbons, which is analogous to the cycling of methane (i.e. production and destruction) described for lake environments (Ellis et al., 1999), although the type of oxidation is expected to be quite dierent.
4. Conclusions
Recognition of biodegradation in gas/oil accumula-tions can be readily achieved using a comprehensive assemblage of chemical and isotopic characteristics. Type 2 gases are determined by the greaterd13C (C
2
-C1) values, elevated C2/C3andi-C4/n-C4ratios and13C
enriched CO2, indicators which are applicable
any-where. While these gas types are fairly widespread they tend to occur under 1500m. By understanding these indicators the risk of misinterpreting ``dry'' and iso-topically-enriched gases should be reduced. Type 2 gases often co-occur with biodegraded oils, thus providing an important tool for hydrocarbon evaluation during exploration and appraisal.
The distinctive CO2 found in these accumulations
may be partly accounted for by dissolution eects of up to 5% but the main enrichment in the ®nal isotopic
composition is due to biochemical fractionation. The circumstantial evidence that type 2 gases, and in parti-cular methanogenic CO2, are often present over
biode-graded oils suggests that the processes of bacterial methane generation and bacterial oil destruction are linked processes. It is proposed that oil biodegradation supports methanogenesis through a coupled mechanism where H2released from the anaerobic degradation of oil
is taken up in the transformation of CO2 to methane
and that these biochemical steps take place simulta-neously. The prevalence of this ``secondary biogenic gas'' type suggests that biodegradation of oil in reser-voirs is a major source for ``dry'' natural gas.
Fig. 9. Proposed model for the formation of ``secondary bio-genic gas'' emphasising the critical interdependence of petro-leum biodegradation and methanogenesis where energy is transferred in the form of H2from the degrading oil to drive
Acknowledgements
I wish to thank B. Krooss and D. Stoddart for their most helpful and constructive comments. Also I appre-ciated the very useful discussions with B. Walker (for-merly of Mobil). Thanks also go to M. Ahmed and R. Quezada for sample preparation and GCMS of the biodegraded oil sample.
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