Reply
Reply to Comment of Smith and Pallasser on
``Factors controlling the origin of gas in
Australian Bowen Basin coals''
C.J. Boreham *, S.D. Golding, M. Glikson
Australian Goelogical Survey Organisation, Marine and Petroleum Division, GPO Box 378, Canberra, ACT 2601, Australia
Firstly, we wish to acknowledge the leadership and contribution that Smith and his various coworkers have made over the last three decades to the understanding of the origin of natural gas and coal seam gas, particularly through the use of stable isotopes. Our intention was in no way to criticise their views. Rather, as expressed in Boreham et al. (1998) our view was and remains ``that the origin of coal seam gases especially methane, is not as straightforward as previously thought''. Indeed, Smith (1999) has recently oered an alternative abio-genic origin for methane and CO2via decomposition of
acetic acid (Smith et al., 1998). The essence of our paper was that laboratory studies, using open system pyrolysis methods with extrapolation to geological heating rates, demonstrate that a carbon isotope composition heavier than ÿ50% for methane is consistent with a thermo-genic origin.
Secondly, we acknowledge the incorrectness of a statement in Boreham et al. (1998) which credited Smith and Pallasser (1996) as reporting that they have ``argued for a predominantly microbiological origin for methane based on reduction of CO2 (sourced from a magmatic
origin) by methanogenic bacteria''. We inadvertently included the phrase ``sourced from magmatic origin'', which has lead to this rebu on the part of Smith and Pallasser. We acknowledge the proposition in Smith and Pallasser (1996), and re-stated in Smith (1999) and Ahmed and Smith (1999), that the origin of the iso-topically light methane in the coal seam gases is from the biogenic reduction of CO2(d13C ÿ23%) where the
source of the CO2 is organic, derived from
decarbox-ylation of the coal. The view of Smith and Pallasser
(1996, p. 891) is that the magmatic-derived CO2 (d13C
72%and >10 mol%) is inert to attack by methano-gens. However, we have diculty with this model since it is hard to see how a microorganism can distinguish the source of CO2, although Smith (1999) has oered
some possible reasons. Another weakness of the Smith and Pallasser (1996) model is that they attribute the downhole increase in the13C content of methane to an
increasing mix of heavy methane (d13C
ÿ5%) relative to the isotopically light biogenic methane. However, they oer no source for this unusually heavy methane.
The main concern expressed in the Comment of Smith and Pallasser is our handling of data appearing in Smith and Pallasser (1996 and references therein) and reproduced by Boreham et al. (1998) in their Fig. 2 and Table 1. This ®gure presented a Rayleigh model ®t to the data to show that the data in Smith and Pallasser (1996) are inconsistent with a `closed system' approach. With this model, the small systematic isotopic changes in the carbon isotopic compositions of the product methane and the residual CO2cannot be reconciled with
the large kinetic isotope eect generally associated with the methanogenic process.
As noted above, Smith and Pallasser (1996, p. 891) have argued that there is no genetic relationship between CO2of magmatic origin and isotopically light
methane. Speci®cally, they state ``where it (CO2) occurs
in seam gas in highly variable proportions, the relatively constant isotopic composition of the CH4 shows that
neither reduction of CO2 to CH4, nor isotopic
equili-bration between CO2and CH4has occurred''. However,
this observation is also consistent with microbial reduc-tion of magmatic CO2where the kinetic isotope eect is
55% and the CO2availability is non-limiting; i.e. an
open system. Additionally, coal seam gases in the Syd-ney Basin (Fig. 1) with CO2contents <10 mol% show
0146-6380/01/$ - see front matter#2001 Published by Elsevier Science Ltd. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 4 0 - 6
Organic Geochemistry 32 (2001) 207±210
www.elsevier.nl/locate/orggeochem
* Corresponding author. Tel.: 2-6249-9488; fax: +61-2-6249-9933.
both isotopically heavy (enriched in 13C) and light
(depleted in13C) CO
2compared to CO2derived from a
magmatic source (where CO2>10 mol%). If the heavy
CO2is indeed a marker for methanogenesis and
repre-sents the remaining CO2, then the `parent' CO2cannot
solely be the isotopically light CO2 since both have
similarly low mol% (Fig. 1). The parent CO2must have
had an originally higher mol%, possibly within the range identi®ed as having a magmatic origin. Hence, the magmatic CO2could play an active role in the bacterial
alteration process.
In their Comment, Smith and Pallasser make parti-cular reference to our lack of recognition of their large dataset of over 300 seam gas analyses and the restriction of our experience to experimental studies. With respect to the former, the purpose of our paper was not to review the available isotope datasets for coal seam gas compositions in the Bowen Basin, but rather to present new experimental results bearing on the origin of these gases. With respect to the latter our recent investigation used a range of methodologies to establish the thermal history of a set of Bowen Basin coals as well as their composition, gas chemistry and isotope composition (Glikson et al., 1995, 1999; Golding et al., 1999, 2000; Uysal et al., 2000a,b).
We present here for completeness a re-analysis of the data from Smith et al. (1984), the majority of which appears in Smith and Pallasser (1996). The frequency distribution of thed13C for methane from coals of the
Bowen and Sydney Basins (Fig. 2) shows a similar average isotope value for methane. However, in the Bowen Basin there is a much higher proportion of sam-ples with d13C CH
4 heavier than ÿ50%, suggesting a
higher thermogenic component in this region. In the Sydney Basin, samples with low CO2 contents show a
wide range in carbon isotopes (Fig. 1). The isotopically light CO2 is most likely sourced from the associated
organic matter (Smith and Pallasser, 1996). Those sam-ples with positive carbon isotope values are, we assert, residuals of methanogenesis, necessitating a higher initial CO2(see discussion above). In the Bowen Basin,
no seam gases with low CO2 contents have yet been
analysed for their isotopic composition. However, these two processes are obviously also active in the Bowen Basin if the light and heavy isotopes for CO2 in the
natural gases are used as a guide. The situation is slightly dierent in other sedimentary basins. In the Carnarvon and Gippsland basins, Australia's two main natural gas producing provinces, no methanogenic uti-lisation of CO2has been observed (although recent
evi-dence does suggest methanogenic utilisation of CO2 in
the Carnarvon Basin; Crostella and Boreham, 2000). The molecular and isotopic compositions are solely governed by the degree of mixing of organic-derived (thermogenic) and inorganic-derived (mantle and/or igneous) CO2(Fig. 3).
A further complication for the Bowen Basin coals is that vitrinite re¯ectance and clay mineral diagenesis studies indicate thermal maturation occurred largely as a result of a short-lived hydrothermal event in the Late Triassic rather than during maximum burial in the
Fig. 1. Plot of carbon isotopes of CO2versus mol% CO2for coal seam gases from the Bowen and Sydney basins (&,&data from Smith et al., 1984;~data from Boreham, 1995).
Middle Triassic (Uysal et al., 2000a,b). Whereas isotopic exchange between CO2and CH4has not been
demon-strated in laboratory pyrolysis experiments (Sackett and Chung, 1979), the extent of isotopic exchange in geo-thermal systems is a function of cooling rate and the ®nal temperature of the rising geothermal ¯uid (Giggenbach, 1982). Carbon isotopic equilibrium between CO2and CH4in geothermal systems is unlikely
at temperatures less than 250C although equilibrium
compositions from higher temperature parts of the sys-tem may be preserved with rapid sys-temperature decline (Giggenbach, 1982; Ohmoto, 1986). The latter possibi-lity is supported by the carbon and oxygen isotope sys-tematics of carbonate mineralisation in Bowen Basin coals, which indicate that much of the carbonate was deposited from a mixed CO2/CH4bearing ¯uid of
rela-tively constant carbon isotopic composition (Boreham et al., 1998; Golding et al., 2000).
We contend that the majority of the large coal seam gas resources-in-place, estimated using an economic cut-o depth of 1200 m (Miyazaki and Korsch, 1993), are of thermogenic origin. Coal seam gases at shallow depths (<500 m) may be more biased towards a bacterial input. However, a major contribution from a thermo-genic origin for these shallow seams as proposed in Boreham et al. (1998) and, as originally suggested by Rigby and Smith (1982) and Smith et al. (1982), may have been subsequently underrated in Smith and Pal-lasser (1996). Other factors not yet considered that may contribute to, though not solely explain, the isotopic depletion of the methane and the leanness in wet gas content are secondary processes such as (i) diusion of methane through and within the coal matrix, which could lead to large isotopic fractionation (Krooss et al., 1999; Prinzhofer et al., 1999), (ii) preferential `¯ushing' of wet gas components through invasion of the mag-matic CO2 (analogous to dry gas stripping of
con-densable C2+ hydrocarbons; Price, 1995), and (iii)
selective loss of wet gas components due to uplift to near-surface conditions; notwithstanding a possible abiogenic origin (Smith, 1999). Obviously, more detailed work is required to identify the origin of coal seam gases in Australian coal-bearing basins.
References
Ahmed, M., Smith, J.W., 1999. Microbial alteration of coals in biogenic methane generation. In: Schoell, M., Claypool, G. (Convenors), Natural Gas Formation and Occurrence, AAPG Hedberg Conference, 6±10 June 1999, Durango, CO (extended abstract).
Boreham, C.J., 1995. Origin of petroleum in the Bowen and Surat Basins: geochemistry revisited. Australian Petroleum Exploration Association Journal 35, 579±612.
Boreham, C.J., Golding, S.D., Glikson, M., 1998. Factors controlling the origin of gas in Australian Bowen Basin coals. Organic Geochemistry 29, 347±362.
Crostella, A., Boreham, C.J., 2000. Origin, distribution and migration patterns of gas in the Northern Carnarvon Basin. Petroleum Exploration Society of Australia 28, in press (December).
Giggenbach, W.F., 1982. Carbon-13 exchange between CO2 and CH4under geothermal conditions. Geochimica et Cos-mochimica Acta 46, 159±165.
Glikson, M., Golding, S.D., Lawrie, G., Szabo, L.S., Fong, C., Baublys, K., Saxby, J.D., Chats®eld, P., 1995. Hydrocarbon generation in Permian coals of Queensland, Australia: source of coalseam gases. In: Follington, I.L., Beeston, J.W. and Hamilton, L.H. (Eds.), Proceedings Bowen Basin Sympo-sium, Mackay, Queensland, pp. 205±216.
Glikson, M., Boreham, C.J., Thiede, D.S., 1999. Coal compo-sition and mode of maturation, a determining factor in quantifying hydrocarbon species generated. In: Mastalerz, M., Glikson, M., Golding, S.D. (Eds.), Coalbed Methane: Scienti®c, Environmental and Economic Evaluation. Kluwer Academic Publishers, The Netherlands, pp. 155±185. Golding, S.D., Baublys, K.A., Glikson, M., Uysal, I.T.,
Bore-ham, C.J., 1999. Source and timing of coal seam gas genera-tion in Bowen Basin coals. In: Mastalerz, M., Glikson, M., Golding, S.D. (Eds.), Coalbed Methane: Scienti®c, Environ-mental and Economic Evaluation. Kluwer Academic Pub-lishers, The Netherlands, pp. 257±269.
Golding, S.D., Collerson, K.D., Uysal, U.T., Glikson, M., Bau-blys, K., Zhao, J.X., 2000. Nature and source of carbonate mineralisation in coals of the Bowen Basin, eastern Australia: implications for the origin of coal seam methane and other hydrocarbons sourced from coal. In: Glikson, M., Mastalerz, M. (Eds.), Organic Matter and Mineralisation: Thermal Altera-tion, Hydrocarbon Generation and Role in Metallogenesis. Kluwer Academic Publishers, The Netherlands, pp. 296±313. Krooss, B.M., Schoemer, S., Zhang, T., Gaschnitz, R., Gerling,
P., 1999. On the appraisal of fractionation processes during natural gas migration. In: Schoell, M., Claypool, G. (Con-venors), Natural Gas Formation and Occurrence, AAPG Hedberg Conference, 6±10 June 1999, Durango, CO (exten-ded abstract).
Fig. 3. Plot of carbon isotopes of CO2versus mol% CO2for Australian natural gases (data from AGSO's organic geo-chemistry database, ORGCHEM, unpublished).
Miyazaki, S., Korsch, R.J., 1993. Coalbed methane resources in the Permian of eastern Australia and their tectonic setting. Australian Petroleum Exploration Association Journal 33 (1), 161±175.
Ohmoto, H., 1986. Stable isotope geochemistry of ore deposits, In: Valley, J.W., Taylor, H.P., O'Neil, J.R. (Eds.), Stable Isotopes in High Temperature Geological Processes. Reviews in Mineralogy 16, 491±559.
Price, L.C., 1995. Origins, characteristics, controls, and eco-nomic viabilities of deep-basin gas resources. Chemical Geology 126, 335±349.
Prinzhofer, A., Charlette, I., Caja, M., Mello, M.R., Guzman-Vega, M., 1999. Could gas really fractionate isotopically during natural migration? In: Schoell, M., Claypool, G. (Convenors), Natural Gas Formation and Occurrence, AAPG Hedberg Conference, 6±10 June 1999, Durango, CO (extended abstract).
Rigby, D., Smith, J.W., 1982. A reassessment of stable carbon isotopes in hydrocarbon exploration. ErdoÈl und Kohle Erd-gas Petrochem 35, 415±417.
Sackett, W.M., Chung, H.M., 1979. Experimental con®rmation of the lack of carbon isotope exchange between methane and carbon oxides at high temperature. Geochimica et Cosmo-chimica Acta 43, 273±276.
Smith, J.W., 1999. The development of an understanding of the origins of the Sydney and Bowen Basin gases. In: Mastalerz,
M., Glikson, M., Golding, S.D. (Eds.), Coalbed Methane: Scienti®c, Environmental and Economic Evaluation. Kluwer Academic Publishers, The Netherlands, pp. 271±277. Smith, J.W., Pallasser, R.J., 1996. Microbiological origin of
Australian coalbed methane. American Association Petro-leum Geologists Bulletin 80, 891±897.
Smith, J.W., Gould, K.W., Rigby, D., 1982. The stable isotope geochemistry of Australian coals. Organic Geochemistry 5, 111±131.
Smith, J.W., Botz, R.W., Gould, K.W., Hart, G., Hunt, J.W., Rigby, D., 1984. Outburst and Gas Drainage Investigations. Report 321. National Energy Research, Development and Demonstration Program, Department of Resources and Energy, Commonwealth of Australia (unpublished). Smith, J.W., Pallasser, R.J., Pang, L.S.K., 1998. Thermal
reac-tions of acetic acid:13C/12C partitioning between CO2 and CH4. Organic Geochemistry 29, 79±82.
Uysal, I.T., Glikson, M., Golding, S.D., Audsley, F., 2000a. The thermal history of the Bowen Basin, Queensland, Aus-tralia: vitrinite re¯ectance and clay mineralogy of Late Per-mian coal measures. Tectonophysics 323, 105±129.
Uysal, I.T., Golding, S.D., Baublys, K., 2000b. Stable isotope geochemistry of authigenic clay minerals from Late Permian coal measures, Queensland, Australia: implications for the evolution of the Bowen Basin. Earth and Planetary Science Letters 180, 149±162.
