Comment on Boreham et al . ( 1998 ) , ` ` Factors controlling the origin of gas in Australian Bowen Basin coals ' ' ( Organic Geochemistry 29 , 347 ± 376 )

It is with some concern that we note in Boreham et al. (1998) detailed reference to our earlier work on the origin of coal bed methane (CH4) in the Sydney and Bowen Basins of Australia (Smith and Pallasser, 1996), together with a totally erroneous description of the conclusions we drew from our data. Due to their error, the detailed discussion, Fig. 2 and Table 1 provided by Boreham et al. (1998) to illustrate shortcomings in our propositions are in themselves incorrect. In the past we have paid particular interest to the origins and dC contents of coal-associated carbon dioxide (CO2) as a guide to the environments and processes of methane generation. In essence it appears that the experience of Boreham and co-workers has been largely related to the gaseous products of laboratory carbonisation tests and with those resulting from direct coal/intrusive interaction in regions of thermal metamorphism. Other mechanisms responsible for much of the CO2 generation (<500 m) in the Eastern Australian coal®elds are not considered, although long since described (e.g. Gould et al., 1981). One of these sources is the coal itself which, via oxidation, decarboxylation, etc., at shallow levels of burial, generates CO2 with a dC value similar to that of the coal (ÿ23 1% PDB). It is this coal-associated CO2 that we believe undergoes kinetic reduction to the ``light'', dry CH4 and the resulting C enriched, isotopically related, residual CO2. C (CO2±CH4) values of 60 10% PDB are commonly measured in these product gases (Smith and Pallasser, 1996). Such a fractionation is in itself a feature strongly associated with biogenic rather than physical processes. The second source of CO2 is of external magmatic origin (ÿ7 2% PDB) and may be present in proportions sucient to completely displace methane from the coal seam. In our recent description of this magmatic CO2 (Smith and Pallasser, 1996, p. 891), we report that where it 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 equilibration between CO2 and CH4 has occurred; that is, the introduced CO2 behaves as an inert diluent''. Nevertheless, although this carbon dioxide is identi®ed as having no active role in our model for seam gas generation, Boreham et al. (1998, p. 348) state that we have ``argued for a predominantly microbiological origin for methane based on reduction of CO2 (sourced from a magmatic origin) by methanogenic bacteria''. In this regard, our much earlier data (Smith et al., 1984), quoted in Table 1 of Boreham et al. (1998), show the largely magmatic CO2 component of a gas from Nattai to vary from 5 to 95% v/v and the CH4 content to vary correspondingly from 95 to 5% v/v. In such circumstances, the absence of signi®cant variation between the dC values of both components over this compositional range again clearly illustrates a complete lack of interaction between CO2 and CH4. Accordingly, it should not surprise Boreham and co-workers that isotopic di€erences between CO2 and the CH4 are commonly unrelated to equilibrium values and that correspondingly, under such non-equilibrium conditions, calculated values of (the isotopic enrichment) are meaningless. In support of these ®ndings, exhaustive laboratory studies of the interaction between CO2 and CH4 under a wide range of conditions at temperatures to 500 C provided no evidence of chemical or isotopic exchange between these gases (Sackett and Chung, 1979). Unfortunately, it seems that the conclusions of Boreham et al. (1998) were reached without reference to the bulk of this work (including Smith and Pallasser, 1996), in which the