Reply to Comment of Smith and Pallasser on ``Factors controlling the origin of gas in Australian Bowen Basin coals''

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 o€ered an alternative abiogenic origin for methane and CO2 via 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 thermogenic 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 isotopically light methane in the coal seam gases is from the biogenic reduction of CO2 (dC ÿ23%) where the source of the CO2 is organic, derived from decarboxylation of the coal. The view of Smith and Pallasser (1996, p. 891) is that the magmatic-derived CO2 (dC 7 2% and >10 mol%) is inert to attack by methanogens. However, we have diculty with this model since it is hard to see how a microorganism can distinguish the source of CO2, although Smith (1999) has o€ered some possible reasons. Another weakness of the Smith and Pallasser (1996) model is that they attribute the downhole increase in the C content of methane to an increasing mix of heavy methane (dC ÿ5%) relative to the isotopically light biogenic methane. However, they o€er 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 CO2 cannot be reconciled with the large kinetic isotope e€ect generally associated with the methanogenic process. As noted above, Smith and Pallasser (1996, p. 891) have argued that there is no genetic relationship between CO2 of 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 equilibration between CO2 and CH4 has occurred''. However, this observation is also consistent with microbial reduction of magmatic CO2 where the kinetic isotope e€ect is 55% and the CO2 availability is non-limiting; i.e. an open system. Additionally, coal seam gases in the Sydney Basin (Fig. 1) with CO2 contents <10 mol% show