Geochemical influences on methanogenic groundwater from a low rank coal seam gas reservoir: Walloon Subgroup, Surat Basin

Hydrochemical data responds at a much slower rate to changes in groundwater conditions than does the propagation of hydraulic pressure, and therefore may provide more insight to groundwater flow paths. In low rank coal measures, where gas is biogenic, it is important to understand the fluid-rock and microbial interactions that affect the spatial and temporal distribution of groundwater composition. Pressure data may not reflect true groundwater conditions pre-anthropogenic influence, nor does it provide information on the main drivers of groundwater composition, actual aquifer behaviour or even prove groundwater flow. This study uses a process-based approach to interpret a combination of tracer (Cl-36, C-14, Sr-87/Sr-86, O-18/O-16) and hydrochemical data obtained from coal seam gas production wells to identify the main geochemical processes and thus controls on the groundwater composition in different coal seam producing areas of the Walloon Subgroup, Surat Basin, Australia. This is arguably one of the largest coal seam gas producing regions in the world. Tracer data measured in this study show that the Walloon Subgroup behaves as a stagnant aquitard, as indicated by the almost total loss of cosmogenic tracers over relatively short groundwater flow distances (similar to 15 km), suggestive of very low ground water flow velocities. The range of Cl-36 is 9.0 to 23.8 (x 10(-15)) while the Cl-36 values across the Undulla anticline in the eastern edge of the basin, are essentially the same (12.2-14.7) within analytical error. It is argued that these isotopic values represent secular equilibrium for the Walloon Subgroup. Radiometric carbon (C-14) levels across all three production areas (Roma, Undulla Nose, Kogan Nose) are also too low (range = 0.12-1.95 pMC) for viable field interpretation largely owing to the long residence time of the groundwater and the local activity of methanogens. Groundwater flow velocity was estimated to be <0.1 m/y, which is significantly less than the 0.7 m/y recently reported for the underlying Hutton Sandstone. As a result of the low groundwater flow velocities, trends in geochemistry are visible only in production regions proximal to the subcrop. At flow distances greater than 10-15 km from subcrop, several low-temperature interactions (cation exchange, silicate weathering, matrix diffusion and hyperfiltration) start to influence groundwater composition. Shallow subsurface chemical and microbial reactions may initially dominate the geochemical composition of the meteoric groundwater, but this is then overprinted by the actions of sulfate reducers and methanogens, resulting in groundwater with the typical geochemical characteristics similar to other coal bed methane groundwater in basins across the world (low SO4, Ca, Mg and high HCO3, Na, Cl). As distance and depth increase further, low temperature fluid-rock interactions then begin to influence the groundwater composition. This holistic, process-based approach applied to a combination of cosmogenic and stable isotopes, and standard hydrochemical data interpreted against basin lithology has enabled a more comprehensive picture on the behaviour of the groundwater of the Walloon Subgroup and is applicable to the study of other sedimentary basins.