The Walloon Subgroup coal seam gas (CSG) play is poorly studied despite ramping production and significant in situ bioreactor potential. This study uses a four-fold sequential approach to target optimum locations for methanogenesis along the core production fairway in the eastern Surat Basin. A sedimentary framework model forms the foundation to the study, based on the interpretation of open-file wireline log data. A desktop study using derived model layers formulates refined hypotheses on gas distribution and origins. Finer-scale field studies integrating geochemical (gas, water, host coal) and geological data then test these hypotheses. Lastly, a conceptual microbial methane exploration model integrates all of the results with parallel work on the in situ bioreactor potential of the Surat Basin.
The Middle Jurassic Walloon Subgroup thins to the south through a combination of depositional thinning and truncation. Results of regional basin analysis support deposition within a southerly prograding fluvial system or clastic wedge. Regional wireline log correlation upholds the interpretation of an upper (Juandah) and lower (Taroom) coal measures separated by a relatively coal-barren unit (Tangalooma Sandstone), similar to industry findings. Within these units, lateral variation in coal character is high, precluding a regionally agreed coal group or seam correlation. Nonetheless, the coal measures are laterally extensive and considerably thick, facilitating an assessment of down-hole gas trends in a regional stratigraphic context. Gas content either (1) increases; (2) increases, then decreases; or (3) decreases with depth. The majority of Walloon Subgroup CSG wells display a parabolic (Trend 2) profile, which inflects around the Tangalooma Sandstone, regardless of depth. As such, a number of hypotheses for gas content distribution and origins are proposed and tested.
Stable isotopic analysis of 4 desorbed gas profiles suggests that the Walloon Subgroup CSG play is a formation-scale finite reservoir system. Down-hole δ13C-CH4 profiles show a positively parabolic trend that echoes gas content. Although an intermediate (-52.8‰) mean δ13C-CH4 composition is consistent with mixed microbial/thermogenic origins, relatively low down-hole variability in δD-CH4, systematic 13C-enrichment in methane and CO2, and dry gas compositions are evidence that Walloon Subgroup CSG is dominated by secondary microbial methane generated by the reduction of CO2. Carbon [Δ13C(CO2-CH4)] and deuterium isotopic differences [ΔD(H2O-CH4)], and cross-plots of δD-H2O and δ18O-H2O also imply kinetic isotope effects related to microbially-mediated carbonate reduction. Consistently, methane and CO2 carbon isotopic compositions are decoupled from those of the coals.
Hydro-geological factors and processes govern gas content distribution and origins in the eastern Surat Basin. Methanogenesis is linked to meteoric recharge, coal seam permeability and cleat development. Higher gas contents and systematically enriched CH4 and CO2 carbon isotopic compositions in the central coal seams (oldest-youngest: in the upper Taroom Coal Measures, Tangalooma Sandstone lower Juandah Coal Measures) are best explained by increased rates of microbial CO2 reduction and closed system substrate depletion. In this sense, Walloon Subgroup δ13C-CH4 and δ13C-CO2 values reflect the extent of microbial methanogenesis, rather than the degree of thermogenic influence or the dominant methanogenic pathway. Differing down-hole gas content profiles are interpreted to reflect flowpath length and inter- and intra-formation permeability deterioration. Individually, these gas content profiles are overprinted by geographic changes in hydrology (e.g. coal permeability, water quality, extent of meteoric recharge, flushing), which further influence microbe ingress, microbial cell concentration, groundwater residence time and gas retention.
These findings are relevant for applying microbially enhanced coal bed methane (MECoM), and potentially, CO2 sequestration in the Surat Basin. There is a building case to trial MECoM in the central, more permeable Walloon coal seams in a depleted/underperforming well on the Undulla Nose structural high. In this ‘sweet spot’, the coals are highly permeable and better-cleated and the surface drainage direction parallels regional structural dip, enabling greater meteoric and microbial recharge. Moreover, in places, the Undulla Nose is overlain by thick floodplain alluvium that may act as a hydrodynamic seal to retain pressure and further enhance recharge.
Overall, this work has shown that spatial variability of methanogenesis is site-specific, requiring a holistic approach to the prediction of gas characteristics within a given basin. In the Walloon Subgroup, lithological heterogeneity influences the extent of secondary microbial methane generation, which is a primary control on gas content, gas isotopic composition, and probably, the degree of saturation. Stratigraphic changes in coal character influence fracture and banding permeability, which affects microbe ingress, bioavailability, and hydrology. Geographic structural and surface drainage domains overprint these parameters.