Microbial controls on the origin and evolution of coal seam gases and production waters of the Walloon Subgroup; Surat Basin, Australia

The Walloon Subgroup coal seam gas (CSG) play in the Surat Basin, Queensland, is Australia's pre-eminent onshore gas field. Concerted multi-disciplinary research is underway investigating the distribution, origin and composition of waters and gases in this dominantly microbial CSG reservoir, to guide both continued production and potential microbially enhanced coal bed methane (MECoM) applications. However, prior to the present research, a detailed study of co-produced waters and gases from across the Surat Basin was not available in the public domain. This study tested whether co-produced water compositional and stable isotopic data show relationships with production gas stable isotope compositions, to elucidate further evidence for microbial CO2 reduction and explore the down-dip geochemical evolution of Walloon coal bed waters and gases. A total of 41 wells were sampled with 50 water and 25 gas samples spanning the 3 major production areas of the Surat Basin. Detailed isotopic and hydrochemical analysis of these samples revealed distinct spatial trends between the different production locales. Water compositions were distinct for each of the production regions reflecting the different lithologies of adjacent recharge zones, differing fluid-rock interactions, likely different microbial consortia, and the extent of methanogenesis. On the western side of the basin near Roma, waters were the 'freshest' with the lowest median values for alkalinity (861 mg/L), and Cl- (588 mg/L) and a delta C-13(DIC) of 14.2 parts per thousand. On the eastern side of the basin, the Kogan Nose waters were the most saline with the highest median values for Na+ (1955 mg/L), Cl- (2280 mg/L) and delta C-13(DIC) (20.0 parts per thousand). Also in the east, in the present gas fairway, the Undulla Nose waters had the highest median alkalinity (1841 mg/L) and were found to have a Na+ excess (median = 1050 mg/L) and a lower than expected median delta C-13(DIC) (14.0 parts per thousand). Co-mingled, produced methane carbon isotope values (delta C-13 -57.0 parts per thousand to -44.5 parts per thousand) from both the upper (Juandah) and lower (Taroom) coal measures plot within the mixed thermogenic/microbial genetic field. By contrast, deuterium isotopic difference [Delta H-2(H2O-CH4)] values and cross-plots of delta H-2-H2O and delta O-18-H2O suggest that microbially mediated CO2 reduction is the dominant methane generation process in situ. At a given depth, the Undulla Nose waters in the east are more depleted in H-2 and O-18 than elsewhere in the Surat Basin, which may suggest these samples have been more heavily impacted by water-rock-microbial reactions. C-14 values from the 3 production regions (0.115 to 1.769 pmC; age: 32,400 to >50,000 years before present (B.P.)) suggest that Walloon coals likely recharged in the last similar to 50,000 years (limit of radiocarbon dating). Consistent with these dates, delta H-2-H2O and delta O-18-H2O values for the Surat Basin (delta H-2 - 32 parts per thousand to -56 parts per thousand, delta O-18 - 5.9 parts per thousand to -9.0 parts per thousand) echo the stable isotopic composition of meteoric waters during the initial part of the last glacial period in southeast Queensland. Based on a strong correlation between delta H-2-CH4 and delta H-2-H2O, we suggest that methane was generated since the Late Pleistocene. PCA analysis showed a degree of positive correlation between total alkalinity and both the delta C-13(DIC) (median 14.2 parts per thousand) and delta C-13-CH4 (median 52.1 parts per thousand) vectors that is consistent with finite reservoir effects. The results inform ongoing studies of gas distribution and origins and MECoM potential in the Surat Basin, and underpin a broader study examining aquifer interactions. (C) 2015 Elsevier B.V. All rights reserved.