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Near-surface leakage detection is often a crucial part of verifying the success of CO 2 sequestration projects, and it cannot be achieved without a detailed description of the natural state of an operational site prior to injection. Without baseline studies, it is difficult to define CO 2 anomalies outside natural variation in the subsurface that may be related to leakage. CO 2 concentrations and δ 13 C-CO 2 values (of DIC, dissolved gas, and soil gas) were investigated to establish a baseline modelling of natural chemical processes leading to CO 2 occurrence and δ 13 C-CO 2 fractionation in the soil-groundwater interface on an example at the Glenhaven site in Queensland, Australia. This was analysed within the context of the hydrogeology and hydrogeochemistry of a shallow aquifer system (<20 m depth). The Glenhaven site and its five monitoring station are located in an area which is a candidate for a future small scale carbon storage pilot project. We combine major ion, stable isotope data of groundwater and soil gas as well as physical hydrogeology data with extensive isotope models to explain the pathways and origins of groundwater and CO 2 . The results indicate shallow aquitard claystone units at the site restrict vertical groundwater leakage and inter-aquifer mixing. Groundwater chemistry is controlled by transpiration of plants, cation exchange, weathering and meteoric recharge to produce Na-Cl and Na-HCO 3 -Cl type waters. δ 18 O and δ 2 H values suggest a shallow meteoric recharge source largely unaffected by evaporation. CO 2 concentrations in soil and dissolved in groundwater range between 631 and 47,516 ppm and δ 13 C values average −17 and −21‰. The mixing of three potential CO 2 sources is discussed: 1) the aerobic decay of C 3 plant matter, 2) the aerobic decay of C 4 plant matter and, 3) shallow coal related CO 2 . Despite this, the variability of CO 2 concentrations and equivocal sources is not likely to hinder leakage detection in the future of site monitoring. [ABSTRACT FROM AUTHOR]

This paper uses hydrochemical and multi-isotope analysis to investigate geological controls on coal seam gas (CSG) saturation domains and gas well production performance in a high-rank (vitrinite reflectance (Rv) > 1.1) CSG field in the north-western Bowen Basin, Australia. New hydrochemical and stable isotope data were combined with existing geochemical datasets to refine hypotheses on the distribution and origins of CSG in two highly compartmentalized Permian coal seams. Stable isotopic results suggest that geographic variations in gas content, saturation and production reflect the extent of secondary microbial gas generation and retention as a function of hydrodynamics. δ13C and δ2H data support a gas mixing hypothesis with δ13C-CH4 increasing from secondary biogenic values to thermogenic values at depth (δ13C −62.2‰ to −46.3‰), whereas correlated methane and carbon dioxide carbon isotope compositions, Δ13C(CO2–CH4) values and δ13CDIC/alkalinity trends are largely consistent with microbial CO2 reduction. In addition, below 200 m, the majority of δ13C-CO2 values are positive (δ13C: −1.2‰ to 7.1‰) and δ13CDIC shows an erratic increase with depth for both seams that is characteristic of evolution via microbial activity. The progression of carbon isotope values along the CO2 reduction fractionation line suggests progressive depletion of the CO2 reservoir with increasing depth. Faults clearly segment coal seams into areas having significantly different production, with results of geochemical analysis suggesting that pooling of biogenic gas and waters and enhanced methanogenesis occur north of a faulted hinge zone. [ABSTRACT FROM AUTHOR]

This study was carried out to document the source and stable isotopic characteristics of coal bed gas and associated coal bed water samples of the Raniganj Basin, West Bengal, India. The Raniganj Basin is one of the largest coal bed gas-producing sites in India. However, the gas isotopic composition of this basin was not well investigated until now. The exploration has been focused on mature coals with a high content of assumed thermogenic methane. Interestingly, the present study has revealed that although the gas is thermogenic, there is a significant effect of microbial activities in gas generation. The gas explored from this Basin contains almost 98–99% methane (CH 4 ) in general with very little portions of other hydrocarbons and thus, this gas can be considered as “dry gas”. It is consistent also with late-stage microbial alteration of thermogenic gases. The δ 13 C and δD values of CH 4 range from −47.4 to −49.7‰ and −207 to −211‰ respectively across the three wells as well as from −48.8 to −50.1‰ and −209 to −211‰ respectively in the gas samples collected from gas gathering station. The Whiticar style cross plot of these two parameters reveals that the gas samples have mixed thermogenic and biogenic sources and the microbial gas was generated through the CO 2 reduction pathway rather than the acetate fermentation pathway. The difference in δD values of water and methane samples [δD (H 2 O-CH 4 )], that varies from 173 to 175‰, may also exhibit that the biogenic methane was produced through reduction of CO 2 . The stable hydrogen and oxygen isotopic signatures of formation water samples (δD and δ 18 O-H 2 O) stand with the mixed origin of the gas. The cross-plot of these two parameters reveals that the water samples lie almost along the Global Meteoric Water Line (GMWL), which may suggest the combined effect of methanogenesis due to mixing of coal seam formation water with modern meteoric water. In summary, the produced coal bed gas from the Raniganj Basin, although being largely thermogenic in origin, has been influenced by secondary microbial alteration. [ABSTRACT FROM AUTHOR]

Abstract: Intensive carbonate and clay mineral authigenesis took place throughout the Late Permian Bowen–Gunnedah–Sydney basin system in eastern Australia. We conducted isotopic and trace element analyses of carbonate and clay minerals from clastic sedimentary rocks of the Gunnedah Basin and the Denison Trough in the Bowen Basin. Rb–Sr isochron age data of the illitic clays are consistent with episodic hydrothermal fluid flow events that occurred in association with Gondwana rifting accompanied by alkaline magmatism at ∼85Ma and ∼95Ma. Stable isotope data of carbonate and clay minerals from the Gunnedah Basin are indicative of meteoric waters from a high-latitude environment as the main fluid source, whereas trace element, Sr and Nd isotope data highlight mixing of meteoric fluids with magmatic and/or crustal components, with a possible input from marine carbonates for some samples. Trace metals, oxygen and strontium isotopes of dawsonites from the Denison Trough are interpreted to have been mobilised by fluids that interacted with evolved clastic sedimentary and marine carbonate end members. According to the carbon isotope data, CO2 for calcite and ankerite precipitation was sourced mainly from thermal degradation of organic matter and magmatism, whereas the CO2 used for dawsonite formation is inferred to have been derived from magmatic and marine sources. In the low permeability environments (particularly in coal seams), the increasing accumulation and oversaturation of CO2 particularly promote the precipitation of dawsonite. [Copyright &y& Elsevier]

Coupling carbon and nitrogen isotopes and petrography of coals and related intra-seam carbonaceous mudstone partings from basins in northern China provides insight into regional palaeoenvironments and palaeoclimate during the late Carboniferous–early Permian. The carbon isotopic composition (δ 13C coal , VPDB) of coal samples from the Taiyuan and Shanxi formations of Qinshui and North China-Bohaiwan basins ranges from −25.3‰ to −22.7‰, with an average of −23.7‰. The average δ 13C coal value is −23.6‰ in the late Carboniferous, −23.4‰ in the early Permian and −23.5‰ in the mid–early Permian. By contrast, equivalent early Permian coals in the southern North China-Bohaiwan Basin to the east were found to be significantly more negative at −25.2‰, likely as a function of regional aridity changes. Related δ 15N in coal seams ranges from +2.3‰ to +4.7‰, with an average of +3.7‰. Within the thick, economically important #15coal seam of the Qinshui Basin, δ 15N coal is significantly more negative than δ 15N in mudstone partings (avg. +7.1‰), implying that the coals underwent a stronger degree of microbial degradation during peat formation. δ 15N of mudstone partings varies stratigraphically, with significantly more positive δ 15N (+8.1‰, +8.3‰) for the lower partings and lower δ 15N (+4.8‰) for the upper parting. This may reflect varying degrees of microbial activity, but could reflect higher thermal maturity in the upper part of the seam as indicated in the coal rank profile. δ 13C coal in this region was used to calculate the carbon isotopic composition of atmospheric CO 2 (δ 13C a). Calculated δ 13C a ranges from −6.0‰ to −3.4‰, with an average of −4.5‰, which is more positive than the δ 13C a of modern atmospheric CO 2 (−8.5‰, Graven et al., 2017). Two δ 13C a excursions are noted: a positive one (magnitude 3.7‰) in the late Carboniferous and a tentative negative excursion (magnitude 6.0‰) in the mid–early Permian. The positive shift coincided with a sea-level transgression in the Qinshui Basin. The rise in sea level may have led to the burial of more terrestrial plant debris that decreased photosynthesis and enriched atmospheric CO 2 in 13C. More data are needed to understand mid-early Permian variation in δ 13C, as this could reflect a regional aridity or humidity effect rather than a global signal. Unlabelled Image • C and N isotope analysis of major coal seams in the Taiyuan and Shanxi formations. • Results compared with the existing terrestrial δ 13C org curve for northern China. • δ 13N coal offers insights on short-term environmental changes and microbial activity. [ABSTRACT FROM AUTHOR]