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Carbon dioxide geological storage involves injecting captured CO2 streams into a suitable reservoir. Subsequent mineral trapping of the CO2 as carbonate minerals is one of the most secure forms of trapping. Injection of CO2 dissolved in water or co-injection of CO2 with water may enhance trapping mechanisms. Produced waters are already re-injected into reservoirs worldwide, and their co-injection with CO2 could enhance mineral trapping in low reactivity rock by providing a source of cations. Sandstone drill core from a reservoir proposed for CO2 storage was experimentally reacted with supercritical CO2 and a synthetic produced water. Micro computed tomography (CT), QEMSCAN, and SEM were performed before and after the reaction. The sandstone sample was predominantly quartz with minor illite/muscovite and kaolinite. The sandstone sub-plug micro-CT porosity was 11.1% and 11.4% after the reaction. Dissolved Ca, Mg, and Sr decreased during the reaction. After the reaction with CO2 and synthetic produced water, precipitation of crystalline carbonate minerals calcite and dolomite was observed in the pore space and on the rock surface. In addition, the movement of pore filling and bridging clays, as well as grains was observed. Co-injection of CO2 with produced waters into suitable reservoirs has the potential to encourage CO2 mineral trapping.

Shale oil and gas is of recent interest in Australia, and the Roseneath, Epsilon and Murteree (REM) shales of the Cooper Basin, Australia, are unconventional gas targets. Shales have a high proport...

The end of the Late Palaeozoic Ice Age (LPIA) ushered in a period of significant change in Earth’s carbon cycle, demonstrated by the widespread occurrence of coals worldwide. In this study, we present stratigraphically constrained organic stable carbon isotope (δ13Corg) data for Early Permian coals (312 vitrain samples) from the Moatize Basin, Mozambique, which record the transition from global icehouse to greenhouse conditions. These coals exhibit a three-stage evolution in atmospheric δ13C from the Artinskian to the Kungurian. Early Kungurian coals effectively record the presence of the short-lived Kungurian Carbon Isotopic Excursion (KCIE), associated with the proposed rapid release of methane clathrates during deglaciation at the terminus of the Late Palaeozoic Ice Age (LPIA), with no observed disruption to peat-forming and terrestrial plant communities. δ13Corg variations in coals from the Moatize Basin are cyclic in nature on the order of 103–105 years and reflect changes in δ13Corg of ~±1‰ during periods of stable peat accumulation, supporting observations from Palaeozoic coals elsewhere. These cyclic variations express palaeoenvironmental factors constraining peat growth and deposition, associated with changes in base level. This study also demonstrates the effectiveness of vitrain in coal as a geochemical tool for recording global atmospheric change during the Late Palaeozoic.

Carbon dioxide (CO2) geological storage traditionally involves capturing a CO2 stream from a point source such as a power station or from cement, steel, or natural gas processing plant, transporting it and compressing it, prior to injection as a supercritical phase into a suitable geological reservoir overlain by a cap-rock or seal. One of the main perceived risks in CO2 geological storage is migration or leakage of the buoyant CO2 stream through the seal, via faults or fractures, or other migration out of the storage complex. Injection of CO2 dissolved in water may be one solution to mitigate the leakage risk. This approach could take advantage of large volumes of wastewater already being reinjected into saline aquifers worldwide but particularly in North America, thus reducing costs. This study examines the potential to “piggyback” off the existing wastewater injection industry as a novel carbon storage option.

Evaporites and carbonates intercalated with volcanic beds are distributed in the Jialingjiang and Leikoupo Formations straddling the boundary of the Lower and Middle Triassic in the Sichuan Basin. ...

Strontium isotopes, water chemistry and whole rock chemistry have been used to investigate the evolution of water from its surface meteoric composition into methane-rich groundwater. Previous studies established that co-produced Surat Basin coal seam gas waters from the Walloon Subgroup (Queensland, Australia) are meteoric in origin and are distinct across the main production regions but the dominant hydrochemical processes have yet to be determined. In this study, strontium isotope ratios (87Sr/86Sr) were measured on production waters from 36 coal seam gas wells, 14 sequentially leached host rock samples and 6 whole rock samples to improve understanding of the water-rock reactions that control spatial variability in solute and isotope chemistry. Strontium isotope ratios of Walloon Subgroup production waters across all production areas are uniformly low in value, ranging from 0.70338 to 0.70456. The majority of values are significantly lower than shallow Walloon formation waters in the recharge zone and extend the lower range of 87Sr/86Sr ratios recorded for Surat Basin groundwaters. These very low 87Sr/86Sr ratios (0.70338) are reached early along the flow paths, within 10 km of the shallow recharge area waters, before then undergoing a gradual increase with flow distance. Mineralogical and geochemical analyses of whole rock samples representing each of the main lithologies across the Walloon Subgroup has identified that the low 87Sr/86Sr ratios are not derived from any one interval but rather the combination of the same few minerals that are present in every unit. The rapid accumulation of relatively non-radiogenic strontium early in the flow path across all three gas producing regions has been attributed to a combination of fluid-rock interactions occurring within the Walloon Subgroup itself. Initial surface 87Sr/86Sr ratio values are quickly dominated through a combination of processes whereby recharge waters with low Sr concentrations are absorbed onto montmorillonite clays via cation exchange and this combined with weathering of plagioclase causes a re-equilibration of the groundwater to lower 87Sr/86Sr ratios. Strontium ratios then start to slowly increase again with flow distance through silicate weathering of K-feldspars and muscovites, which are present at low levels across the basin. Local geological features such as the Hutton-Wallumbilla fault and the Kogan and Undulla anticlines appear to influence flow paths and consequently water and gas composition.

Geological storage of CO2 generally involves injection of a CO2 stream into a high porosity and permeability reservoir, contained by one or more overlying low permeability formations. Sandstone reservoirs and associated cap-rocks of targeted CO2 storage sites therefore have distinct properties such as porosity and mineral contents. Their geochemical response or reactivity to injected supercritical CO2 and associated changes in porosity, and permeability affecting scaling, mineral trapping, injectivity, or migration can therefore be very different. Six drill core samples including quartz-rich sandstones, calcite cemented sandstones, and feldspar or clay-rich cap-rocks from a proposed demonstration site in the Surat Basin, Australia, were characterized before and after reaction with pure supercritical CO2 and low salinity formation water. The quartz-rich sandstones have low reactivity, and maintain high porosities with visible pore connectivity after reaction, they are unlikely to be affected by scaling. Kaolin and fine grain movement observed via μCT and SEM could have the potential to open or plug pores, potentially increasing or decreasing permeability and CO2 injectivity. Calcite cemented sandstones had the greatest measured change in porosity after reaction via calcite dissolution. Narrow angular channels were formed in the calcite cement around framework grains, extending through to the center of the sub-plug in the courser grained rock, and surface roughness increased. Solution pH was however quickly passivated. The highest concentrations of Ca, Mn, Sr, and Mg were released to solution from calcite dissolution. Clay (and feldspar) rich cap-rock core had mainly microporosity and the smallest initial pore throat diameters associated with clays. Small changes to μCT calculated porosities after reaction were related to a decrease in chlorite X-ray density, and dissolution of patchy carbonate minerals. Pores were disconnected in μCT images, except for some created horizontal connection along a sandy lamination in a cap-rock. Dissolved concentrations of Ca, Fe, Si, Sr, Mn, Li and Mg increased via dissolution of both carbonate and silicate minerals. Dissolved Ca, Fe, Mn and Mg from silicate minerals in the cap-rock were available for longer term mineral trapping of CO2. Potential increases in porosity and migration will be highest in the calcite cemented zones, while clay-rich cap-rocks could be expected to maintain integrity. There is a low likelihood of mineral trapping or scaling in the quartz rich lower Precipice Sandstone. Overlying rocks can provide Fe, Mg, Ca for mineral trapping of CO2 as ferroan carbonates such as siderite, ankerite and dolomite over longer time scales when pH is buffered. Changes to porosity, mineral content, and water chemistry after pure CO2 reaction observed here and in other published studies were dependent on mineral content and fluid accessibility. These results could be generalized to other sandstone reservoirs where it is expected to inject CO2. The results can also be used to validate geochemical models to build longer term predictions.

The Precipice Sandstone in the Surat Basin is being appraised for CO2 geological storage owing to its high porosity and permeability and expected high injectivity. Generally it is quartz rich with variable kaolinite, however detailed characterisation of core shows that it contains minor to trace amounts of potentially reactive minerals including carbonates, plagioclase, chlorite, and muscovite, increasing towards the overlying Evergreen Formation top seal. The Evergreen Formation is more variable, with interbedded low porosity and permeability mudstones, fine-grained sandstones, and calcite cemented zones. Injected CO2 dissolves into formation water forming carbonic acid. The geochemical reactivity of drill core samples affects the predicted pH and the dissolution or precipitation of minerals which could permanently trap CO2 as carbonates such as siderite, or dynamically change porosity and permeability altering CO2 migration. Comparative kinetic geochemical modelling of the CO2 reactivity of four representative mineralogies from drill core samples from deeper parts of the central basin indicates that the Evergreen Formation is potentially more reactive to CO2 than the Precipice Sandstone, especially for calcite or siderite containing zones. In the Precipice Sandstone small amounts of albite and siderite dissolved with traces of siderite and kaolinite precipitated. Dissolution of calcite and siderite in the Evergreen Formation favourably buffered acidity, with predicted precipitated minerals including siderite, kaolinite, ankerite, and smectites. The geochemical models indicate overall changes to porosity are however minor. Recent data from capture technologies has reported that CO2 from coal combustion may retain NOx impurities in the form of NO. Simulations with the addition of 30-100 ppm NO in the CO2 stream indicated the generated pH in the quartz rich Precipice Sandstone is similar to that on injection of pure CO2 after 30 years. The precipitation of Fe-rich smectite clays was additionally predicted.

Hydrogen peroxide solution was used to pretreat coal samples for the purpose of improving bioavailability for methanogenesis. Crushed coals of subbituminous (PEN9-003) and medium-volatile-bituminous (C.D.) ranks were oxidized with three different dosages of H2O2 solution over four time periods (1, 4, 7, and 30 days) in the presence of UV light. Bioassays were set up using the oxidation products as the sole carbons substrates for biomethane production. To characterize the change in coal composition after oxidation, concentrations of volatile fatty acids (VFAs) and total organic carbon (TOC) in the filtered liquid sample of the pretreatment reactor were measured. The solid phase was dried and solvent-extracted with a mixture of dichloromethane and methanol. Compounds in the organic extract were analyzed using a GC–MS. H2O2 oxidation has managed to boost the biomethane yield in the subbituminous PEN9-003 coal by a maximum 10 times to 223.7 μmol/g. Production of methane was found to increase exponentially with the concentration of H2O2 relative to coal, but less significantly with pretreatment length. In contrast, the bituminous C.D. coal was barely affected by pretreatment. In addition, oxidation has considerably enhanced solubilization of PEN9-003 coal in water, with contents of VFAs and dissolved TOC surged by up to 28.2 and 22.3 times. The pretreatment has also increased the abundance of solvent-extractable matter in PEN9-003 coal. Oxygen-containing compounds, including aliphatic carboxylic acids, alcohols, ethers, ketones, and oxygenated monocyclic aliphatic and aromatic hydrocarbons were the most affected. The enrichment was due both to formation of new chemical species and an increase in the abundance of existing compounds. In comparison, oxidation of C.D. coal failed to generate any new organic species, nor did it increase the content of dissolved TOC. Biodegradation has resulted in depletion of all dissolved organic carbon, suggesting water-soluble compounds are highly bioavailable. Bioconversion of solvent-extractable matter in PEN9-003 has been improved by a maximum 29.5% upon pretreatment, while that of C.D. coal was lowered. Molecules with heterogeneous moieties, especially those that have been enriched after pretreatment demonstrated the highest bioavailability. Thermal maturation of coal from subbituminous to medium-volatile bituminous rank has been associated with a decrease in extractable matter contents, liptinite proportion, H/C ratio, elemental oxygen, heterogeneous moieties, as well as physical accessibility to microorganisms. They are thought to be responsible for the poor bioavailability and amenability of C.D. coal to H2O2 oxidation. Application of the pretreatment is only likely to be ex-situ, in which coal beneficiation waste may be used as a feed to bioreactors. The study offers a good basis for further process optimization and upscaling.

An examination of the processes contributing to the production of landfill greenhouse gas (GHG) emissions is required, as the actual level to which waste degrades anaerobically and aerobically beneath covers has not been differentiated. This paper presents a methodology to distinguish between the rate of anaerobic digestion (rAD), composting (rCOM) and CH4 oxidation (rOX) in a landfill environment, by means of a system of mass balances developed for molecular species (CH4, CO2) and stable carbon isotopes (δ13C-CO2 and δ13C-CH4). The technique was applied at two sampling locations on a sloped area of landfill. Four sampling rounds were performed over an 18 month period after a 1.0 m layer of fresh waste and 30–50 cm of silty clay loam had been placed over the area. Static chambers were used to measure the flux of the molecular and isotope species at the surface and soil gas probes were used to collect gas samples at depths of approximately 0.5, 1.0 and 1.5 m. Mass balances were based on the surface flux and the concentration of the molecular and isotopic species at the deepest sampling depth. The sensitivity of calculated rates was considered by randomly varying stoichiometric and isotopic parameters by ±5% to generate at least 500 calculations of rOX, rAD and rCOM for each location in each sampling round. The resulting average value of rAD and rCOM indicated anaerobic digestion and composting were equally dominant at both locations. Average values of rCOM: ranged from 9.8 to 44.5 g CO2 m−2 d−1 over the four sampling rounds, declining monotonically at one site and rising then falling at the other. Average values of rAD: ranged from 10.6 to 45.3 g CO2 m−2 d−1. Although the highest average rAD value occurred in the initial sampling round, all subsequent rAD values fell between 10 and 20 g CO2 m−2 d−1. rOX had the smallest activity contribution at both sites, with averages ranging from 1.6 to 8.6 g CO2 m−2 d−1. This study has demonstrated that for an interim cover, composting and anaerobic digestion of shallow landfill waste can occur simultaneously.

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 (CH4) 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 δ13C and δD values of CH4 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 CO2 reduction pathway rather than the acetate fermentation pathway. The difference in δD values of water and methane samples [δD (H2O-CH4)], that varies from 173 to 175‰, may also exhibit that the biogenic methane was produced through reduction of CO2. The stable hydrogen and oxygen isotopic signatures of formation water samples (δD and δ18O-H2O) 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.

Coal bed methane (CBM) has recently been produced from sub-bituminous to high volatile bituminous low rank coals (vitrinite reflectance 0.39 to 0.53%) from the Paleocene Cuervos Formation in the Cesar Sub-basin, Colombia. Understanding CBM gas origin is vital for designing exploration and production strategies, in order to target either shallow economic accumulations of microbial gas or deeper and thermally mature coal seams. In this context, this study aims to determine whether thermogenic gas, biogenic gas, or a mixture of both gas types have contributed to gas yield in the Cesar Sub-basin; it also discusses the relationship between gas origin and tectonics. For the first time, co-produced waters and gas samples from CBM multi-seam production wells and shallow aquifer water-wells were collected in the Boqueron compartment (Cesar Sub-basin) and were analysed via stable isotope composition and molecular geochemistry. Structural compartmentalisation in the Sub-basin as a result of interplate shortening during the Mid-Paleogene has resulted in slightly different coal ranks, saturation, and gas contents between compartments of the Paleocene Cuervos Formation. This event could have interrupted early thermogenesis and allowed the introduction of meteoric waters carrying bacteria consortia responsible for biogenesis. The produced waters from the CBM wells were HCO3 > Na+ > Cl− type, and ranged in TDS (Total dissolved solids) from 2268 to 6602 mg/L (avg. 3887 mg/L). Recently revised genetic diagrams of δ13C-CH4 versus δ2H-CH4 and additional parameters, such as gas dryness ratio and water chemistry, corroborated biogenesis as the main gas origin. Carbon isotopic differences between carbon dioxide and methane (Δ13CH2O–CH4), as well as those of hydrogen isotopes in water and methane (Δ2HH2O–CH4), also indicated a typical microbial CO2 reduction pathway. The positive carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) not only showed a clear differentiation between waters from CBM wells and those from shallow aquifers, but also confirmed the occurrence of methanogenesis when paired with high alkalinity. In addition, the water quality analysis showed increasing sodium and bicarbonate concentrations with depth, which is typical of CBM production basins. Although biogenic gas was identified, tectonic settings do not seem to explain gas origin occurrence, since secondary biogenesis was expected in an uplifted basin such as Cesar Rancheria. Isotopic data was not decisive in differentiating primary versus secondary biogenesis, since the gas isotope compositions plot close to the boundary between the primary and secondary biogenic gas fields. Further studies on isotopic chemistry are required to refine this interpretation and confirm gas generation in the other two compartments.

During CO2 geological storage, low porosity and permeability cap-rock can act as a structural trap, preventing CO2 vertical migration to overlying fresh water aquifers or the surface. Clay and organic matter rich shales, fine-grained sandstones and mudstones often act as cap-rocks and may contain substantial sub-micron porosity. CO2-brine-rock interactions can open or close pore throats through dissolution, precipitation or migration of clay fines or grains. This could affect CO2 migration if the porosity is accessible, with unchanging or decreasing accessible porosity favourable for trapping and integrity. Two cap-rock core samples, a clay and organic-rich mudstone and a more organic-lean feldspar-rich fine grained sandstone, from a well drilled for a CO2 storage feasibility study in Australia were experimentally reacted with impure CO2 (+ SO2, O2) and low salinity brine at reservoir conditions. Mercury injection capillary pressure indicated that the majority of pores in both cores had pore throat radii ~ 5–150 nm with porosities of 5.5–8.4%. After reaction with impure CO2-brine the measured pore throats decreased in the clay-rich mudstone core. Dissolution and precipitation of carbonate and silicate minerals were observed during impure CO2 reaction of both cores via changes in water chemistry. Scanning electron microscopy identified macroporosity in clays, mica and amorphous silica cements. After impure CO2-brine reaction, precipitation of barite, Fe-oxides, clays and gypsum was observed. Ion leaching from Fe-rich chlorite was also apparent, with clay structural collapse, and fines migration. Small-angle neutron scattering measured the fraction of total and non-accessible pores (~ 10–150 nm radii pores) before and after reaction. The fraction of pores that was accessible in both virgin cap-rocks had a decreasing trend to smaller pore size. The clay-rich cap-rock had a higher fraction of accessible pores (~ 0.9) at the smallest SANS measured pore size, than the feldspar rich fine-grained sandstone (~ 0.75). Both core samples showed a decrease in SANS accessible pores after impure CO2-water reaction at CO2 storage conditions. The clay-rich cap-rock showed a more pronounced decrease. After impure CO2-brine reaction the fraction of accessible pores at the smallest pore size was ~ 0.85 in the clay-rich cap-rock and ~ 0.75 in the feldspar-rich fine-grained sandstone. Reactions during impure CO2-brine-rock reaction have the potential to close cap-rock pores, which is favourable for CO2 storage integrity.

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, C, Sr/Sr, O/O) 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 (~15 km), suggestive of very low ground water flow velocities. The range of Cl is 9.0 to 23.8 (x 10) while the Cl 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) 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

In numerous global coal–bearing basins, methane, possessing a biogenic stable isotope composition, is spatially and temporally associated with groundwater recharge. However, beyond groundwater inoculating the subsurface with microorganisms, the biological mechanisms that control the distribution of biogenic methane are poorly understood. In this study, we examined the interactions between bituminous coal and a) methanogenic microbial communities sourced from the goaf of a Bowen Basin underground coal mine and b) pure cultures of methanogens. When coal mine microbial consortium was amended with acetate and a low concentration of coal (1 g in 25 mL medium), methane production was stimulated compared to the addition of acetate alone, though the presence of coal did not affect the methane production from H2/CO2. To test whether methanogens benefited directly from the addition of coal, 1 g of either quartz sand or bituminous coal was added to a pure culture of Methanococcus maripaludis, a hydrogenotrophic methanogen isolated from the coal mine and grown on H2/CO2, and to a pure culture of Methanosarcina barkeri, an acetoclastic methanogen grown on acetate. In this experiment coal was not included as an energy source for microbial growth but to test interactions between coal and methanogens. The presence of coal in the medium did not affect methane production by Ms. barkeri but slightly inhibited methane production by M. maripaludis at the start of the growth phase. Scanning electron microscopy revealed that M. maripaludis cells were attached to both sand grains and coal particles, with preferential attachment to rough surfaces, such as cracks within broken coal pieces and clay-rich areas of coal. When the experiment with M. maripaludis and Ms. barkeri was repeated with a 1:1 volumetric ratio of bituminous coal to medium, which more accurately reflects the environmental conditions of a coal seam, methane production by M. maripaludis and Ms. barkeri was completely inhibited. In addition, M. maripaludis cell numbers declined after inoculation. This suggests that at a high coal: fluid ratio, methanogenesis is inhibited by some component of the coal, e.g., bitumens in the coal or coal-sourced hydrocarbons dissolved into the medium. Based on these laboratory results, we propose that in coal seams, the dilution of inhibitory compounds may be one mechanism by which groundwater recharge promotes biogenic methane production.

The study sets out to characterise the bioavailability of six Surat Basin Walloon coals from different stratigraphic layers in a single borehole to environmental methanogenic consortia. Factors that control bioavailability have also been investigated on grounds of coal petrographic composition and the organic composition of solvent-extractable matter. Finely crushed coal core samples were inoculated with digested sludge culture from domestic wastewater treatment in serum bottles kept anoxically before incubation at mesophilic temperature over 30 days for biomethane production. Degradation of coal compounds was demonstrated via GC–MS characterisation of methanol and dichloromethane (DCM) extracts, in combination with aqueous volatile fatty acids and alcohols and total organic carbon (TOC) analysis on fresh and microbially-digested coal samples. The resulting methane yields ranged from 14 to 33 μmol/g, with an average of 21 μmol/g (0.515 m 3 /t), comparable to those previously reported for subbituminous coals. Organic solvent-extractable materials that accounted for 3.8 to 12% of coal weight were generally dominated by aliphatic compounds, composed of mainly medium-long-chain n -alkanes, n -alcohols and esters. Aromatics were detected up to three fused rings, and are rich in dibenzofuran, alkyl benzene, diphenyls and alkyl PAH (polymeric aromatic hydrocarbon). The abundance of solvent-extractable matter was found to be positively associated with liptinite content, particularly suberinite, sporinite and liptodetrinite. Preservation of these compounds was thought to rely on vitrinite, such as telinite and collotelinite that are rich in micropores, serving as storage for the hydrocarbons. Environmental factors, such as microbe-carrying groundwater might compromise coal extractability by converting coal hydrocarbons to biogas. Bioavailability of coal was shown to be controlled by three factors: 1) Water solubility - Bioassay eliminated an average 98% of aqueous compounds (based on TOC), which were dominated by volatile fatty acids and alcohols, and to a lesser degree, medium-chain (primarily C 10 to C 20 ) n -alcohols, esters and aliphatic amine; 2) Solvent extractability – 34.5% of solvent-extractable compounds were shown to be biodegraded (based on peak intensity in GC–MS), with methanol extracts being more bioavailable than DCM's; 3) Heterogeneous moieties, particularly aliphatic hydroxyl group, ester bond, ether bond and C N bond in aliphatic amine - These functional groups present heteroatoms that can lower the activation energy of nearby bonds, making them vulnerable for microbial cleavage. Compound degradation in bioassays was shown to be clearly associated with methane yield, but only a small proportion degraded was converted to methane. Further improvement may be achieved via proper adaptation of the current microbial community.

Illitic clay is ubiquitous in clastic hydrocarbon reservoirs, and the host for several radiometric isotopes such as the potassium-argon (K-Ar) and rubidium-strontium (Rb-Sr) systems. This study applied the isotope-dilution thermal ionization mass spectrometry technique for small samples (3–4 mg) to conduct illite Rb-Sr isotope dating of five illitic clay samples from the Silurian bituminous sandstone (SBS) intersected by five drillholes in the Tarim Basin, NW China. The 87Rb/86Sr ratio of clays is fractionated mainly by the addition of Rb during the illitization of mixed-layer illite/smectite (I/S), which is the dominant clay species in the Tarim Basin samples. The subsample-scale Rb/Sr isotope values suggest that each subsample may contain I/S particles of slightly variable degrees of illitization. Three of the analyzed samples (H6, KQ1 and TZ67) generated Rb-Sr isochron ages of 141 ± 61 Ma, 332 ± 32 Ma and 235 ± 8 Ma (errors quoted at 2σ), respectively. These results are similar to the corresponding K-Ar ages (125 Ma, 389 Ma and 234 Ma). The isotopic ages are consistent with the timing of hydrocarbon charge which varies in different drillholes as constrained by basin modelling, indicating that a closed-system behavior is attained by the hydrocarbon charge that inhibits the illitization of I/S. The Rb-Sr isotope analyses of the other two samples (YM35-1 and Q1) that did not yield isochron ages suggest the conditions for producing isochrons were not satisfied, which may be caused by disturbance of the isotope system by a post-charge hydrothermal event. The outcomes of this study show the robust potential of Rb-Sr clay subsample geochronology for cross-checking isotopic ages yielded by other systems (e.g. K-Ar system) and constraining the timing of hydrocarbon charge.

Apatite may be a common accessory mineral in coal seams, but interpretation of its origins can vary from syndepositional mineral detritus, early precipitation during diagenesis, and post coalification precipitation and replacement from hydrothermal or other fluids. It is hypothesised that paragenesis is reflected in the modes of occurrence in different stratigraphic units of the Late Permian in the Bowen Basin. From samples analysed in this study, apatite in the youngest unit, the Rangal Coal Measures, most commonly occurs as pore-apatites (i.e. apatites infilling preserved plant cell cavities of fusinite and semifusinite macerals and commonly associated with kaolinite) and less commonly as fracture-apatites (i.e. apatites infilling micro-fractures) and detrital-apatites intermixed with the organic coal layers (Detrital-B). In the tuffaceous Fort Cooper Coal Measures and equivalents, apatite occurs more commonly as detrital-apatites intermixed with the lithic layers (Detrital-A) (Detrital. Within the basal unit, the Moranbah Coal Measures and its equivalents, the mode is commonly encrusting-apatites (i.e. small apatite crystallites encrusting silicate minerals). Almost all apatites in the coal samples tested by electron microprobe microanalysis (EPMA), regardless of stratigraphic location and mode, are fluorapatite. Although there are some locality-driven enrichments, the minor and trace-element chemistry vary more so by mode of occurrence. For comparison, apatites from dykes intersecting the coal measures and from tuffs were also added to the study. These had lower fluorine (F) but measurable chlorine (Cl) contents and are enriched in light rare earth elements (LREE), similar to the Durango apatite that is of magmatic origins. Most of the detrital-A apatites are also enriched in LREE similar to the tuff-apatites, but with variable enrichment in the rare earth and yttrium elements. Detrital-B apatites have minimal Cl but also show the magmatic LREE trend. The pore- and fracture-apatites exhibited two trends. In the samples from tuffaceous coals (e.g., Fort Cooper Coal Measures), the pore- and fracture apatites tended to follow the magmatic LREE trend. In the Rangal Coal Measures, the pore- and fracture-apatites have elevated F contents, minimal Cl contents and depleted LREE contents (or Middle REE enrichment). Depletion of LREE could occur through leaching in an acidic environment, which is suggested by the abundance of kaolinite. Apatite precipitates under more neutral conditions although pore-apatites are commonly considered “early” and fracture-apatites “late” or post coalification. Their geochemical similarity suggests they have similar fluid origins and/or timing, with geothermal fluids moving through the porosity afforded by the structured inertinite group macerals and fractures. Verification of that geothermal source still requires further work and a technique that can analyse the isotopic composition—e.g., carbon (13C/12C), oxygen (18O/16O) and strontium (87Sr/86Sr)—of micron-sized crystals in situ.

A microbial consortium derived from sewage sludge from the treatment of wastewater (Luggage Point Wastewater Treatment Plant, Brisbane, Australia) has been applied to Jameson Cell (J-cell) rejects (R o,max = 0.96 ± 0.008) of a Bowen Basin coal preparation plant to assess the potential for biogenic methane production. A maximum methane yield of 26.20 μmol/g J-cell rejects (0.64 m 3 CH 4 /ton) was observed, suggesting biogenic methane production from coal waste materials is a feasible process if yields can be improved. Molecular analysis performed on the microbial consortium showed similar microbial community compositions to those observed in natural coal bed environments. The study demonstrates that Australian coal waste materials can be used as a viable feedstock for biogenic methane production using microorganisms that are not native within the coal beds.

Carbon capture and storage is considered one of the methods that can help reduce greenhouse gas emissions from point sources. The captured CO2 commonly contains impurities related to the fuel source. Critical to safely storing CO2 with impurities is an understanding of the chemical behavior of the CO2-impurity-water-rock system. Numerical modelling of storage scenarios provides one way of developing that understanding. In this study reactive transport modelling of CO2 with SO2 injection at a proposed storage site in Australia was conducted to evaluate the impact of incorporating calcite dissolution as either an equilibrium or kinetically controlled process. It was determined that proximal to the injector, there is an impact on model outcome for reactions that are strongly coupled like the calcite dissolution/gypsum precipitation reactions. The assumption of calcite equilibrium may lead to over estimating the extent of gypsum precipitation and pH buffering. That overestimation may have significant implications on site leakage risk assessments. (C) 2017 Published by Elsevier B.V.

Reservoir and cap-rock core samples with variable lithology's representative of siliciclastic reservoirs used for CO2 storage have been characterized and reacted at reservoir conditions with an impure CO2 stream and low salinity brine. Cores from a target CO2 storage site in Queensland, Australia were tested. Mineralogical controls on the resulting changes to porosity and water chemistry have been identified. The tested siliciclastic reservoir core samples can be grouped generally into three responses to impure CO2-brine reaction, dependent on mineralogy. The mineralogically clean quartzose reservoir cores had high porosities, with negligible change after reaction, in resolvable porosity or mineralogy, calculated using X-ray micro computed tomography and QEMSCAN. However, strong brine acidification and a high concentration of dissolved sulphate were generated in experiments owing to minimal mineral buffering. Also, the movement of kaolin has the potential to block pore throats and reduce permeability. The reaction of the impure CO2-brine with calcite-cemented cap-rock core samples caused the largest porosity changes after reaction through calcite dissolution; to the extent that one sample developed a connection of open pores that extended into the core sub-plug. This has the potential to both favor injectivity but also affect CO2 migration. The dissolution of calcite caused the buffering of acidity resulting in no significant observable silicate dissolution. Clay-rich cap-rock core samples with minor amounts of carbonate minerals had only small changes after reaction. Created porosity appeared mainly disconnected. Changes were instead associated with decreases in density from Fe-leaching of chlorite or dissolution of minor amounts of carbonates and plagioclase. The interbedded sandstone and shale core also developed increased porosity parallel to bedding through dissolution of carbonates and reactive silicates in the sandy layers. Tight interbedded cap-rocks could be expected to act as baffles to fluids preventing vertical fluid migration. Concentrations of dissolved elements including Ca, Fe, Mn, and Ni increased during reactions of several core samples, with Mn, Mg, Co, and Zn correlated with Ca from cap-rock cores. Precipitation of gypsum, Fe-oxides and clays on seal core samples sequestered dissolved elements including Fe through co-precipitation or adsorption. A conceptual model of impure CO2-water-rock interactions for a siliciclastic reservoir is discussed.

Seal or cap-rock integrity is a safety issue during geological carbon dioxide capture and storage (CCS). Industrial impurities such as SO2, O2, and NOx, may be present in CO2 streams from coal combustion sources. SO2 and O2 have been shown recently to influence rock reactivity when dissolved in formation water. Buoyant water-saturated supercritical CO2 fluid may also come into contact with the base of cap-rock after CO2 injection. Supercritical fluid-rock reactions have the potential to result in corrosion of reactive minerals in rock, with impurity gases additionally present there is the potential for enhanced reactivity but also favourable mineral precipitation. The first observation of mineral dissolution and precipitation on phyllosilicates and CO2 storage cap-rock (siliciclastic reservoir) core during water-saturated supercritical CO2 reactions with industrial impurities SO2 and O2 at simulated reservoir conditions is presented. Phyllosilicates (biotite, phlogopite and muscovite) were reacted in contact with a water-saturated supercritical CO2 containing SO2, or SO2 and O2, and were also immersed in the gas-saturated bulk water. Secondary precipitated sulfate minerals were formed on mineral surfaces concentrated at sheet edges. SO2 dissolution and oxidation resulted in solution pH decreasing to 0.74 through sulfuric acid formation. Phyllosilicate dissolution released elements to solution with ∼50% Fe mobilized. Geochemical modelling was in good agreement with experimental water chemistry. New minerals nontronite (smectite), hematite, jarosite and goethite were saturated in models. A cap-rock core siltstone sample from the Surat Basin, Australia, was also reacted in water-saturated supercritical CO2 containing SO2 or in pure supercritical CO2. In the presence of SO2, siderite and ankerite were corroded, and Fe-chlorite altered by the leaching of mainly Fe and Al. Corrosion of micas in the cap-rock was however not observed as the pH was buffered by carbonate dissolution. Ca-sulfate, and Fe-bearing precipitates were observed post SO2-CO2 reaction, mainly centered on surface cracks and an illite rich illite-smectite precipitate quantified. Water saturated impure supercritical CO2 was observed to have reactivity to rock-forming biotite, muscovite and phlogopite mineral separates. In the cap-rock core however carbonates and chlorite were the main reacting minerals showing the importance of assessing actual whole core.

An integrated analysis of megascopic lithotype, microscopic maceral and mineral composition, stable carbon isotope and interburden sedimentology from a single well was used to interpret the response of the Late Permian Fort Cooper Coal Measures (FCCM) to regional and global environmental influences. The FCCM are differentiated from underlying, relatively high vitrinite Moranbah Coal Measures, and overlying higher inertinite Rangal Coal Measures in the Bowen Basin by their intercalation with abundant tuff and siliciclastic partings and interbeds. Besides this, there is little described about the variation in the organic composition of the FCCM and its causes. The FCCM can be subdivided into a lower aggradational Fair Hill Formation, transgressed by the shallow marine-derived Black Alley Shale that interfinger with/is overlain by the progradational Middle Main Seams and Burngrove Formation coal measures. The coals are dominantly dull with minor bright bands that are more abundant in the Burngrove Formation representing a change in plant composition. The maceral analysis shows that the coals in the Fair Hill Formation and Middle Main Seams are vitrinite-rich (80–90% mmf) albeit with high mineral matter suggesting the formation of precursory peat under rising water levels and with high sediment (tuff) influx and preservation. The coals in the Burngrove Formation have an increased inertinite content (30% mmf) but are also high in mineral matter suggesting a shift to increased decomposition arising from a fluctuating water table, possibly increased aridity and/or microbial activity. Tuffs occur throughout, and although their frequency is higher in the lower Fair Hill Formation, the preservation of thicker tuffs in the upper Burngrove Formation indicates increased intensity of volcanism that could have modified the environment. Variation in carbon isotope compositions show a parabolic trend, from around − 24.1‰ in the Fair Hill Formation to more variable values in the Middle Main Seams with an overall 13 C-enrichment upwards in the Burngrove Formation, prior to the δ 13 C values becoming negative (depleted between − 1 to − 4% from the average − 24.1‰) in the top seams and into the overlying Rangal Coal Measures. The 13 C-depletion trend in the upper part of the section is unexpected in view of corresponding increase in inertinite content of the coals indicating that the δ 13 C values/plant composition and inertinite content are decoupled, unless the inertinite origin is from intense microbial decay. Similar stable carbon isotope depletion trends have been observed elsewhere in an equivalent stratigraphic interval of the Bowen Basin suggesting that the carbon isotope values are responding to global conditions that cause negative excursions in carbon isotopes before the P-T boundary. Overall, the basin was continuously subsiding and peats accumulated with constant interruptions from excessive sedimentation and volcanic eruptions. The low proportion of visible thick bright bands coupled with high telovitrinite (mmf) content suggests a marsh to fen environment, with an open canopy and ponding able to preserve volcanic ash falls.

Carbon and hydrogen isotope compositions of CH 4 generated via methanogenesis in cultures of South Sumatra Basin (SSB) coalbed methane (CBM) formation waters grown on coal, acetate and H 2 + CO 2 were investigated. CH 4 production and molecular analysis confirmed the presence of active microbial communities that are able to convert coal into CH 4 using both acetoclastic and hydrogenotrophic pathways. The representative bacterial sequences were dominated by Bacteroidetes , Firmicutes and Deltaproteobacteria , while Methanosaeta and Methanosarcina were the most prevalent archaeal methanogens present in the cultures. CH 4 produced in this study's culturing experiments has δ 13 C values in the range of − 50 ‰ to − 20 ‰, with most values falling outside the current understanding of the carbon isotopic boundaries for biogenic CH 4 (− 110 ‰ to − 30 ‰). However, the corresponding apparent carbon isotopic α factor ( α c = 1.02 ± 0.006), and isotopic effect ( e c = − 20.1 ‰ ± 15.3) showed that CH 4 in SSB cultures was predominantly produced by acetoclastic methanogenesis, which is consistent with the results of molecular DNA analysis. In addition, the calculated contribution of CO 2 reduction from the δ 13 C values of coal-treated cultures was overall 4 production in the SSB cultures. The outcome of this experimental study also suggests that δ 2 H-CH 4 values may not provide a reliable basis for distinguishing methanogenic pathways, while apparent carbon isotopic fractionation factor ( α c ) and isotope effect ( e c ) are considered more useful indicators of the methanogenic pathway. The high δ 13 C-CH 4 values (≥ 30 ‰) and the dominance of Methanosaeta over Methanosarcina indicate that methanogens within the SSB cultures were operating at low substrate concentrations. An unusually positive δ 13 C-CH 4 suggests a substrate depletion effect, which is thought to be related to a decrease in the relative abundance of key bacterial coal degraders with formation water inoculum storage time. Closer observation of δ 13 C-CH 4 values during the growth of cultures within a single experiment also showed a 13 C-enrichment trend over time. At log phase of growth, the CH 4 produced was 13 C-depleted when compared to the stationary phase that also indicates substrate depletion effects. Finally, the δ 13 C-CH 4 values encountered in this study (as high as − 20 ‰) highlight the possible positive extension of δ 13 C-CH 4 values of acetoclastic methanogenesis from those currently reported in the literature for natural and experimental samples (as high as − 30 ‰).

The Late Jurassic Walloon Subgroup (recently dated as Oxfordian) is a productive, subbituminous coal seam gas source in the Surat Basin and can be subdivided from bottom to top into the Taroom Coal Measures, the Tangalooma Sandstone, the Lower and Upper Juandah Coal Measures, which have different coal character. The lower Taroom coals are commonly thick, associated with sandstones and interpreted to form as base level is rising, creating sodden anoxic conditions for peat accumulation. The middle Tangalooma to Lower Juandah contains fewer and thinner coals, and transitions upwards from a sandstone to siltstone dominated sequence responding to inundation with the development of floodplain lakes. The strata then coarsen upward in both grain size and coal thickness in the Upper Juandah Coal Measures, which may be eroded by an overlying unit, the Springbok Sandstone. This unconformable surface is basin wide and depending on age, can be tied into global changes in climate and base level. Existing models for peat growth under changing base level and the variability in terms of the conditions of peat formation through time, as well as throughout the basin, are tested. Environment of peat deposition and changes therein, are investigated by petrographic analysis of the Walloon coals, coupled with high resolution lithotype logging of core and organic stable carbon isotope analysis. Fine microlayering and abundance of root suberinite, telo- and detrovitrinite indicate precursory peat formation in a mostly herbaceous marsh to fen environment, in which bigger trees are either infrequent or absent, except for the lower seams of the Taroom Coal Measures and the upper seams of the Lower Juandah Coal Measures, where bright bands are thicker (≥ 10 mm) and more frequent. No extended periods of dehydration-oxidation ( Stable carbon isotopes of coal show a distinct positive shift in the Lower Juandah Coal Measures that sets in well before the increased inertinite content in the Upper Juandah Coal Measures. The enrichment in 13 C could be linked to a change in climate during the high stand depositional cycle, marking the onset of late stage falling, where base level begins to drop, later creating exposures and water stress. A shift to a less humid climate in the Upper Juandah Coal Measures could have favoured the conditions for desiccation, mouldering and bush fires, which is reflected in the coal's maceral composition. The Surat Basin δ 13 C isotope trend follows the global trend found in marine carbonate samples from the same age interval that corroborates increasing enrichment towards the top of the coal measures (approximately middle Oxfordian), followed by a shift to more negative compositions, which corresponds to the onset of the Springbok Sandstone deposition on an unconformable surface.

The Surat Basin hosts various industries that extract groundwater including coal seam gas (CSG), feedlots, and agriculture. With water drawdown, gas has been observed in some bores drawing groundwater from different aquifers across the Basin. While methane can occur naturally in aquifers, biogenic CSG has been extracted from the Walloon Coal Measures raising questions on the sources of gas in overlying aquifers. Current standard monitoring uses a direct fill approach to measure dissolved methane concentrations in vials; however, this approach may lose gas present above solubility levels. Water and gas sampling was performed on bores in the Springbok, Gubberamunda, Mooga, Hutton, and Precipice Sandstones, the Orallo Formation, and the Condamine Alluvium. Water bores and CSG production wells from the Walloon Coal Measures were also sampled. We compared direct fill with a closed sampling method for dissolved gas, plus a method for sampling the total free and dissolved gas in the field. Higher dissolved and total methane concentrations were measured using closed sampling methods, especially in gassy bores. The majority of our sampled aquifer gases and waters have stable isotopic signatures distinct from CSG, where methane had likely been formed insitu in shallow aquifers by primary microbial CO2 reduction or fermentation processes. In several gassy bores, the source of the methane could not be clearly identified. This study indicates that (1) current monitoring methods may be underestimating methane concentrations above identified thresholds especially in gassy bores and (2) a combination of isotopic techniques may distinguish methane sources and interaquifer disconnectivity in the majority of cases.

The Herrin coal (Western Kentucky No. 11) “ragged edge” is composed of peculiar organic materials, including voids, which defy conventional maceral classification. These materials seem to have originated from natural pyrolysis caused by hot fluids migrating through low-rank coal. However, the observed petrographic patterns and the reflectance analysis were not enough to confirm this hypothesis. In order to obtain for the first time an approximate composition and temperature value of the fluids that may have thermally affected the Herrin coal, the origin of the “ragged edge” of the Herrin coal seam was studied via a combination of analytical techniques mainly focused on the carbonates including optical microscopy, cathodoluminescence, X-ray fluorescence, X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, electron microprobe, and Raman microspectroscopy. The sum of indicators is collectively considered evidence that ascending hot fluids thermally affected the coal. These indicators include the ambient reflectance increase due to forced coalification; liptinite fading and “cryptic liptinite” formation at the bottom of the coal seam; the carbonate stable isotopes model temperature that indicates a range of 150 °C to 200 °C, and the Raman spectra proxy of the thermally affected particles indicating a temperature between 160 and 220 °C.

An injected CO2 or greenhouse gas (GHG) stream will dissolve into formation water forming carbonic acid, especially in the case of low salinity reservoir storage. Industrial GHG streams from coal combustion, cement or streel production may contain accessory gases including N2, Ar, SOx, NOx, or O2, several including SOx and NOx form stronger acids. The dissolved GHG stream will have reactivity to some rock forming minerals, potentially modifying porosity or water chemistry, or mineral trapping CO2. The Precipice Sandstone in the Surat Basin, Australia, is a low salinity target reservoir for CO2 storage. The Surat CCS project proposes to a demonstration-scale injection test of 60,000 t per year of a GHG stream captured from black coal PCC into the quartz rich Lower Precipice Sandstone. The dissolved GHG stream is expected to interact with the Lower and Upper Precipice Sandstone and sink to interact with the underlying Moolayember Formation of the Bowen Basin. However little was known about the lithologies of the Moolayember Formation which unconformably underlies the projects proposed injection site. Drill cores from the Moolayember Formation were sampled and characterized: indicating a high proportion of carbonate cements, potentially reactive clays, feldspars, and trace amounts of sulphides and coal. Complex carbonate assemblages and secondary textures indicate this may already be a valuable example of a natural analogue of CO2 alteration. Kinetic geochemical modelling was performed to predict the CO2 or CO2-SO2-NO reactivity of several lithologies. This indicted that after 30 years, low concentrations of SOx and NO (100 ppm) had minimal effect on the pH which was buffered to similar values relative to pure CO2 for these reactive mineralogies. The pH was instead controlled by the carbonate mineral content, which buffered acidity. Reaction of clay and feldspar rich lithologies resulted in dissolution of K-feldspar, chlorite, and plagioclase to form kaolinite, smectite, and additionally siderite mineral trapping CO2. SO2 was predicted to be mineral trapped as pyrite or alunite. The net predicted mineral volumes or porosities however did not change significantly. Experiments at reservoir conditions and further characterization are planned to validate and improve model predictions.

An understanding of the long-term reactivity of different rock types to injected CO2 is needed for sequestration site assessment. Relative to saline aquifer studies, the long term reactivity of CO2 in low salinity aquifers has received little attention. Currently in Australia, the Surat Basin is being appraised for its large-scale CO2 storage potential within low salinity aquifers. Sixteen core samples from the Precipice Sandstone and Evergreen Formation – the notional target reservoir and seal complex – were characterized for mineral content; helium, mercury-injection and micro CT porosities; air permeability; and, imaged with SEM-EDS. Samples consisted of quartz rich reservoir sandstones, feldspar and clay rich or calcite cemented sandstones (secondary reservoir), mudstones (sealing complex), and oolitic ironstones (sealing complex) derived from braided river, fluvial-deltaic, and restricted marine shoal depositional environments, respectively. The reservoir sandstone samples characterized here had measured total porosity that ranged from 11 to 23% with pore throats mainly between 90 and 100 μm, and core air permeability from 558 to 3397 mD. In the Precipice Sandstone reservoir sample μCT plugs, 98% of the pore space was connected with calculated vertical permeability 145–4611 mD and horizontal 4291–8200 mD. Feldspar and clay rich sandstone and mudstone samples from the overlying Evergreen Formation had porosity that ranged between 0.2 and 22.9%, with a wide range of pore throat sizes from ~0.005 to 30 μm, and permeability from 0.2 to 28.1 mD, respectively. Ironstone and mudstone samples from the Westgrove Ironstone Member (Evergreen Formation) had porosity from 0.7 to 9.7% and a low permeability of 0.04 mD. Kinetic geochemical CO2 reactivity models made predictions over two time-scales: 30 or 1000 years. Selected models also accounted for the potential presence of 10 ppm SO2 gas. The Precipice Sandstone quartz-rich reservoir sandstones had consistently small amounts of reactive minerals and mineral trapping or scaling of the reservoir was not predicted over 30 years, with the pH approximately 4.5 after 30 years. Samples from the Evergreen Formation included feldspar and clay rich sandstones and mudstones, several contained variable amounts of carbonate cement. Their response to CO2 was more influenced by mineral content than rock type. Plagioclase feldspars and Fe-rich chlorite were the main silicate minerals that reacted to produce siderite and ankerite mineral trapping up to 2.57 kg/m3 CO2. In the very unlikely event that CO2 rich fluids migrated upwards as far as the Westgrove Ironstone Member, chlorite is predicted to alter to siderite. This study indicates that the Precipice Sandstone reservoir in the study region has a low likelihood of mineral scaling which is favorable to avoid CO2 injectivity issues. Mineral trapping as ankerite and siderite could be expected to trap CO2 in the chlorite and plagioclase rich Evergreen Formation seal lithologies. Further work is suggested on validating long term predictions with observation data from natural analogue studies.

Demand for natural gas is expected to increase faster than any other fossil fuel over the coming decades. Australian coal bed methane (CBM) resources are among the largest in the world and are already being utilized to meet increasing demand. The majority of methane contained within CBM producing coal beds is microbially generated and low rank coals are often associated with higher bioavailability. However, the results of previous studies are conflicting, and it is unclear how or if coal rank has an effect on microbial community structure. Here, enrichment cultures grown on coals spanning 13 different ranks (lignite to bituminous) were characterized with 16S rRNA gene amplicon sequencing and combined with volatile fatty acid (VFA) and headspace methane measurements to understand the effect of coal rank on CBM microbial communities. Our results show that there is a significant negative correlation between final methane yield and rank, suggesting that the bioavailability of the coal organic material decreases with increasing thermal maturity. The concentration of VFAs generally increased with decreasing rank and revealed that community composition was significantly correlated with the concentration of specific VFAs (e.g. acetate). These data suggest that the observed higher methane production from lower rank coals is linked to increased concentrations of low molecular weight acids desorbing from the coal. The increase in low molecular weight VFAs was associated with the enrichment of specific taxa known to specialize in the degradation of low molecular weight acids and alcohols.

This paper presents an experimental and numerical investigation on anisotropic permeability of coal in order to evaluate the effects of the anisotropic permeability variation on CO 2 sequestration in coal and CO 2 -sequestration enhanced coalbed methane (CO 2 -ECBM) recovery from coal seams. An alternative permeability model has been developed to improve reservoir simulation for CO 2 sequestration in coal and coalbed methane (CBM) recovery and provide better understanding of the CO 2 -ECBM process. Combined with experimental observations, the permeability model adopts a discontinuum medium approach, by which coal is treated as a discontinuum medium containing anisotropic matrixes and cleats. The permeability variations and anisotropic permeability ratios under isotropic net stresses were tested with relatively large coal samples to provide experimental information for model parameterization and validation. The model was further incorporated into a 3D coal reservoir model for reservoir simulations to investigate the impact of the anisotropic permeability of coal on coalbed methane (CBM) recovery. The simulations show good agreements with the experimental data, revealing that the developed model is superior for describing stress- and sorption-induced permeability variations in coals compared with commonly used permeability models that treat coal as isotropic media and use constant values for stress-dependent parameters. The results suggest that anisotropic permeability has significant effects on gas production and CO 2 breakthrough time, implying that it is a critical parameter in determining well pattern and orientation of horizontal wells.

Methane cycling gets more diverse The production and consumption of methane by microorganisms play a major role in the global carbon cycle. Although these processes can occur in a range of environments, from animal guts to the deep ocean, these metabolisms are confined to the Archaea. Evans et al. used metagenomics to assemble two nearly complete archaeal genomes from deep groundwater methanogens (see the Perspective by Lloyd). The two reconstructed genomes are members of the recently described Bathyarchaeota and not the phylum to which all previously known methane-metabolizing archaea belonged. Science , this issue p. 434 , see also p. 384

Carbon dioxide captured and stored geologically from industrial sources such as coal fired and oxyfuel power stations or cement processing may contain impurity gases such as SO 2 . Carbonates including siderite and ankerite are some of the most reactive minerals present in siliciclastic reservoirs. The reactivity of crushed siderite-ankerite to pure supercritical CO 2 and impure SO 2 -CO 2 dissolved in water was compared in a combined experimental and kinetic modelling study. Ankerite dissolution and fine-grained Fe-bearing mineral precipitation was observed with pure CO 2 -water reaction at 80°C and 200bar. Dissolved Ni, Cr, and Zn concentrations initially increased but subsequently decreased by 192h. The solution pH was predicted to be buffered from 3.1 to 4.4, with siderite and trace Fe-oxide precipitation. With the presence of SO 2 in the CO 2 stream, greater ankerite dissolution and additionally siderite dissolution were observed and predicted initially. Solution pH initially decreased on gas injection and was subsequently buffered from 2.5 to 5 during reaction, with the predicted pH3.9. Gypsum and amorphous FeS were precipitated along with an Fe-Mn carbonate cement, potentially re-precipitated siderite. Dissolved Cr concentration initially increased but subsequently decreased by 192h, with Cr signatures observed in precipitated FeS. Conversion of ankerite or calcite to gypsum initially (rather than anhydrite) could decrease rock porosity. Theoretically complete conversion of 10% ankerite or 15% calcite could reduce horizontal permeability in potential CO 2 storage cap-rock by 3-170mD favourably self-sealing and reducing gas migration. Changes to rock permeability through geochemical reactions however require validation data from experimental measurements. These results have implications for the co-injection of flue gas impurities during CO 2 storage in reservoirs containing ferroan carbonates.

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 δ13CDIC of 14.2‰. 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 δ13CDIC (20.0‰). 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 δ13CDIC (14.0‰). Co-mingled, produced methane carbon isotope values (δ13C − 57.0‰ to − 44.5‰) from both the upper (Juandah) and lower (Taroom) coal measures plot within the mixed ‘thermogenic/microbial’ genetic field. By contrast, deuterium isotopic difference [Δ2H(H2O–CH4)] values and cross-plots of δ2H–H2O and δ18O–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 2H and 18O than elsewhere in the Surat Basin, which may suggest these samples have been more heavily impacted by water–rock–microbial reactions. 14C 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 ~ 50,000 years (limit of radiocarbon dating). Consistent with these dates, δ2H–H2O and δ18O–H2O values for the Surat Basin (δ2H − 32‰ to − 56‰, δ18O − 5.9‰ to − 9.0‰) 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 δ2H–CH4 and δ2H–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 δ13CDIC (median 14.2‰) and δ13C–CH4 (median − 52.1‰) 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.

This study examines the relationship between borehole temperature logs and gas distribution in coal seams, both spatially and with depth. Temperature logs are often utilized in hydrogeology to monitor groundwater flow which can introduce methanogenic consortia into coal seams, resulting in the generation and accumulation of coal seam gases. Areas of hydraulic connectivity, characterized by open cleats and fractures, provide a pathway for the meteoric influx, whereas tight, mineralized sections of strata prohibit vertical flow and have the potential to trap coal seam gases, or to limit the influx of methanogens and the generation of secondary, biogenic methane. The combination of these concepts raises the possibility of utilizing temperature logs for mapping coal seam gas distributions and assisting exploration activities. Wireline temperature logs are inexpensive to obtain as part of any exploration, production or monitoring program, but provide information pertaining to flow regimes and in situ geological environments. A case study is presented from the Sydney Basin of Australia to demonstrate the types of analyses and interpretations relating to coal seam gas distribution that may be gleaned from temperature log datasets.

Some industrial CO2 streams stored geologically may contain impurities including SO2, O2, and NOX. The impacts of these reactive gases in CO2–water–rock interactions have received limited attention relative to pure CO2 studies. An experimental and geochemical modelling study of reservoir and cap-rock core samples from a potential CO2 storage site in the Surat Basin, Queensland, Australia is described. Reservoir and cap-rock samples were reacted with low salinity fluid saturated with an impure CO2 stream containing 0.16% SO2 at reservoir simulated in situ conditions. This study is relevant to the storage of impure CO2 streams from industrial sources in sandstone reservoirs. Experimentally, dissolution of SO2 from the supercritical gas phase into the aqueous phase showed a kinetic control. The acidification from sulphuric acid generation, and the resulting silicate and carbonate dissolution were generally observed in all experiments. Significant dissolved cation concentrations in solution (up to ~ 843 mg/kg) indicated a higher potential for CO2 trapping, especially from mineralogically reactive core samples. The extent of mineral dissolution was dependent on the rock mineralogy. Precipice Sandstone samples showed the lowest reactivity indicating its suitability as a reservoir unit. However this study indicates the potential for localised changes to water quality in the near wellbore region that necessitate further investigation. Corrosion of the carbonates calcite, siderite and ankerite, and silicates including iron-rich chlorite, plagioclase, and K-feldspar was observed on rock surfaces in potential seal rocks. Calcite cement partially dissolved and acicular gypsum precipitated on the surface of a sandstone formation that could act as a potential secondary sealing unit. The formation of dense sulphates may self-seal some cap-rock. Simulations showed a good history match with the experiments, with the main predicted reactive phases including calcite, ankerite, siderite and chlorite +/− Fe oxides on the time scale of the experiments. Redox potential of the system was important; where oxidised iron was present SO2 oxidation to sulphuric acid was predicted to dominate in the simulations; otherwise pyrite and elemental S formation were also predicted (as observed experimentally). The findings of this study are generally applicable to understanding likely impacts to water quality from impure CO2 injection in CCS sites in Australia and internationally. Data has been provided which may be used to improve longer term modelling predictions.

The Pretty Hill Formation of the Otway Basin (Australia) has been studied as a natural analogue for geological storage of anthropogenic CO2 in order to examine the effects that CO2 concentration and reservoir heterogeneity have on CO2-related reactions. New petrographic data are presented, which validate the use of Hylogger™ as a tool to investigate high-resolution vertical changes in reservoir mineralogy. The integrated data set confirms earlier interpretations, showing that chlorite has been altered to kaolinite and siderite/ankerite in reservoir facies exposed to moderate and high CO2 concentrations, while chlorite remains the dominant clay mineral in all parts of the formation where CO2 content is low. Differences have been observed in the degree of CO2-related reaction relative to CO2 concentration and reservoir heterogeneity. Where CO2 content is very high (c. 98 mol%) and associated with high water saturations, both chlorite and detrital feldspars have undergone complete reaction in the reservoir facies, resulting in quartzose sandstones with a kaolinite matrix, and with siderite as the dominant carbonate precipitate. Conversely, where CO2 content is moderate (c. 29–57 mol%) and within the gas leg of the reservoir, chlorite has undergone significant reaction, but much of the original feldspar is preserved, suggesting relatively minor reaction. Carbonate cements from the moderate CO2 gas-leg comprise calcite, siderite and ankerite, occurring as cemented zones associated with rock heterogeneities and the present-day gas–water contact. Heterogeneities within the gas-leg are likely to have associated pore fluid contacts, whereby relatively high water saturations will be present in the fine-grained baffles and seals. The most advanced feldspar reaction occurs locally at the contact between baffles and reservoir rock, while reactions have been significantly impeded in the finer grained units due to their low permeabilities. Stable isotope data presented for carbonate cements analysed from wells with low and moderate CO2 levels show no clear distinction. Relatively early formed calcite has δ13C values that require an organic carbon source, suggesting precipitation unrelated to the reservoir CO2 in the Otway Basin. In contrast, diagenetically late calcite and siderite samples display two distinct δ13C groups (dependent on carbonate type), where the calculated fluid carbon isotope compositions are similar to documented magmatic CO2 reservoired in the nearby Caroline Field. This suggests that magma-derived CO2 may have been more prevalent through the Pretty Hill Formation than previously thought. Although the CO2 has not been contained over the long term in the low CO2 sites, it may have caused the local dissolution of carbonate and laumontite cement, and also contributed a source of carbon for late-stage calcite cements. These studies illustrate the importance of understanding both the reservoir composition and vertical heterogeneity of potential storage systems. Fluid–mineral reactions are likely to be advanced within stacked reservoir facies and impeded within siltstone layers, while the distribution of carbonate cement may increase the reservoir heterogeneity by the formation of cemented siltstone/sandstone layers, thereby creating impermeable barriers or baffles to CO2.

The interactions between CO2, water and rock in low-salinity host formations remain largely unexplored for conditions relevant to CO2 injection and storage. Core samples and sub-plugs from five Jurassic-aged Surat Basin sandstones and siltstones of varying mineralogy have been experimentally reacted in low-salinity water with supercritical CO2 at simulated in situ reservoir conditions (P=12MPa and T=60°C) for 16days (384h), with a view to characterising potential CO2-water-rock interactions in fresh or low-salinity potential siliclastic CO2 storage targets located in Queensland, Australia. CO2-water-rock reactions were coupled with detailed mineral and porosity characterisation, obtained prior to and following reaction, to identify changes in the mineralogy and porosity of selected reservoir and seal rocks during simulated CO2 injection. Aqueous element concentrations were measured from fluid extracts obtained periodically throughout the experiments to infer fluid-rock reactions over time. Fluid analyses show an evolution of dissolved concentration over time, with most major (e.g. Ca, Fe, Si, Mg, Mn) and minor (e.g. S, Sr, Ba, Zn) components increasing in concentration during reaction with CO2. Similar trends between elements reflect shared sources and/or similar release mechanisms, such as dissolution and desorption with decreasing pH. Small decreases in concentration of selected elements were observed towards the end of some experiments; however, no precipitation of minerals was directly observed in petrography. Sample characterisation on a fine scale allowed direct scrutiny of mineralogical and porosity changes by comparing pre- and post-reaction observations. Scanning electron microscopy and registered 3D images from micro-computed tomography (micro-CT) indicate dissolution of minerals, including carbonates, chlorite, biotite members, and, to a lesser extent, feldspars. Quantitative mineral mapping of sub-plugs identified dissolution of calcite from carbonate cemented core, with a decrease in calcite content from 17vol.% to 15vol.% following reaction, and a subsequent increase in porosity of 1.1vol.%. Kinetic geochemical modelling of the CO2-water-rock experiments successfully reproduced the general trends observed in aqueous geochemistry for the investigated major elements. After coupling experimental geochemistry with detailed sample characterisation and numerical modelling, expected initial reactions in the near-well region include partial dissolution and desorption of calcite, mixed carbonates, chloritic clays and annite due to pH decrease, followed in the longer-term by dissolution of additional silicates, such as feldspars. Dissolution of carbonates is predicted to improve injectivity in the near-well environment and contribute to the eventual re-precipitation of carbonates in the far field.