Presentations

This multi-year study reveals a series of systems rich in He and H2 gas, and a complex CH4 cycle with multiple abiotic and biological sources. Surface gas seeps along rivers are dominated by microbial CH4 related to near-surface processes in tropical sediments and wetlands. In contrast, samples from gas exploration wellheads have He (up to 1% by vol) and high concentrations of H2 (up to 25-40% by vol but consistently between at least 7-11%) for samples measured between 2012 and 2019. Here for the first time an exploration gas well discharging at surface shows evidence of the type of deep cratonic gases typically associated with the deep mines of the Witwatersrand Basin and Canadian Shield1. Specifically, some wells show a significant component of abiotic alkanes with high associated concentrations of H2, higher hydrocarbons, and isotopic and geochemical characteristics associated with abiotic organic synthesis2. Noble gas analyses confirm a crustal rather than mantle source. Particularly notable is the elevated 21Ne/22Ne end-member identified in at least one gas well, and from other areas of the São Francisco Craton3, that both show the characteristric elevated neon end-member value first identified in ancient fracture fluids from deep mines in Canada and South Africa5,5.

Overall, these results demonstrate the H2-rich gases in the Precambrian to early Paleozoic cratonic rocks of Brazil share important characteristics with the deep gas and ancient fluids first described in the deep mines of the Canadian Shield and Witwatersrand Basin. The exploration gas wells reflect a complex mixture of discharging gas associated with abiotic organic synthesis and H2 production (likely related to radiolysis and/or serpentinization), and local mixing with what are likely more surficial sources of microbial CH4.

1Sherwood Lollar et al. (2021) GCA 294:295-314. 2Warr et al. (2021) GCA 294:315-334. 3Magalhães et al, Goldschmidt Abstract (2021). 4Lippmann-Pipke et al. (2011) Chem Geo 283:287-296. 5Holland et al. (2013) Nature 497:357-360. 

This multi-year study reveals a series of systems rich in He and H2 gas, and a complex CH4 cycle with multiple abiotic and biological sources. Surface gas seeps along rivers are dominated by microbial CH4 related to near-surface processes in tropical sediments and wetlands. In contrast, samples from gas exploration wellheads have He (up to 1% by vol) and high concentrations of H2 (up to 25-40% by vol but consistently between at least 7-11%) for samples measured between 2012 and 2019. Here for the first time an exploration gas well discharging at surface shows evidence of the type of deep cratonic gases typically associated with the deep mines of the Witwatersrand Basin and Canadian Shield1. Specifically, some wells show a significant component of abiotic alkanes with high associated concentrations of H2, higher hydrocarbons, and isotopic and geochemical characteristics associated with abiotic organic synthesis2. Noble gas analyses confirm a crustal rather than mantle source. Particularly notable is the elevated 21Ne/22Ne end-member identified in at least one gas well, and from other areas of the São Francisco Craton3, that both show the characteristric elevated neon end-member value first identified in ancient fracture fluids from deep mines in Canada and South Africa5,5.

Overall, these results demonstrate the H2-rich gases in the Precambrian to early Paleozoic cratonic rocks of Brazil share important characteristics with the deep gas and ancient fluids first described in the deep mines of the Canadian Shield and Witwatersrand Basin. The exploration gas wells reflect a complex mixture of discharging gas associated with abiotic organic synthesis and H2 production (likely related to radiolysis and/or serpentinization), and local mixing with what are likely more surficial sources of microbial CH4.

1Sherwood Lollar et al. (2021) GCA 294:295-314. 2Warr et al. (2021) GCA 294:315-334. 3Magalhães et al, Goldschmidt Abstract (2021). 4Lippmann-Pipke et al. (2011) Chem Geo 283:287-296. 5Holland et al. (2013) Nature 497:357-360. 

The São Francisco Basin is a Neoproterozoic, intracratonic basin, spanning the states of Minas Gerais and Bahia, Brazil. It is bounded to the east and west by the Araçuai and Brasiliano orogenic belts. In most areas, the basement of the basin is only identified on seismic sections and the composition remains unknown, but is likely to be composed of suites of TTG (tonalite-trondhjemite-granodiorite). Hydrocarbon exploration in the basin over the last two decades has revealed that natural gas is common in the basin, but restricted to small, low-permeability reservoirs. Interestingly, early analyses of some of these gases revealed the presence of significant concentrations of hydrogen in the natural gas. Surface exhalations of gas are also common across the basin. 

A project is underway to investigate the source, generation mechanism, and migration (if any) of the H2-bearing gases. New samples and data from a natural gas exploration well near Corinto, Minas Gerais, confirm notably high concentrations of H2 (~10%) and He (>1%). Stable isotopes indicate an abiotic origin for the methane, while He-isotopes show a strong crustal signature. Neon isotope data suggest the presence of an Archaean crustal component in the gases, indicating that a portion of the gas likely originated in the crystalline basement of the basin. 

This new data confirms that crystalline Archaean rocks have the potential to be a significant geological source of H2, and that geological H2 can provide an essential building block for methane generation.

The Storage Security Calculator (SSC) is a tool to simulate the long-term (10kyr) security of CO2 storage at a basin scale. Simulations show that CO2 storage in regions with moderate abandoned well densities and that are regulated using current best practice will retain 96% of the injected CO2 over 10,000 years in more than half of cases, with maximum leakage of 9.6% in fewer than 5% of cases. Poorly unregulated storage is less secure, but over 10,000 years, less than 27% of injected CO2 leaks in half of the simulations; up to 34% leaks in just 5% of cases. This leakage is primarily through undetected and poorly abandoned legacy wells, and could be reduced through effective leak identification and prompt remediation of leakage. Natural subsurface immobilisation means that this leakage will not continue indefinitely. Regulators can most effectively improve CO2 storage security by identifying and monitoring abandoned wells, and perform reactive remediation should they leak. Geological storage of CO2 is a secure, resilient and feasible option for climate mitigation even in overly pessimistic poorly regulated storage scenarios and thus CO2 storage can effectively contribute to meeting the Paris 2015 target.

Inherent fingerprints, composed of the noble gas and stable carbon and oxygen isotopic composition of captured CO2 streams, are potentially powerful tracers for use in Carbon Capture and Storage technology that avoids the expense and complication of adding chemical tracers to the injected CO2. We will present a synthesis of the recent work undertaken in this area by our group and the key implications of our results for monitoring future CO2 storage projects using inherent tracers. We will highlight the high quality systematic measurements of the carbon and oxygen isotopic and noble gas fingerprints recently measured in anthropogenic CO2 captured from combustion power stations and fertiliser plants, using amine capture, oxyfuel and gasification processes, and derived from coal, biomass and natural gas feedstocks. We find that δ13C values are primarily controlled by the δ13C of the feedstock while δ18O values are predominantly similar to atmospheric O2. Noble gases are of low concentration and exhibit relative element abundances different to expected reservoir baselines and air, with isotopic compositions that are similar to air or fractionated air. The use of inherent tracers for monitoring and verification was assessed by analysing CO2 samples produced from two field storage sites after CO2 injection. These experiments at Otway, Australia, and Aquistore, Canada, highlight the need for robust baseline data. Noble gas data indicates noble gas stripping of the formation water and entrainment of Kr and Xe from an earlier injection experiment at Otway, and inheritance of a distinctive crustal radiogenic noble gas fingerprint at Aquistore. This fingerprint can be used to identify unplanned migration of the CO2 to the shallow subsurface or surface.

Noble gases are inherent geochemical tracers suitable for monitoring CO2 migration within the reservoir and to the shallow surface. The physical and chemical processes contributing to and modifying the noble gas contents of CO2 are explored using the data from two natural CO2 fields in the Otway Basin of SE Australia and three CO2-rich springs in Victoria. We focus on identifying the origin of the gases and the genetic link between gases stored in reservoir traps and those emanating at the surface from the  natural mineral springs. Addition of radiogenic 4He accounts for the 3He/4He variation in well gas samples. The variation of noble gas concentrations in the spring samples are explained by solubility fractionation during degassing. The combination of these two processes allows to link the spring and well gas samples to a common initial end-member. This technique can be utilised to investigate links between injected CO2 and shallow emissions detected at the surface in the CCS setting.

Carbon Capture and Storage (CCS) can help nations meet their Paris CO2 reduction commitments cost-effectively. However, lack of confidence in geologic CO2 storage security remains a barrier to CCS implementation. Leak rates of 0.01% yr-1, equivalent to 99% retention of the stored CO2 after 100 years, are referred to by many stakeholders as adequate to ensure the effectiveness of CO2 storage. Here, we present a numerical program that calculates CO2 storage security and leakage to the  atmosphere over 10kyr. This links processes of geologically measured CO2 subsurface retention (residual and dissolution trapping), and CO2 leakage estimates (based on measured surface fluxes from appropriate analogues). We model 12 GtCO2 of cumulative storage based on the EU’s 2050 target, commencing injection in 2020, and calculate CO2 retention for well-regulated onshore and offshore scenarios, and for a hypothetical onshore, poorly regulated scenario. Simulations using base-case, expert chosen values for model input parameters give total leakage after 10,000 years of between 2 and 23% of the stored CO2, equating to simplified time-averaged linear leak rates of between 0.0002% yr-1 and 0.002% yr-1, respectively. Uncertainty on the results, introduced through uncertainties in the input parameters, is quantified using Monte Carlo analysis. These Monte Carlo results show that CO2 storage in regions with moderate abandoned well densities and that are regulated using current best practice will retain 96% of the injected CO2 over 10,000 years in more than half of the cases, and will lose 9.6% of the injected CO2 in fewer than 5% of cases. 

Sensitivity analysis indicates that well density is a key control on storage security. Sensitivity tests also highlight that the most significant uncertainty in the model is how the leakage rate evolves over time; the leakage rate is expected to decrease over time once injection ceases, as the mass of mobile CO2 in the reservoir decreases via residual and chemical trapping, and leakage. 

Our new multi-parameter program can, for the first time, successfully simulate the storage of Gt of CO2 regionally across multiple sites. As expected, we find that poorly regulated storage is less secure. This leakage is primarily through undetected and poorly abandoned legacy wells, and could be reduced through identification and remediation of leakage if a comprehensive site screening and monitoring program is deployed. Importantly, natural subsurface trapping mechanisms mean that this leakage will not continue indefinitely. Consequently, even with mitigation actions restricted solely to repair of abandoned wells that blow out, regions with a legacy of poorly regulated subsurface operations can reliably and robustly store and retain 73% of injected CO2. 

Our calculated leakage values are well below 0.01% yr-1. We therefore show geological storage of CO2 to be a secure, resilient and feasible option for climate mitigation even in poorly regulated storage scenarios. Hence, deployment of carbon capture and storage can be recommended to all governments as part of their actions to comply with the Paris 2015 target of keeping the global mean temperature well below 2 C.

Inherent tracers, the isotopic and trace gas composition of captured CO2 streams, are potentially powerful tracers for use in CCS technology [1,2]. Despite this potential, the inherent tracer fingerprint in captured CO2 streams has yet to be robustly investigated and documented [3]. Here, we will present the first high quality systematic measurements of the carbon and oxygen isotopic and noble gas fingerprints measured in anthropogenic CO2 captured from combustion power stations and fertiliser plants, using amine capture, oxyfuel and gasification processes, and derived from coal, biomass and natural gas feedstocks. 

We will show that δ13C values are mostly controlled by the feedstock composition, as expected. The majority of the CO2 samples exhibit δ18O values similar to atmospheric O2 although captured CO2 samples from biomass and gas feedstocks at one location in the UK are significantly higher. Our measured noble gas concentrations in captured CO2 are generally as expected2, typically being 2 orders of magnitude lower in concentration than in atmospheric air. Relative noble gas elemental abundances are variable and often show an opposite trend to that of a water in contact with the atmosphere. 

Expected enrichments in radiogenic noble gases (4He and 40Ar) for fossil fuel derived CO2 were not always observed due to dilution with atmospheric noble gases during the CO2 generation and capture process. Many noble gas isotope ratios indicate that isotopic fractionation takes place during the CO2 generation and capture processes, resulting in isotope ratios similar to fractionated air. We conclude that phase changes associated with CO2 transport and sampling may induce noble gas elemental and isotopic fractionation, due to different noble gas solubilities between high (liquid or supercritical) and low (gaseous) density CO2. 

Data from the Australian CO2CRC Otway test site show that δ13C of CO2 will change once injected into the storage reservoir, but that this change is small and can be quantitatively modelled in order to determine the proportion of CO2 that has dissolved into the formation waters. Furthermore, noble gas data from the Otway storage reservoir post-injection, shows evidence of noble gas stripping of formation water and contamination with Kr and Xe related to an earlier injection experiment. Importantly, He data from SaskPower’s Aquistore illustrates that injected CO2 will inherit distinctive crustal radiogenic noble gas fingerprints from the subsurface once injected into an undisturbed geological storage reservoir, meaning this could be used to identify unplanned migration of the CO2 to the surface and shallow subsurface [4].

References

 [1] Mayer et al., (2015) IJGGC, Vol. 37, 46-60 http://dx.doi.org/10.1016/j.ijggc.2015.02.021

[2] Gilfillan et al., (2014) Energy Procedia, Vol. 63, 4123-4133 http://dx.doi.org/10.1016/j.egypro.2014.11.443

[3] Flude et al., (2016) Environ. Sci. Technol., 50 (15), pp 7939–7955 DOI: 10.1021/acs.est.6b01548

[4] Gilfillan et al., (2011) IJGGC, Vol. 5 (6) 1507-1516 http://dx.doi.org/10.1016/j.ijggc.2011.08.008

In the long term, captured CO2 will most likely be stored in large saline formations and it is highly likely that CO2 from multiple operators will be injected into a single saline formation. Understanding CO2 behavior within the reservoir is vital for making operational decisions and often uses geochemical techniques. Furthermore, in the event of a CO2 leak, being able to identify the owner of the CO2 is of vital importance in terms of liability and remediation. 

Addition of geochemical tracers to the CO2 stream is an effective way of tagging the CO2 from different power stations, but may become prohibitively expensive at large scale storage sites. Here we present results from a project assessing whether the natural isotopic composition (C, O and noble gas isotopes) of captured CO2 is sufficient to distinguish CO2 captured using different technologies and from different fuel sources, from likely baseline conditions.

Results include analytical measurements of CO2 captured from a number of different CO2 capture plants and a comprehensive literature review of the known and hypothetical isotopic compositions of captured CO2 and baseline conditions. Key findings from the literature review suggest that the carbon isotope composition will be most strongly controlled by that of the feedstock, but significant fractionation is possible during the capture process; oxygen isotopes are likely to be controlled by the isotopic composition of any water used in either the industrial process or the capture technology; and noble gases concentrations will likely be controlled by the capture technique employed. Preliminary analytical results are in agreement with these predictions.

Comparison with summaries of likely storage reservoir baseline and shallow or surface leakage reservoir baseline data suggests that C-isotopes are likely to be valuable tracers of CO2 in the storage reservoir, while noble gases may be particularly valuable as tracers of potential leakage.

Sorry, no abstract available.

In the long term, captured CO2 will most likely be stored in large saline formations and it is highly likely that CO2 from multiple operators will be injected into a single saline formation. Understanding CO2 behaviour within the reservoir is vital for making operational decisions and often uses geochemical techniques. Furthermore, in the event of a CO2 leak, being able to identify the owner of the CO2 is of vital importance in terms of liability and remediation. Addition of geochemical tracers to the CO2 stream is an effective way of tagging the CO2 from different operators, but may become prohibitively expensive at large scale storage sites. Here we present results from a project assessing whether the natural isotopic composition (C, O and noble gas isotopes) of captured CO2 is sufficient to distinguish CO2 captured using different technologies and from different fuel sources from each other and from likely baseline conditions.

Results include analytical measurements of CO2 sampled from a number of different CO2 capture plants and a comprehensive literature review of the known and hypothetical isotopic compositions of captured CO2 and baseline conditions.

Key findings from the literature review suggest that the carbon isotope composition will be most strongly controlled by that of the feedstock, but significant fractionation is possible during the capture process; oxygen isotopes are likely to be controlled by the isotopic composition of any water used in either the industrial process or the capture technology; and noble gases concentrations will likely be controlled by the capture technique employed. Preliminary analytical results are in agreement with these predictions.

Comparison with summaries of likely storage reservoir baseline data and shallow or surface leakage reservoir baseline data suggests that C-isotopes are likely to be valuable tracers of CO2 in the storage reservoir, while noble gases may be particularly valuable as tracers of potential leakage.

Sorry, abstract not available

In the long term, captured CO2 will most likely be stored in large saline formations and it is highly likely that CO2 from multiple operators will be injected into a single saline formation. Understanding CO2 behaviour within the reservoir is vital for making operational decisions and often uses geochemical techniques. Furthermore, in the event of a CO2 leak, being able to identify the owner of the CO2 is of vital importance in terms of liability and remediation. Addition of geochemical tracers to the CO2 stream is an effective way of tagging the CO2 from different operators, but may become prohibitively expensive at large scale storage sites. Here we present results from a project assessing whether the natural isotopic composition (C, O and noble gas isotopes) of captured CO2 is sufficient to distinguish CO2 captured using different technologies and from different fuel sources, from each other and from likely baseline conditions.

Results include a comprehensive literature review of the known and hypothetical isotopic compositions of captured CO2 and baseline conditions and analytical measurements of CO2 sampled from a number of different CO2 capture plants.

Key findings from the literature review suggest that the carbon isotope composition will be most strongly controlled by that of the feedstock, but significant fractionation is possible during the capture process; oxygen isotopes are likely to be controlled by the isotopic composition of any water used in either the industrial process or the capture technology; and noble gases concentrations will likely be controlled by the capture technique employed. Preliminary analytical results are in agreement with these predictions.

Comparison with summaries of likely storage reservoir baseline data and shallow or surface leakage reservoir baseline data suggests that C-isotopes are likely to be valuable tracers of CO2 in the storage reservoir, while noble gases may be particularly valuable as tracers of potential leakage.

The contemporaneous Rotoiti and Earthquake Flat ignimbrites, erupted from the Taupo Volcanic zone, New Zealand, form a distinctive tephrostratigraphic horizon in the Southern Pacific. Radioisotopic dating results for these eruptions remain controversial, with published ages ranging from 35.1 ± 2.8 ka [1] to 71 ± 6 ka [2], with 61.0 ± 1.5 ka [3] often being cited as the most widely accepted age. These eruptions are difficult to date as their age is near the limit for various radiometric dating techniques, which are complicated by a large proportion of inherited material (xenocrysts) and a lack of phases suitable for dating.

Glass-bearing plutonic blocks erupted with the Rotoiti and Earthquake Flat ignimbrites have previously been interpreted as deriving from a slowly cooled and incompletely solidified magma body that was sampled by the eruptions. They contain large vugs lined with euhedral quartz, sanidine and biotite crystals, indicating that these crystals grew in a gas or aqueous fluid rich environment and are interpreted to have formed shortly before or during eruption. Here we will present Ar-40/Ar-39 ages for sanidines and biotites extracted from vugs in lithic blocks erupted as part of the Earthquake Flat ignimbrite. We show that, even for vug-lining material, inherited ages remain a problem and are the likely source of the wide variation in published radiometric ages. Nevertheless, many of the Ar-40/Ar-39 ages are much younger than the 61 ka age [3] and are more consistent with the recent stratigraphic, C-14 and U-238/Th-230+(U-Th)/He ages that have been suggested (e.g. [4,5]).

1. Whitehead, N. & Ditchburn, R. New Zealand Journal of Geology and Geophysics 37, 381–383 (1994).

2. Ota, Y., Omura, A. & Iwata, H. New Zealand Journal of Geology and Geophysics 32, 327–331 (1989).

3. Wilson, C. J. N. et al. Quaternary Science Reviews 26, 1861–1870 (2007).

4. Molloy, C., Shane, P. & Augustinus, P. Geological Society of America Bulletin 121, 1666–1677 (2009).

5. Danišík, M. et al. Earth and Planetary Science Letters 349-350, 240–250 (2012).