G. K. W. Dawson

Carbon dioxide-rich coals of the Oaky Creek area, central Bowen Basin: a natural analogue for carbon sequestration in coal systems

High-CO2-containing coal seams in the Oaky Creek area of the Bowen Basin, eastern Australia provide natural analogues of the processes likely to occur as a result of CO2 injection and storage in coal systems. We conducted mineralogical, stable and radiogenic isotope and major element analyses of mudstones and sandstones adjacent to the coal seams and stable isotope and compositional studies of coal seam gas desorbed from the coals to establish the impact of the high CO2 levels and the mechanisms that keep the CO2 naturally sequestered. Siderite is the earliest carbonate phase present and occurs with kaolinite in mudstones and sandstones. It is interpreted to have formed under low-temperature, reducing conditions where methanogenesis has produced residual 13C-enriched CO2. Enhanced kaolinite concentrations adjacent to a low-CO2-containing coal seam reflect interaction with acidic fluids produced during the coalification of organic matter. Stable isotope data for carbonates and Rb–Sr isochron ages for illitic clays indicate that illitic clay–carbonate assemblages adjacent to both coal seams formed as a result of meteoric hydrothermal activity in the Upper Triassic with more intensive mineralogical reactions evident in the high-CO2 coals. The present-day CO2 in the high-CO2 coals at Oaky Creek was emplaced in the Upper Triassic based on dating of illitic clay minerals from the high-CO2 well and is magmatic or deep crustal in origin. Methane in the coals is of mixed origin, with secondary biogenic CH4 formed by microbial reduction of CO2 predominant in the high-CO2 coals. This suggests that methanogenesis may provide an additional sequestration mechanism for CO2 in coal seams.

Mineralogical characterisation of a potential reservoir system for CO2 sequestration in the Surat Basin

The Jurassic-aged Precipice Sandstone and Hutton Sandstone in the Surat Basin (Queensland) are potential targets for CO2 sequestration that exhibit considerably different mineralogies. Hyperspectral logging was complemented with petrographic studies to reveal core-scale mineralogical distributions. Scanning electron microscopy and X-ray diffraction analyses of the bulk rock composition and clay (< 2 μm) fraction provided an adjunct to mineral identification. The Precipice Sandstone is interpreted to be geochemically poorly reactive with a clean, quartzose mineralogy dominated by quartz and kaolinitic clays. These clays coat grains and partially fill pores, having formed from the extensive leaching of unstable feldspars and lithics, and are commonly associated with Fe-oxide/hydroxide. The mineralogy of the Evergreen Formation is geochemically reactive and contains significant feldspar, smectite, chlorite, carbonate and kaolinite, and should therefore serve as an effective intraformational seal. Low permeability in the Evergreen Formation is attributed to extensive porosity loss following burial compaction, which is evident from the formation of pressure-dissolution features and a lithic-derived clay-rich pseudo matrix in several samples. The Hutton Sandstone is a heterogeneous potential reservoir, with geochemically reactive intervals containing smectite, chlorite and Fe–Mn–Mg–Ca carbonates, in addition to feldspars and lithics. Variable clay matrix content, porosity and locally significant cementation within the Hutton Sandstone indicate a mineralogically heterogeneous unit that could have the capacity to buffer emplaced CO2. This potentially provides an additional vertical component of capture and storage to the potential reservoir system, in addition to the traditional regional seal provided by the Evergreen Formation.

SO2 and O2 co-injection with potential carbon storage target sandstone from a fresh-water aquifer

Oceanic anoxic events (OAEs) were a frequent occurrence in the Cretaceous greenhouse ocean. Based on a variety of paleoredox indicators, euxinic water column conditions are commonly invoked for these OAEs. However, in a high resolution study of OAE3 deep sea sediments [1], revised paleoredox indicators suggest that euxinic conditions fluctuated with anoxic ferruginous conditions on orbital timescales. Building upon this, we here present new data for a continental shelf setting at Tarfaya, Morocco, that spans a period prior to, and during, the onset of OAE2. We again find strong evidence for orbital transitions from euxinic to ferruginous conditions. The presence of this distinct cyclicity during OAE2 and OAE3 in shallow and deep water settings, coupled with its occurrence on the anoxic shelf prior to the global onset of anoxia, suggests that these fluctuations were a fundamental feature of anoxia in the Cretaceous ocean. The observed redox cyclicity has major implications for the cycling of phosphorus, and hence the maintenance and longevity of OAEs. However, despite this significance, controls on the observed redox cyclicity are essentially unknown. Here, we utilize S isotope measurements (pyrite S and carbonate-associated S) from the deep sea and shelf settings to model oceanic sulphate concentrations across the redox transitions. Perhaps surprisingly, we find no evidence to suggest that ferruginous conditions arose due to extensive drawdown of seawater sulphate (as pyrite-S and organic-S) under euxinic conditions. Instead, S isotope systematics in the deep sea imply increased sulphate concentrations during ferruginous intervals. Based on these observations and other major element data, we infer that the redox cyclicity instead relates to orbitally-paced fluctuations in continental hydrology and weathering, linking the redox state of the global ocean to climate-driven processes on land. [1] Marz et al (2008) GCA, 72, 3703-3717.

Berea Sandstone permeability pre and post reaction with supercritical CO2 in 1% NaCl brine

Iron and Manganese Reduction and Associated Phosphorus Release in Coastal Baltic Sea Sediment