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

摘要

High-CO_2-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 CO_2 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 CO_2 levels and the mechanisms that keep the CO_2 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 ~(13)C-enriched CO_2. Enhanced kaolinite concentrations adjacent to a low-CO_2-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-CO_2 coals. The present-day CO_2 in the high-CO_2 coals at Oaky Creek was emplaced in the Upper Triassic based on dating of illitic clay minerals from the high-CO_2 well and is magmatic or deep crustal in origin. Methane in the coals is of mixed origin, with secondary biogenic CH_4 formed by microbial reduction of CO_2 predominant in the high-CO_2 coals. This suggests that methanogenesis may provide an additional sequestration mechanism for CO_2 in coal seams.