The influence of petrological properties and burial history on coal seam methane reservoir characterisation, Sydney Basin, Australia

Abstract

Abstract Gas content of coals continuously change throughout their burial histories as a result of the changing state of equilibrium of the coal–gas system caused by variations in P–T conditions and coal rank. To fully evaluate the prospectivity of a coalbed methane resource, numerous coal properties, burial history, P–T conditions, hydrology and the likelihood of secondary biogenic gas generation need to be considered with respect to gas sorption capacity, gas contents and permeability. Previous studies have given differing interpretations on relationships between rank and maceral composition with sorption capacity. The maximum gas storing capacities for Sydney Basin coals is inversely related to rank up to medium volatile bituminous, but a coked, contact metamorphosed coal has an elevated capacity. Comparison of sorption capacities of coals having similar ranks and variable maceral group composition, indicate that rank has a dominating effect over any effects of organic matter type. For the Sydney Basin coals, the in-situ gas contents, on average, increase with depth up to about 600 m and with further increases in depth to 900 m, the gas contents tend to plateau or even decrease. Such a trend probably is consistent with the combined effects of pressure and temperature on the gas sorption capacity during the geological history. R-mode cluster analyses of the coal and gas properties yield a positive correlation between gas contents and inertinite abundance. This is related to undersaturation of the vitrinite-rich coals, possibly due to higher permeability and consequent leakage of more gas from vitrinite-rich coals than from inertinite-rich coals. Although a large amount of methane and other hydrocarbon gases would have been generated in the Sydney Basin at maximum burial during the Early Cretaceous, a large proportion of the gas might not have been sorbed within the coal due to limited gas sorption capacities and enhanced diffusivity at high temperatures. Upon uplift, gas that migrated from deeper in the sequence or from shallower biological activity may have been sorbed into the coals. Without secondary gas replenishment however, many of these coals remain significantly undersaturated. The areas that contain considerable amounts of secondary biogenic gas are highly prospective for coalbed methane production partly because of the higher gas contents, but also because of the higher permeability, which is required for access of the microbes and nutrients in meteoric waters. To fully evaluate prospectivity of coalbed methane resources, numerous coal properties, burial history, geologic setting and the likelihood of secondary biogenic gas generation need to be considered with respect to gas sorption capacity, gas contents and permeability.