NATURAL ACCUMULATION OF CO2 IN COALS FROM THE SOUTHERN SYDNEY BASIN—IMPLICATIONS FOR GEOSEQUESTRATION

Abstract

The southern Sydney Basin is an ideal natural analogue for CO2 geosequestration because of the widespread CO2 occurrence, extensive data sets available and general knowledge of gas distribution. The CO2 mainly occurs adsorbed in coal, incorporated into carbonate minerals and dissolved in formation water. On this basis, an area of ~900 km2 has been chosen for detailed examination.

Gas in the coal seams of this area contain mainly CH4 and CO2, the CO2 content ranging from <1 to 20 m3/tonne. The 13C values indicate multiple sources including thermal maturation of coal, microbial alteration of pre-existing gases and magmatic activity as the main source. The highest concentrations of adsorbed CO2 occur mainly in anticlines and between ~300 and 600 m possibly reflecting inverse relatonship with CO2 solubility in formation water. Carbon dioxide appears to have migrated from the deep-seated magmatic sources along faults and permeable strata, towards structural highs and stratigraphically shallower coal seams.

Calculations indicate that about 78 x 106 tonnes of CO2 are presently stored in coaly intervals in the study area. Assuming a storage capacity of 20 m3/t for these coal seams, the total CO2 storage capacity for the coaly intervals is ~880 x 106 tonnes. Using the study area as an analogue for enhanced coal seam methane production, 175 x 106 tonnes of CO2 could be stored, assuming a 50% CH4 recovery factor and an average CO2 sorption capacity 1.5 times that for CH4.