Defining CO2 Migration Pathways in the Eastern Otway Basin
Matthew J. McINTYRE
Bachelor of Science (Petroleum Geology and Geophysics) 2015
Australian School of Petroleum
The University of Adelaide
The source of natural charge and compositional variability for the carbon dioxide (CO2) component found within gas accumulations of the eastern Otway Basin is poorly understood. Existing, often unreliable carbon isotopic data suggests a volcanically derived CO2 source for these gas fields. However, lack of evidence for volcanic rocks and unfavourable lateral flow migration analysis in proximity to (and in connectivity with) the area where most onshore gas fields occur, together suggest this to be an unlikely scenario. This study aimed to identify the presence of faults able to transmit CO2 and their influence on past CO2 migration to constrain the source of natural CO2 and compositional variability of gas accumulations in the eastern Otway Basin.
The role of sealing faults in the geographic partitioning of CO2 within natural gas fields is assessed in this project, firstly by interpretation of 3D seismic survey data and log data for 21 wells, then by modelling of fault clay content and drainage systems analysis. Three key stratigraphic horizons and 12 major NW – SE trending normal faults were interpreted and subject to structural and petrophysical modelling. In order to identify the magnitude of lateral flow migration, present-day topographic and fault relationships were derived from modelled drainage systems for the top Waarre Formation, allowing classification of major flow lines and areas of high and low flow accumulation.
Major NW – SE trending normal faults were found to provide a strong structural control on the distribution of gas fields in the region. Available gas composition data show that gas fields associated with major NW – SE trending fault surfaces with shale gouge exceeding sealing threshold values (50% - 60%) are found to have low CO2 concentrations.
A geographically more plausible source of inorganic CO2 is from thermal decomposition of underlying dolomitic cements within basement rocks. I propose this occurred as a result of contact metamorphism and fault movement related to the reactivation of basement structures during the final separation of Australia and Antarctica. CO2 generated this way could then have migrated along basement related structures as the primary migration fairway to gas fields within the eastern Otway Basin.