Seismic Interpretation Of The Eastern Gippsland Basin With Application To Fault Seal Analysis In Carbon Dioxide Storage Leads
Doctor of Philosophy, 2010
University of Adelaide
Geological storage of carbon dioxide (CO2) is a mitigation option for reducing greenhouse gases. To date, CO2 storage-related research in the Gippsland Basin, has focussed on detailing how the stratigraphy and facies, but not how the faults may affect CO2 storage/fluid-flow and fault-trap integrity. This thesis addresses this latter deficiency through a 3-D seismic-based structural interpretation of CO2 storage leads (CO2SL) identified in the eastern part of the basin. Further underpinning this study are over 800 tops/markers collated and/or interpreted from 95 wells.
The primary goals of this study are two-fold: first, to ascertain how structural events, basin tectonic phases and associated sedimentary fill, influence fault-trap integrity, and second, how the fault network across fault-block complexes, minor faults, fault tips, branch lines, juxtaposition of reservoir intervals and modelling of shale smear across-fault influence fault-trap integrity.
With respect to CO2 storage, and based on the structural interpretation undertaken, the basin’s rift-drift subphase and the associated Halibut Subgroup sedimentary section, is the best overall storage option. Of the 200 or so faults interpreted, 20% and 67% have fault tips that arrest at the Campanian Seahorse and Early Eocene Mackerel Unconformities, respectively, but fault tips do not arrest at the Oligocene Latrobe Unconformity, as has been previously interpreted. The implication is that the intra-Halibut Subgroup faults are not kinematically and/or hydraulically linked to the minor faults present in the overlying regional seal, thus providing some assurance of fault-trap integrity.
Twelve CO2SLs are identified and screened; areal closures are 16.9 ± 13 km2 (excluding outliers), CO2 flow-path distances are 7.4 ± 4.2 km and, flow-path heights are 0.55 ± 0.14 km. The fault network across the fault-block complexes is interpreted to be reduced, providing some assurance that CO2 flow-paths are relatively unimpeded.
The high proportion of across-fault sand-on-sand windows and, poorly developed shale smear, precludes any significant amounts of CO2 from being trapped against any significant length of any of the larger fault planes. It is established that most faults have a moderate to high likelihood of fault reactivation, as most trend subparallel to one of the conjugate planes of shear failure; however, the likelihood can be low across portions of the fault plane where the strike deviates from trend (up to ± 33°). When splay faults are considered, there is an increase in the overall likelihood of fault reactivation for 32% of the branch-line cases considered, a reduction for 26% of cases and, negligible effect for the remaining 42%.
This study conclusively demonstrates that the primary factor affecting fault-trap integrity in the Halibut Subgroup is the high number of across-fault sand-on-sand windows; by comparison, the contribution of fault tips, branched faults and presence of shale smear is secondary. Any breach of containment by CO2 flow across-fault will adversely affect adjacent fault-block complexes, raising the concern that CO2 storage in this part of the basin, or similar geological areas in this or other basins, may be difficult to contain geographically. The implications would be profound for other offshore basins that are more poorly ranked than the Gippsland Basin.