A revised seismic & stratigraphic framework for the early Mesozoic Crayfish Group - onshore Otway Basin, Victoria; implications for understanding the development of depositional environments within a continental rift basin

The aim of this study was to construct a new stratigraphic framework for the Victorian section of the onshore Otway Basin, while answering unresolved questions about the basin’s early rift history. These include the order in which individual depocentres were formed, the timing of deposition of laterally varying stratigraphic packages, which units were confined to individual rift depocentres and how the transition between depositional environments was controlled by rift related tectonism. The approach first involved the integration of stratigraphic correlations, petrophysical interpretation, drill-core descriptions and biostratigraphy with seismic facies analysis. This resulted in the identification of at least two new lithostratigraphic members and the construction of a new stratigraphic framework for the sediments. Structural analysis was then undertaken by building a 3D structural model which was used to calculate cumulative displacement rate across depocentre bounding faults and sediment accumulation rates within the main depocentres. This analysis suggests that changes in rates of fault activity were the primary control on temporal shifts in depositional environments. The results were then enhanced by building a series of continental sequence stratigraphy models for each depocentre. These demonstrated that a series of age correlatable regional tectonic events controlled common variations in the vertical stacking order of stratigraphy in all the depocentres. Sequence stratigraphy interpretation also allowed detailed palaeogeography mapping which in turn led to the construction of a high-resolution volumetric model of the Crayfish Group members. This resulted in a revised chronostratigraphic framework for the onshore Otway Basin, which provided a basis for explaining the relationship between fluvio-lacustrine sedimentation and tectonism during the early history of intercontinental rift systems. Finally, the resulting geological model was used to evaluate its consequences on the thermal history and source rock maturation of the basin. This was done by building a series of 1D and 3D thermal models. These models allowed investigation of the potential for both conventional and unconventional petroleum systems and carbon capture and storage potential. They demonstrate that the highest quality source rocks in the system are mature for hydrocarbon generation but are restricted to a series of ancient lacustrine environments within the central Otway Basin. They also demonstrate that highest quality reservoirs are buried deeply enough and are hot enough to store carbon dioxide in its supercritical phase within many of the existing drilled structural culminations.