New insights into the offshore Otway Basin

Geoscience Australia has undertaken a regional seismic mapping study of the offshore Otway Basin extending across the explored inner basin to the frontier deep-water region. Seismic interpretation covers over 18,000 line-km of new and reprocessed data acquired in the 2020 Otway Basin seismic program, over 40,000 line-km of legacy 2D seismic data and GA’s new 2023 Otway 3D post-stack Mega Merge seismic dataset. This work provides a new perspective on regional structural architecture and basin evolution and has important implications for hydrocarbon prospectivity of this region. This seminar was two short talks centring on the Otway Basin. <u>Post-stack 3D merging to fast-track regional interpretation - offshore Otway Basin case study, presented by Merrie-Ellen Gunning</u> This case study was to produce a regularised and seamless 3D dataset of the highest possible quality, for the offshore Otway Basin, within two-months. The input migrated volumes varied by data extent, migration methodology, angle range and grid orientation. Fourteen input volumes totalling 8,092 km2 were post-stack merged and processed to produce a continuous and consistent volume, enabling more efficient and effective interpretation of the region. The surveys were regularised onto a common grid, optimised for structural trend, prior to survey matching. A mis-tie analysis algorithm, applied over a time window optimised for interpretation of key events, was used to derive corrections for timing, phase and amplitude, using a reference. This was followed by time-variant spectral and amplitude matching to improve continuity between volumes. Additional enhancements including noise removal and lateral amplitude scaling were also applied. The final merged volume offers significant uplift over the inputs, providing better imaging of structure and events and dramatically improving the efficiency and quality of interpretation. This enables rapid reconnaissance of the area by explorers. <u>Structural architecture of the offshore Otway Basin presented by Chris Nicholson</u> We present new basin-scale isochore maps that show the distribution of the Cretaceous depocentres. Maps for the Lower Cretaceous Crayfish and Eumeralla supersequences, together with those recently published for the Upper Cretaceous Shipwreck and Sherbrook Supersequences, completes the set of isochore maps for the main tectonostratigraphic basin intervals. Mapping of basement involved faults has revealed structural fabrics that have influenced depocentre development. The tectonostratigraphic development of depocentres and maps of deep crustal units delineate crustal thinning trends related to late Cretaceous extension phases. This work highlights the need to review and update structural elements. For example, the boundary between the Otway and Sorell basins is now geologically constrained. The refinements to the tectonostratigraphic evolution of the Otway Basin presented here have important implications for the distribution and potential maturity of petroleum systems, especially with regard to heat flow associated with crustal extension.