The Otway Basin is a broadly northwest–southeast trending basin and forms part of a rift system that developed along Australia’s southern margin. It represents an established hydrocarbon province with mostly onshore and shallow-water offshore discoveries. However, the outboard deep-water Otway Basin, with water depths up to 6300m, is comparatively underexplored and can be considered a frontier area. Following the completion of a basin-wide seismic depth-imaging program (Part 1) and insights from the revised seismic interpretation (Part 2), we have developed a comprehensive petroleum system modelling (PSM) study by integrating these data and findings (Part 3). Together, the studies have resulted in an improved understanding of the hydrocarbon prospectivity of the deep-water areas of the basin. Given the sparsity of data outboard, almost all legacy PSM studies have been focused either on the onshore or shallow-water areas of the basin and primarily on their thick Lower Cretaceous depocentres. The limitations of legacy seismic datasets resulted in a high degree of uncertainty in the derivative interpretations used as input into PSM studies. In addition, the paucity and poor quality of data in the deep-water area reduced confidence in the understanding of the basin evolution and spatial distribution of depositional environments through time. The newly acquired 2D seismic survey and reprocessed legacy data, with calibration via several wells across the basin, have improved confidence in our understanding of the tectonostratigraphic evolution of the basin (Part 2). The study presented herein integrates products from the work in Part 2 into a petroleum system model with the primary objective being to better understand the petroleum systems across the deep-water Otway Basin.
The Otway Basin is a northwest-southeast trending passive margin rift basin over 500km long and forms part of the Jurassic-Cretaceous Australian Southern Rift System. Exploration for oil and gas has, to date, focused on the onshore shelfal portions targeting thick early Cretaceous depocentres. Outboard, under deep-water areas, potential hydrocarbon resources in thick late Cretaceous depocentres remain significantly under-explored, with limited sparse legacy 2D seismic lines and no wells drilled to date. As a result, little is known about the potential continuation of the proven hydrocarbon plays, or indeed the presence of new plays, in the outboard areas. The 2020 Otway Basin seismic program was carried out with the key objectives being to infill data gaps outboard through the acquisition of new 2D seismic lines and improve the quality of legacy datasets inboard through reprocessing. A comprehensive broadband processing and depth imaging workflow was designed to address the inherent subsurface challenges that have inhibited legacy imaging campaigns. The results of this seismic program are improving the interpretability of the full stratigraphic sequence whilst also unravelling deeper crustal elements. This is providing a better understanding of the distribution of regional stratigraphic sequences, both laterally across the basin, and from the shelf to deep-water areas for the first time. As a result, new insights are being gained on both the basin evolution and potential for working petroleum systems.
The Schlumberger Multiclient Exmouth 3D survey was acquired over the Exmouth sub-basin, North West Shelf Australia and covers 12 600 km2. One of the primary objectives of this survey was to produce a wide coverage of high quality imaging with advanced processing technology within an agreed turnaround time. The complexity of the overburden was one of the imaging challenges that impacted the structuration and image quality at the reservoir level. Unlike traditional full-waveform inversion (FWI) workflow, here, FWI was introduced early in the workflow in parallel with acquisition and preprocessing to produce a reliable near surface velocity model from a smooth starting model. FWI derived an accurate and detailed near surface model, which subsequently benefitted the common image point (CIP) tomography model updates through to the deeper intervals. The objective was to complete the FWI model update for the overburden concurrently with the demultiple stages hence reflection time CIP tomography could start with a reasonably good velocity model upon completion of the demultiple process.
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