VGP Technical Report 48 - Petroleum systems modelling, Otway Basin, Victoria.
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Product description:About the Victorian Gas Program: The Victorian Gas Program (VGP) is a comprehensive science-led program, incorporating geoscientific and environmental research to assess the risks, benefits and impacts of potential onshore conventional gas exploration and production. The program is also investigating the potential for further discoveries of onshore conventional and offshore gas in the Otway and Gippsland geological basins and assessing the feasibility of additional onshore underground gas storage in depleted reservoirs around the Port Campbell area. The VGP includes an extensive, proactive and phased community and stakeholder engagement program, through which the results of the scientific studies are being communicated. Executive summary: An integrated petroleum systems model of the Victorian Otway Basin has been constructed to quantify the generation, migration and accumulation of petroleum, to inform a prospectivity assessment and resource estimate for use by the Geological Survey of Victoria (GSV) as part of the Victorian Gas Program (VGP). Petroleum systems analysis (PSA) provides a framework for integrating the different elements that make up a petroleum system, such as charge, trap, reservoir and seal, into a coherent model. This model can then be used to assess the hydrocarbon prospectivity of an area. PSA typically involves source rock characterisation (e.g. net thickness, richness and quality, and kinetics), basin modelling (burial, thermal and pressure history, migration patterns, charge timing) and fluid characterisation (correlation to source rock, charge episodes, secondary processes, pressure-volume-temperature (PVT) behaviour). For this study, over fifty one-dimensional (1D) basin models were built to provide a framework for a basin-wide pseudo three-dimensional (3D) petroleum systems model. The burial and thermal histories of the 1D models were calibrated using present-day measured and corrected temperatures and vitrinite reflectance. The 1D models were integrated with a 3D geological model of basin architecture, that was built from surfaces interpreted and gridded from 2D and 3D seismic surveys. The resulting pseudo-3D petroleum systems model was calibrated using the modelling results from the 1D models. The petroleum systems elements, source rock, reservoir and seal were defined based on published data. This regional model covers both the onshore Otway Basin and a large offshore area. Two major petroleum systems were modelled. The Austral 1 petroleum system consists of Late Jurassic to Early Cretaceous fluvial-lacustrine source rocks of the Casterton Formation and the Crayfish Subgroup, with reservoirs and intraformational seals in the Pretty Hill and Laira formations. The Austral 2 petroleum system consists of coals and carbonaceous source rocks in the Early Cretaceous Eumeralla Formation, with the Waarre Formation providing reservoirs and the Belfast Mudstone, the main regional seal. For both systems, modelling resulted in quantification of total generated and expelled hydrocarbons as well as the spatial distribution of these modelled volumes. Maps of these correlate very well with the locations of discovered fields and with oil and gas shows. Austral 2 is the most prolific petroleum system in the Otway Basin, generating mostly gas. Due to the abundance of source material and the thickness of the Eumeralla Formation, the system has generated and expelled large volumes of hydrocarbons since the Albian (around 110 million years ago) and is presently still generating in key areas such the Port Campbell Embayment and the Shipwreck Trough. Since gas is difficult to retain in reservoirs over long geological time periods, most of it has been lost through leakage and water washing. Nevertheless, volumes expelled over the last 10 million years still exceed many times the volumes of accumulations discovered to date. The Austral 1 petroleum system has been mostly active in the Penola Trough. The system has generated and expelled hydrocarbons since the Aptian/Albian (around 120 million years ago) and is presently expelling at low rates in depocentres such as the Penola Trough. Volumes expelled over the last 20 million years still exceed many times the volumes of accumulations discovered to date. Monte Carlo simulations were run to ascertain the uncertainties in the expelled hydrocarbon volumes, which result from uncertainties in the main parameters that define the petroleum system. The total modelled expelled volume of gas for Austral 2 in the Victorian Otway Basin between 10 and 0 million years is estimated at 634 Tcf (P50), of which around 82 Tcf is onshore and 499 Tcf is offshore. The total modelled volume of gas for Austral 1 in the Victorian Otway Basin between 20 and 0 million years is estimated at 89 Tcf (P50), of which around 64 Tcf is onshore and 24 Tcf is offshore. The calculated volumes are at least two orders of magnitude higher than what has been discovered to date in the Victorian Otway Basin. These findings highlight that the basin still has significant potential for future discoveries of conventional gas. Bibliographic reference: van Aarssen, B.G.K., 2020. Petroleum systems modelling, Otway Basin, Victoria. VGP Technical Report 48. Geological Survey of Victoria. Download: The downloadable version of this report is supplied in PDF format (PDF 53.4 MB) & Att A1 (ZIP 949 MB). Related products:
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