VGP Technical Report 55 - The application of seismic inversion methods to geophysical prospecting in the Port Campbell Embayment Onshore Otway Basin
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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: The Port Campbell Embayment is located in the eastern onshore Otway Basin and is host to gas discoveries in the Waarre Formation. Since the acquisition of multi-fold seismic data began in the 1990s in the Port Campbell Embayment, the understanding of the basin architecture and stratigraphy in the area has improved. This resulted in an increase in the number of exploration wells and discoveries in the Port Campbell Embayment. This study uses the multi-fold seismic data from three 3D seismic surveys acquired in the Port Campbell Embayment to first, refine the structural and stratigraphic interpretation across the area, and second, to apply two geophysical inversion methods to identify potential gas accumulations. This report is part of a series of studies undertaken by CSIRO as part of a collaborative research program with the Geological Survey of Victoria (GSV). Seismic methods for detecting gas fields rely on the identification of high contrast amplitudes in 3D seismic volumes (bright spots) where the high amplitude of the reflected seismic waves correlate with the presence of gas. However, as the bright spots are the result of rapid changes in the density and the contents of the rocks, similar features may also be the result of CO2-rich mixtures, gas/water mixes or changes in rock type. In addition, not all gas accumulations will produce obvious bright spots. To overcome the limitations of detecting gas fields from seismic data alone, seismic inversion is used to link the interpretation and properties of seismic data with the petrophysical interpretation and properties of well data. The result is a dataset of rock properties with a resolution between that of well and seismic data covering the area of a seismic survey. Other useful seismic attributes are also generated as part of this process. Two inversion models were used in this study to generate rock property and seismic attribute volumes, and from these, seismic anomaly maps at the Waarre Formation level have been produced over the Nirranda merged 3D survey. The first inversion method, an acoustic inversion model, was calibrated to 21 wells with available, relevant log data. Based on the correlated well logs, an Acoustic Impedance (AI) volume comprising the target Waarre and Eumeralla formations was produced. Additionally, a property known as Relative Acoustic Impedance (RAI) was calculated to account for local changes in rock properties, such as gas saturation or increased porosity. The second inversion method used CSIRO’s joint facies/fluid inversion (JIFI) inversion technology. This technology inverts for elastic properties and litho-fluid indicators and results in a classification of seismic facies (gas versus fluid filled rocks). This involved creating facies trend models for the seismic elastic parameters for each of the key intervals. Known fields were tested against the RAI maps, and there was a good correlation between the areas where the RAI values are highest and the mapped extent of previously discovered fields. A number of anomalies were identified using both methods. Many of the anomalies, indicating the presence of gas, remain untested and are of significant size, warranting further investigation. The individual prospectivity of the anomalies will depend on more detailed, site-specific, investigation to assess trapping mechanisms and seal quality, but the results of this study indicate a favourable exploration scenario for the Port ampbell Embayment. Bibliographic reference: Dance, T., Gunning, J., Caird, A., Pervukhina, M., Glubokovskikh, S., & Mora, A.S., 2021. The application of seismic inversion methods to geophysical prospecting in the Port Campbell Embayment, Onshore Otway Basin, Victoria. Victorian Gas Program Technical Report 55. Geological Survey of Victoria. Department of Jobs, Precincts and Regions. Download: The downloadable version of this report is supplied in PDF format (83.5 MB), Att A1 (ZIP 22 MB) & Att A2 (ZIP 901 MB). | ||||||
